Corneal myofibroblast biology and pathobiology: generation, persistence, and transparency

Myofibroblast transdifferentiation: The dark force in ocular wound healing and fibrosis

Wound healing is one of the most complex biological processes to occur in life. Repair of tissue following injury involves dynamic interactions between multiple cell types, growth factors, inflammatory mediators and components of the extracellular matrix (ECM). Aberrant and uncontrolled wound healing leads to a non-functional mass of fibrotic tissue. In the eye, fibrotic disease disrupts the normally transparent ocular tissues resulting in irreversible loss of vision. A common feature in fibrotic eye disease is the transdifferentiation of cells into myofibroblasts that can occur through a process known as epithelial-mesenchymal transition (EMT). Myofibroblasts rapidly produce excessive amounts of ECM and exert tractional forces across the ECM, resulting in the distortion of tissue architecture. Transforming growth factor-beta (TGFβ) plays a major role in myofibroblast transdifferentiation and has been implicated in numerous fibrotic eye diseases including corneal opacification, pterygium, anterior subcapsular cataract, posterior capsular opacification, proliferative vitreoretinopathy, fibrovascular membrane formation associated with proliferative diabetic retinopathy, submacular fibrosis, glaucoma and orbital fibrosis. This review serves to introduce the pathological functions of the myofibroblast in fibrotic eye disease. We also highlight recent developments in elucidating the multiple signaling pathways involved in fibrogenesis that may be exploited in the development of novel anti-fibrotic therapies to reduce ocular morbidity due to scarring.

Unravelling the interplay of sphingolipids and TGF-β signaling in the human corneal stroma

Purpose

To delineate the role of Sphingolipids (SPLs) in the human cornea and their cross-talks with transforming growth factor beta (TGF-β) in order to develop novel, non-invasive therapies.

Methods

Human corneal fibroblasts (HCFs) were harvested from healthy donors, stimulated with Vitamin C to promote extracellular matrix assembly, treated with exogenous sphingosine-1-phosphate (S1P) or sphingosine kinase inhibitor 2 (SPHK I₂) and isolated after 4 weeks for further analysis.

Results

Data showed that S1P led to a significant decrease in cellular migration where SPHK I₂ just delayed it for 24h. Significant modulation of the sphingolipid pathway was also noted. Sphingosine kinase-1 (SphK1) was significantly downregulated upon exogenous stimulation with S1P at a concentration of 5μM and Sphingosine kinase-2 (SphK2) was also significantly downregulated at concentrations of 0.01μM, 0.1μM, and 5μM; whereas no effects were observed upon stimulation with SPHK I_2. S1PR3 was significantly downregulated by 0.1μM and 5μM S1P and upregulated by 5μM and 10μM SPHK I₂. Furthermore, both S1P and SPHK I₂ regulated corneal fibrosis markers such as alpha-smooth muscle actin, collagen I, III, and V. We also investigated the interplay between two TGF-β isoforms and S1P/SPHK I₂ treatments and found that TGF-β1 and TGF-β3 were both significantly upregulated with the 0.1μM S1P but were significantly downregulated with the 5μM S1P concentration. When TGF-β1 was compared directly to TGF-β3 expression, we observed that TGF-β3 was significantly downregulated compared to TGF-β1 in the 5μM concentration of S1P. No changes were observed upon SPHK I₂ treatment.

Conclusion

Our study delineates the role of sphingolipids in the human cornea and highlights their different activities based on the cell/tissue type.

Tissue and cellular biomechanics during corneal wound injury and repair

Corneal wound healing is an enormously complex process that requires the simultaneous cellular integration of multiple soluble biochemical cues, as well as cellular responses to the intrinsic chemistry and biophysical attributes associated with the matrix of the wound space. Here, we document how the biomechanics of the corneal stroma are altered through the course of wound repair following keratoablative procedures in rabbits. Further we documented the influence that substrate stiffness has on stromal cell mechanics. Following corneal epithelial debridement, New Zealand white rabbits underwent phototherapeutic keratectomy (PTK) on the right eye (OD). Wound healing was monitored using advanced imaging modalities. Rabbits were euthanized and corneas were harvested at various time points following PTK. Tissues were characterized for biomechanics with atomic force microscopy and with histology to assess inflammation and fibrosis. Factor analysis was performed to determine any discernable patterns in wound healing parameters. The matrix associated with the wound space was stiffest at 7days post PTK. The greatest number of inflammatory cells were observed 3days after wounding. The highest number of myofibroblasts and the greatest degree of fibrosis occurred 21days after wounding. While all clinical parameters returned to normal values 400days after wounding, the elastic modulus remained greater than pre-surgical values. Factor analysis demonstrated dynamic remodeling of stroma occurs between days 10 and 42 during corneal stromal wound repair. Elastic modulus of the anterior corneal stroma is dramatically altered following PTK and its changes coincide initially with the development of edema and inflammation, and later with formation of stromal haze and population of the wound space with myofibroblasts. Factor analysis demonstrates strongest correlation between elastic modulus, myofibroblasts, fibrosis and stromal haze thickness, and between edema and central corneal thickness.

STATEMENT OF SIGNIFICANCE

Tissue biomechanics during the course of corneal wound healing is documented for the first time through atomic force microscopy, and is correlated with advanced clinical imaging and immunohistochemistry. Parameters obtained from the study are applied in a multivariate statistical model to cluster the data for better classification and monitor the wound repair process. Elastic modulus of the anterior corneal stroma is dramatically altered following wounding and correlates initially with the development of edema and inflammation, and later with formation of stromal haze and population of the wound space with myofibroblasts. Importantly, the occurrence of myofibroblasts is preceded by changes in tissue mechanics, which is important to consider in light of crosslinking procedures applied to treat corneal diseases.

Galectin-3 Inhibition by a Small-Molecule Inhibitor Reduces Both Pathological Corneal Neovascularization and Fibrosis

Purpose

Corneal neovascularization and scarring commonly lead to significant vision loss. This study was designed to determine whether a small-molecule inhibitor of galectin-3 can inhibit both corneal angiogenesis and fibrosis in experimental mouse models.

Methods

Animal models of silver nitrate cautery and alkaline burn were used to induce mouse corneal angiogenesis and fibrosis, respectively. Corneas were treated with the galectin-3 inhibitor, 33DFTG, or vehicle alone and were processed for whole-mount immunofluorescence staining and Western blot analysis to quantify the density of blood vessels and markers of fibrosis. In addition, human umbilical vein endothelial cells (HUVECs) and primary human corneal fibroblasts were used to analyze the role of galectin-3 in the process of angiogenesis and fibrosis in vitro.

Results

Robust angiogenesis was observed in silver nitrate-cauterized corneas on day 5 post injury, and markedly increased corneal opacification was demonstrated in alkaline burn-injured corneas on days 7 and 14 post injury. Treatment with the inhibitor substantially reduced corneal angiogenesis and opacification with a concomitant decrease in α-smooth muscle actin (α-SMA) expression and distribution. In vitro studies revealed that 33DFTG inhibited VEGF-A-induced HUVEC migration and sprouting without cytotoxic effects. The addition of exogenous galectin-3 to corneal fibroblasts in culture induced the expression of fibrosis-related proteins, including α-SMA and connective tissue growth factor.

Conclusions

Our data provide proof of concept that targeting galectin-3 by the novel, small-molecule inhibitor, 33DFTG, ameliorates pathological corneal angiogenesis as well as fibrosis. These findings suggest a potential new therapeutic strategy for treating ocular disorders related to pathological angiogenesis and fibrosis.

Antifibrotic Actions of Peroxisome Proliferator-Activated Receptor γ Ligands in Corneal Fibroblasts Are Mediated by β-Catenin-Regulated Pathways

Wound healing after corneal injury typically involves fibrosis, with transforming growth factor β1 (TGF-β1) as one of its strongest mediators. A class of small molecules-peroxisome proliferator-activated receptor γ (PPARγ) ligands-exert potent antifibrotic effects in the cornea by blocking phosphorylation of p38 mitogen-activated protein kinase (MAPK). However, why this blocks fibrosis remains unknown. Herein, we show that PPARγ ligands (rosiglitazone, troglitazone, and 15-deoxy-Δ12,14-prostaglandin J2) decrease levels of β-catenin. We also show that β-catenin siRNA and the Wingless/integrated (Wnt) inhibitor pyrvinium block the ability of corneal fibroblasts to up-regulate synthesis of α-smooth muscle actin (α-SMA), collagen 1 (COL1), and fibronectin (FN) in response to TGF-β1. Activation of TGF-β receptors and p38 MAPK increased glycogen synthase kinase 3β (GSK3β) phosphorylation, whereas a chemical inhibitor of p38 MAPK (SB203580) reduced the phosphorylation of GSK3β, decreasing active β-catenin levels in both cytoplasmic and nuclear fractions. Finally, lithium chloride, a GSK3 inhibitor, also attenuated the TGF-β1-induced increase in α-SMA, COL1, and FN expression. All in all, our results suggest that TGF-β1 stimulation increases active β-catenin concentration in cultured corneal fibroblasts through p38 MAPK regulation of canonical Wnt/β-catenin signaling, increasing α-SMA, COL1, and FN synthesis. Thus, PPARγ ligands, by blocking TGF-β1-induced p38 MAPK phosphorylation, prevent increases in both total and active β-catenin through p38 MAPK-GSK3β signaling.

Sustained activation of ERK1/2 MAPK in Schwann cells causes corneal neurofibroma

Recent studies have shown that constitutive activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in Schwann cells (SCs) increases myelin thickness in transgenic mice. In this secondary analysis, we report that these transgenic mice develop a postnatal corneal neurofibroma with the loss of corneal transparency by age six months. We show that expansion of non-myelinating SCs, under the control of activated ERK1/2, also drive myofibroblast differentiation that derives from both SC precursors and resident corneal keratocytes. Further, these mice also harbor activated mast cells in the central cornea, which contributes to pathological corneal neovascularization and fibrosis. This breach of corneal avascularity and immune status is associated with the growth of the tumor pannus, resulting in a corneal stroma that is nearly four times its normal size. In corneas with advanced disease, some axons became ectopically myelinated, and the disruption of Remak bundles is evident. To determine whether myofibroblast differentiation was linked to vimentin, we examined the levels and phosphorylation status of this fibrotic biomarker. Concomitant with the early upregulation of vimentin, a serine 38-phosphorylated isoform of vimentin (pSer38vim) increased in SCs, which was attributed primarily to the soluble fraction of protein-not the cytoskeletal portion. However, the overexpressed pSer38vim became predominantly cytoskeletal with the growth of the corneal tumor. Our findings demonstrate an unrecognized function of ERK1/2 in the maintenance of corneal homeostasis, wherein its over-activation in SCs promotes corneal neurofibromas. This study is also the first report of a genetically engineered mouse that spontaneously develops a corneal tumor.

Regeneration of Defective Epithelial Basement Membrane and Restoration of Corneal Transparency After Photorefractive Keratectomy

PURPOSE

To study regeneration of the normal ultrastructure of the epithelial basement membrane (EBM) in rabbit corneas that had -9.00 D photorefractive keratectomy (PRK) and developed late haze (fibrosis) with restoration of transparency over 1 to 4 months after surgery and in corneas that had incisional wounds.

METHODS

Twenty-four rabbits had one of their eyes included in one of the two procedure groups (-9.00 D PRK or nearly full-thickness incisional wounds), whereas the opposite eyes served as the unwounded control group. All corneas were evaluated with slit-lamp photographs, transmission electron microscopy, and immunohistochemistry for the myofibroblast marker alpha-smooth muscle actin and collagen type III.

RESULTS

In the -9.00 D PRK group, corneas at 1 month after surgery had dense corneal haze and no evidence of regenerated EBM ultrastructure. However, by 2 months after surgery small areas of stromal clearing began to appear within the confluent opacity (lacunae), and these corresponded to small islands of normally regenerated EBM detected within a larger area of the excimer laser-ablated zone with no evidence of normal EBM. By 4 months after surgery, the EBM was fully regenerated and the corneal transparency was completely restored in the ablated zone. In the incisional wound group, the two dense, linear corneal opacities were observed at 1 month after surgery and progressively faded by 2 and 3 months after surgery. The EBM ultrastructure was fully regenerated at the site of the incisions, including around epithelial plugs that extended into the stroma, by 1 month after surgery in all eyes.

CONCLUSIONS

In the rabbit model, spontaneous resolution of corneal fibrosis (haze) after high correction PRK is triggered by regeneration of EBM with normal ultrastructure in the excimer laser-ablated zone. Conversely, incisional wounds heal in rabbit corneas without the development of myofibroblasts because the EBM regenerates normally by 1 month after surgery. [J Refract Surg. 2017:33(5):337-346.].

Effects of TGFβ1, PDGF-BB, and bFGF, on human corneal fibroblasts proliferation and differentiation during stromal repair

In an effort to improve the regenerative nature of corneal repair, this study reports the use of an in vitro human corneal fibroblasts (HCFs) wound model after treatment with three of the main growth factors (GFs) involved in corneal healing: transforming growth factor beta 1 (TGFβ1), platelet-derived growth factor BB-isoform (PDGF-BB), and basic fibroblast growth factor (bFGF) in order to delve in cell proliferation and differentiation processes. HCFs were mechanically wounded. The individual effect of TGFβ1, PDGF-BB, and bFGF on cell proliferation and differentiation during the repair process was studied at different time points until wound closure. Wound dimensions and morphological changes were evaluated by microscopy. Cell proliferation and myofibroblast differentiation were analyzed by immunofluorescence cytochemistry. Changes in cell morphology were apparent at Day 4. PDGF-BB- and bFGF-treated cells had fibroblast-like morphology. TGFβ1 stimulated proliferation in the wound edge and surrounding area, induced myofibroblast differentiation and inhibited cellular migration. PDGF-BB induced rapid wound closure due to proliferation, high motility, and late myofibroblast differentiation. The time course of closure induced by bFGF was similar to that for PDGF-BB, but was mostly due to proliferation in the wound area, and inhibited myofibroblast differentiation. Each of the GFs induced increases in responses promoting stromal repair differently. This study provides insight regarding how to optimize the outcome of stromal repair following corneal injury.

Molecular Hydrogen Effectively Heals Alkali-Injured Cornea via Suppression of Oxidative Stress

The aim of this study was to examine the effect of molecular hydrogen (H₂) on the healing of alkali-injured cornea. The effects of the solution of H₂ in phosphate buffered saline (PBS) or PBS alone topically applied on the alkali-injured rabbit cornea with 0.25 M NaOH were investigated using immunohistochemical and biochemical methods. Central corneal thickness taken as an index of corneal hydration was measured with an ultrasonic pachymeter. Results show that irrigation of the damaged eyes with H₂ solution immediately after the injury and then within next five days renewed corneal transparency lost after the injury and reduced corneal hydration increased after the injury to physiological levels within ten days after the injury. In contrast, in injured corneas treated with PBS, the transparency of damaged corneas remained lost and corneal hydration elevated. Later results-on day 20 after the injury-showed that in alkali-injured corneas treated with H₂ solution the expression of proinflammatory cytokines, peroxynitrite, detected by nitrotyrosine residues (NT), and malondialdehyde (MDA) expressions were very low or absent compared to PBS treated injured corneas, where NT and MDA expressions were present. In conclusion, H₂ solution favorably influenced corneal healing after alkali injury via suppression of oxidative stress.

Single-cell analysis reveals IGF-1 potentiation of inhibition of the TGF-β/Smad pathway of fibrosis in human keratocytes in vitro

Corneal wound healing is often affected by TGF-β–mediated fibrosis and scar formation. Guided fibrosis with IGF-1 and antifibrotic substances might maintain corneal transparency. Primary human corneal keratocytes under serum-free conditions were used as a model of corneal stromal wounding, with markers of corneal fibrosis and opacity studied under TGF-β2 stimulation. Single-cell imaging flow cytometry was used to determine nuclearization of Smad3, and intracellular fluorescence intensity of Smad7 and the corneal crystallin aldehyde dehydrogenase 3A1. Extracellular matrix proteoglycans keratocan and biglycan were quantified using ELISAs. On the TGF-β2 background, the keratocytes were treated with IGF-1, and suberoylanilidehydroxamic acid (SAHA) or halofuginone ± IGF-1. IGF-1 alone decreased Smad3 nuclearization and increased aldehyde dehydrogenase 3A1 expression, with favorable extracellular matrix proteoglycan composition. SAHA induced higher Smad7 levels and inhibited translocation of Smad3 to the nucleus, also when combined with IGF-1. Immunofluorescence showed that myofibroblast transdifferentiation is attenuated and appearance of fibroblasts is favored by IGF-1 alone and in combination with the antifibrotic substances. The TGF-β/Smad pathway of fibrosis and opacity was inhibited by IGF-1, and further with SAHA in particular, and with halofuginone. IGF-1 is thus a valid aid to antifibrotic treatment, with potential for effective and transparent corneal wound healing.

Pathological and Immunohistochemical Alterations of the Cornea in Congenital Corneal Opacification Secondary to Primary Congenital Glaucoma and Peters Anomaly

PURPOSE

To examine the immunohistochemical alterations in the corneal stroma in Peters anomaly (PA) and congenital glaucoma (CG) compared with age-matched normals and acquired adult corneal scarring (AACS).

METHODS

The clinical features of PA and CG patients who underwent penetrating keratoplasty were recorded. Immunohistochemistry of cornea and control tissue (normal and acquired corneal scars) was performed with antibodies against collagen types I, III, keratan sulfate, lumican, decorin, and smooth muscle actin followed by semiquantitative analysis of immunolabeling.

RESULTS

Clinical features in 2 groups were consistent with PA and CG. Microscopy showed thickened stromal collagen bundles in PA (n = 15), CG (n = 11), and AACS (n = 20) compared with normals (n = 18). PA and CG had distinct immunophenotypes compared with controls. Type I collagen labeling was more intense in CG compared with PA (intensity grading (IG) 2.73 vs. 2.07; P < 0.001). Decorin, lumican, and keratan sulfate labeling was significantly less intense in PA versus AACS (IG; 1.91, 0.38, 1.75 in PA and 2.7, 1.11, 2.61 in AACS. respectively; P = 0.002, P = 0.001 and P = 0.004) and normals (IG 1.92, 1.06, 2.59 respectively; P < 0.001, P < 0.001 and P = 0.005). Collagen I labeling was less intense in CG versus AACS (IG 2.73 vs. 3.09) (P = 0.007). Collagen III labeling was more intense in PA/CG than in normals (IG 0.9, 0.64, 0.62 retrospectively) (P < 0.001 in both).

CONCLUSIONS

The immunophenotype of the corneal scar in PA and CG differs from normal corneas and so does PA from AACS. The similarities between CG and AACS suggest that CG scarring has an acquired component.

Effect of bone marrow and adipose tissue-derived mesenchymal stem cells on the natural course of corneal scarring after penetrating injury

In the present study, we investigate and compare the efficacy of bone marrow- and adipose tissue-derived mesenchymal stem cell (MSCs) in corneal wound healing. A penetrating injury was created in the right corneas of Wistar rats (n = 40). Ten microliters of phosphate-buffered solution (PBS) containing 2 × 10(5) green fluorescent protein (GFP) labeled bone-marrow-derived MSCs to group 1 (n = 15), 10 μl of PBS containing 2 × 10(5) GFP-labeled adipose-tissue-derived MSCs to group 2 (n = 15), 10 μl PBS was injected into anterior chamber in group 3 (n = 10, control). Corneal opacity scoring, in vivo confocal microscopy, and histopathological evaluation were done at the end of 8 weeks. Immunofluorescence sections were evaluated to detect transplanted cells. Immune staining was performed to measure the expression levels of keratocan, aldehyde dehydrogenase (ALDH) and CD34. The gene expression levels of tumor necrosis factor (TNF-α), the interleukin 6 receptor (IL-6R), interleukin 12b (IL-12b), and transforming growth factor beta (TGF-β1) was measured on corneas. The establishment of stem cells in the corneas of the transplanted groups was confirmed by immunofluorescence staining. The expression of keratocan, ALDH, and CD34 increased in the transplanted groups (p < 0.05). The density of keratocytes increased significantly in both transplanted groups according to the in vivo confocal microscopy data (p < 0.05). The expression of TNF-α, IL-6R, and IL-12b decreased significantly in the transplanted groups (p < 0.05). Based on our findings, we consider that allogeneic stem cells facilitate the regeneration of corneal stroma and can be a cell source for stromal repopulation in diseased cornea.

Inflammatory Biomarkers Profile as Microenvironmental Expression in Keratoconus

Keratoconus is a degenerative disorder with progressive stromal thinning and transformation of the normal corneal architecture towards ectasia that results in decreased vision due to irregular astigmatism and irreversible tissue scarring. The pathogenesis of keratoconus still remains unclear. Hypotheses that this condition has an inflammatory etiopathogenetic component apart from the genetic and environmental factors are beginning to escalate in the research domain. This paper covers the most relevant and recent published papers regarding the biomarkers of inflammation, their signaling pathway, and the potentially new therapeutic options in keratoconus.

The effect of growth factor supplementation on corneal stromal cell phenotype in vitro using a serum-free media

In order to expand cells quickly and in high numbers for corneal tissue engineering applications corneal stromal cells, or keratocytes, are often cultured in the presence of serum. However, keratocytes become fibroblastic when exposed to serum leading to a downregulation of corneal stromal specific markers. The purpose of this current study was to determine if corneal stromal cells, made fibroblastic by serum, could display native quiescent keratocyte characteristics when cultured under serum-free conditions supplemented by different growth factors. Markers specific to a native keratocyte phenotype such as keratocan and aldehyde dehydrogenase 3A1 (ALDH3A1) and those specific to a fibrotic phenotype such as α-smooth muscle actin (αSMA) and collagen type III were examined. Cells were cultured in monolayer, self-assembled pellets or collagen hydrogels. Growth factors known to modulate keratocyte phenotype were chosen to supplement the serum free media, specifically insulin-like growth factor 1 (IGF-1) and transforming growth factor beta 1 and 3 (Tβ1 and Tβ3). The effects of serum-free media, growth factors and culture system on cell proliferation and morphology and extracellular matrix (ECM) synthesis were evaluated. The expression of keratocyte markers was evaluated by real-time PCR, immunofluorescent staining and western blotting. In addition, cell migration was tested using scratch assays. When serum was removed from the cells they displayed a reduction in proliferation and ECM synthesis (not significant), in addition to a significant decrease in migratory capacity (p < 0.05). Serum-free media promoted increased expression of keratocan (130.68 ± 47.44-fold increase; p < 0.05) and collagen type I (15.58 ± 9.49-fold increase; p < 0.05). However, there was no significant change in ALDH3A1 and αSMA expression, while collagen type III expression was significantly increased (44.66 ± 25.61-fold increase; p < 0.05). In addition, cells retained an elongated fibroblastic morphology. In monolayer, the addition of Tβ1 and Tβ3 to serum free media resulted in reduced expression of keratocan, ALDH3A1 and collagen type I and III, increased expression of αSMA (p < 0.05) and an increase in cell proliferation and ECM synthesis. Pellet cultured cells demonstrated a significant increase in ALDH3A1 and collagen type I over 14 days relative to day 5 (p < 0.05), however the expression of fibrotic markers was also enhanced. Cells in collagen hydrogels did not increase expression of keratocyte markers in serum free conditions and underwent contraction in Tβ1 and Tβ3 supplemented media. These results demonstrate that corneal fibroblasts only partially express the phenotypic characteristics of keratocytes when cultured in serum-free medium. While growth factors did not significantly enhance this phenotype, it appears that pellet or self-assembled culture could be more beneficial to promoting a keratocyte phenotype.

Smad-independent TGF-β2 signaling pathways in human trabecular meshwork cells

Aberrant expression and signaling of Transforming Growth Factor (TGF)-β is strongly associated with development of elevated intraocular pressure (IOP) and primary open-angle glaucoma (POAG). In cells of the trabecular meshwork, a key component of the conventional outflow pathway, TGF-β is well-known to promote expression of multiple ocular hypertensive mediators, including genes associated with fibrosis as well as cellular contractility. These effects are mediated by induction of canonical (Smad) as well as non-canonical (MAPK, Rho GTPase) signaling cascades. In the present review, we will highlight the non-canonical, Smad-independent signaling pathways activated by TGF-β2 in human TM cells, as well as the genes known to be induced by non-canonical TGF-β2 signaling.

Modulation of Smad signaling by non-TGFβ components in myofibroblast generation during wound healing in corneal stroma

Corneal scarring/fibrosis disturbs normal transparency and curvature of the tissue and thus impairs vision. The lesion is characterized by appearance of myofibroblasts, the key player of the fibrogenic reaction, and excess accumulation of extracellular matrix. Inflammatory/fibrogenic growth factors or cytokines expressed in inflammatory cells that infiltrate into injured tissues play a pivotal role in fibrotic tissue formation. In this article the pathogenesis of fibrosis/scarring in the corneal stroma is reviewed focusing on the roles of myofibroblast, the key player in corneal stromal wound healing and fibrosis, and cytoplasmic signals activated by the fibrogenic cytokine, transforming growth factor β (TGFβ). Although it is established that TGFβ/Smad signal is essential to the process of keratocyte-myofibroblast transformation in a healing corneal stroma post-injury. This article emphasizes the involvement of non-TGFβ molecular mechanisms in modulating Smad signal. We focus on the roles of matricellular proteins, i.e., osteopontin and tenascin C, and as cellular components, the roles of transient receptor potential (TRP) cation channel receptors are discussed. Our intent is to draw attention to the possibility of signal transduction cascade modulation (e.g., Smad signal and mitogen-activated protein kinases, by gene transfer and other related technology) as being beneficial in a clinical setting to reduce or even prevent corneal stromal tissue fibrosis/scarring and inflammation.

The corneal fibrosis response to epithelial-stromal injury

The corneal wound healing response, including the development of stromal opacity in some eyes, is a process that often leads to scarring that occurs after injury, surgery or infection to the cornea. Immediately after epithelial and stromal injury, a complex sequence of processes contributes to wound repair and regeneration of normal corneal structure and function. In some corneas, however, often depending on the type and extent of injury, the response may also lead to the development of mature vimentin+ α-smooth muscle actin+ desmin+ myofibroblasts. Myofibroblasts are specialized fibroblastic cells generated in the cornea from keratocyte-derived or bone marrow-derived precursor cells. The disorganized extracellular matrix components secreted by myofibroblasts, in addition to decreased expression of corneal crystallins in these cells, are central biological processes that result in corneal stromal fibrosis associated with opacity or "haze". Several factors are associated with myofibroblast generation and haze development after PRK surgery in rabbits, a reproducible model of scarring, including the amount of tissue ablated, which may relate to the extent of keratocyte apoptosis in the early response to injury, irregularity of stromal surface after surgery, and changes in corneal stromal proteoglycans, but normal regeneration of the epithelial basement membrane (EBM) appears to be a critical factor determining whether a cornea heals with relative transparency or vision-limiting stromal opacity. Structural and functional abnormalities of the regenerated EBM facilitate prolonged entry of epithelium-derived growth factors such as transforming growth factor β (TGF-β) and platelet-derived growth factor (PDGF) into the stroma that both drive development of mature myofibroblasts from precursor cells and lead to persistence of the cells in the anterior stroma. A major discovery that has contributed to our understanding of haze development is that keratocytes and corneal fibroblasts produce critical EBM components, such as nidogen-1, nidogen-2 and perlecan, that are essential for complete regeneration of a normal EBM once laminin secreted by epithelial cells self-polymerizes into a nascent EBM. Mature myofibroblasts that become established in the anterior stroma are a barrier to keratocyte/corneal fibroblast contributions to the nascent EBM. These myofibroblasts, and the opacity they produce, often persist for months or years after the injury. Transparency is subsequently restored when the EBM is completely regenerated, myofibroblasts are deprived of TGFβ and undergo apoptosis, and the keratocytes re-occupy the anterior stroma and reabsorb disordered extracellular matrix. The aim of this review is to highlight factors involved in the generation of stromal haze and its subsequent removal.

Corneal Wound Healing Requires IKB kinase β Signaling in Keratocytes

IkB kinase β (IKKβ) is a key signaling kinase for inflammatory responses, but it also plays diverse cell type-specific roles that are not yet fully understood. Here we investigated the role of IKKβ in the cornea using Ikkβ^ΔCS mice in which the Ikkβ gene was specifically deleted in the corneal stromal keratocytes. The Ikkβ^ΔCS corneas had normal morphology, transparency and thickness; however, they did not heal well from mild alkali burn injury. In contrast to the Ikkβ^F/F corneas that restored transparency in 2 weeks after injury, over 50% of the Ikkβ^ΔCS corneas failed to fully recover. They instead developed recurrent haze with increased stromal thickness, severe inflammation and apoptosis. This pathogenesis correlated with sustained myofibroblast transformation with increased α smooth muscle actin (α-SMA) expression, higher levels of senescence β-Gal activity and scar tissue formation at the late stage of wound healing. In addition, the Ikkβ^ΔCS corneas displayed elevated expression of hemo-oxygenase-1 (HO-1), a marker of oxidative stress, and activation of stress signaling pathways with increased JNK, c-Jun and SMAD2/3 phosphorylation. These data suggest that IKKβ in keratocytes is required to repress oxidative stress and attenuate fibrogenesis and senescence in corneal wound healing.

A cell-based screening assay to identify pharmaceutical compounds that enhance the regenerative quality of corneal repair

The goal of this study was to develop and validate a simple but quantitative cell-based assay to identify compounds that might be used pharmaceutically to give tissue repair a more regenerative character. The cornea was used as the model, and some specific aspects of repair in this organ were incorporated into assay design. A quantitative cell-based assay was developed based on transcriptional promoter activity of fibrotic marker genes ACT2A and TGFB2. Immortalized corneal stromal cells (HTK) or corneal epithelial cells (HCLE) were tested and compared to primary corneal stromal cells. Cells were transiently transfected with constructs containing the firefly luciferase reporter gene driven by transcriptional promoters for the selected fibrotic marker genes. A selected panel of seven chemical test compounds was used, containing three known fibrosis inhibitors: lovastatin (LOV), tyrphostin AG 1296 (6,7-dimethoxy-3-phenylquinoxaline) and SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole), and four potential fibrosis inhibitors: 5-iodotubercidin (4-amino-5-iodo-7-(β-D-ribofuranosyl)-pyrrolo(2,3-d)pyrimidine), anisomycin, DRB (5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole) and latrunculin B. Transfected cells were treated with TGFB2 in the presence or absence of one of the test compounds. To validate the assay, compounds were tested for their direct effects on gene expression in the immortalized cell lines and primary human corneal keratocytes using RT-PCR and immunohistochemistry. Three "hits" were validated LOV, SB203580 and anisomycin. This assay, which can be applied in a high throughput format to screen large libraries of uncharacterized compounds, or known compounds that might be repurposed, offers a valuable tool for identifying new treatments to address a major unmet medical need. Anisomycin has not previously been characterized as antifibrotic, thus, this is a novel finding of the study.

Differential expression of epithelial basement membrane components nidogens and perlecan in corneal stromal cells in vitro

PURPOSE

The purpose of this study was to examine the expression of corneal epithelial basement membrane (EBM) components in different corneal stromal cell types. In vitro model systems were used to explore the expression of EBM components nidogen-1, nidogen-2, and perlecan that are the primary components in the lamina lucida and the lamina densa that defectively regenerate in corneas with stromal opacity after in -9.0 D photorefractive keratectomy (PRK).

METHODS

Primary rabbit corneal stromal cells were cultured using varying serum concentrations and exogenous growth factors, including fibroblast growth factor (FGF)-2 and transforming growth factor (TGF)-β1, to optimize the growth of each cell type of interest. The expression of the keratocyte-specific marker keratocan and the myofibroblast-specific marker α-smooth muscle actin (α-SMA) were analyzed with real-time PCR, western blot, and immunocytochemical staining to evaluate the specificity of the cell types and select optimal conditions (high keratocan and low α-SMA for keratocytes; low keratocan and high α-SMA for myofibroblasts; low keratocan and low α-SMA for corneal fibroblasts). The expression of the EBM components nidogen-1, nidogen-2, and perlecan was evaluated in each corneal cell type using real-time PCR, immunostaining, and western blotting. In agreement with previous studies, serum-free DMEM was found to be optimal for keratocytes, DMEM with 10% serum and 40 ng/ml FGF-2 yielded the best marker profile for corneal fibroblasts, and DMEM with 1% serum and 2 ng/ml TGF-β1 was found to be optimal for myofibroblasts.

RESULTS

Nidogen-1 and nidogen-2 mRNAs were highly expressed in keratocytes, whereas perlecan was highly expressed in myofibroblasts. In keratocytes, nidogen-2 and perlecan proteins were expressed predominantly in intracellular compartments, whereas in myofibroblasts expression of both EBM components was observed diffusely throughout the cell. Although the perlecan mRNA levels were high in the myofibroblasts, the qualitative protein expression was different from that of the keratocytes. Corneal fibroblasts produced a low amount of each EBM component.

CONCLUSIONS

We have demonstrated qualitative and quantitative differences in the expression of nidogen-1, nidogen-2, and perlecan by keratocytes compared to myofibroblasts that may contribute to defective regeneration of the lamina lucida and the lamina densa of the EBM associated with late stromal haze after high-correction PRK.

Topical delivery of aqueous micellar resolvin E1 analog (RX-10045)

PURPOSE

The primary objective of this study were to optimize aqueous micellar solution of isopropyl ester prodrug of resolvin (RX-10045), study in vivo ocular compatibility and tissue distribution following topical administration.

METHODS

An optimized ratio of hydrogenated castor-oil and octoxynol-40 (1.0:0.05 wt%) was prepared to entrap RX-10045 in the hydrophobic core of micelles. RX-10045 aqueous micelles were subjected to characterization. In vitro stability studies were performed at 4 °C, 25 °C and 40 °C. In vivo studies were conducted in New Zealand albino rabbits following topical drop administration.

RESULTS

Aqueous RX-10045 micellar solutions were successfully prepared. Micelles had a mean diameter of ∼12 nm with low negative surface charge. RX-10045 demonstrated high stability in citrate buffer (0.0 1M) at 40 °C. Hackett-McDonald ocular irritation scores were extremely low comparable to negative control. No significant difference in intraocular pressure was noted. Electroretinography studies did not reveal any retinal damage after multiple dosing of RX-10045 micellar solution. Ocular tissue distribution studies demonstrated appreciable drug concentrations in anterior ocular tissues. Moreover, RX-10008 (active metabolite of RX-10045) was detected in retina/choroid upon topical drop instillation.

CONCLUSIONS

A clear, stable, aqueous 0.1% RX-10045 micellar formulation was successfully prepared. Micellar solution was well-tolerated and did not have any measurable tissue damage in rabbit ocular tissues. Micelles appear to follow conjunctival/scleral pathway to reach back-of-the-eye tissue (retina). Topical aqueous formulations may be employed to treat posterior ocular diseases. Such micellar topical formulations may be more patient acceptable over invasive routes of administrations such as intravitreal injection/implants.

Transforming Growth Factor Beta 1 Modulates the Functional Expression of the Neurokinin-1 Receptor in Human Keratocytes

Purpose: Transforming growth factor beta 1 (TGF-β1) is a cytokine involved in a variety of processes, such as differentiation of fibroblasts into myofibroblasts. TGF-β1 has also been shown to delay the internalization of the neurokinin-1 receptor (NK-1 R) after its activation by its ligand, the neuropeptide substance P (SP). NK-1 R comprises two naturally occurring variants, a full-length and a truncated form, triggering different cellular responses. SP has been shown to affect important events in the cornea – such as stimulating epithelial cell proliferation – processes that are involved in corneal wound healing and thus in maintaining the transparency of the corneal stroma. An impaired signaling through NK-1 R could thus impact the visual quality. We hypothesize that TGF-β1 modulates the expression pattern of NK-1 R in human corneal stroma cells, keratocytes. The purpose of this study was to test that hypothesis.

Methods: Cultures of primary keratocytes were set up with cells derived from healthy human corneas, obtained from donated transplantation graft leftovers, and characterized by immunocytochemistry and Western blot. Immunocytochemistry for TGF-β receptors and NK-1 R was performed. Gene expression was assessed with real-time polymerase chain reaction (qPCR).

Results: Expression of TGF-β receptors was confirmed in keratocytes in vitro. Treating the cells with TGF-β1 significantly reduced the gene expression of NK-1 R. Furthermore, immunocytochemistry for NK-1 R demonstrated that it is specifically the expression of the full-length isotype of the receptor that is reduced after treatment with TGF-β1, which was also confirmed with qPCR using a specific probe for the full-length receptor.

Conclusions: TGF-β1 down-regulates the gene expression of the full-length variant of NK-1 R in human keratocytes, which might impact its signaling pathway and thus explain the known delay in internalization after activation by SP seen with TGF-β1 treatment.

Corneal Complications During and After Vitrectomy for Retinal Detachment in Photorefractive Keratectomy Treated Eyes

To evaluate the occurrence of late-onset corneal haze (LOCH) after vitrectomy for rhegmatogenous retinal detachment (RRD) in photorefractive keratectomy (PRK)-treated eyes. This observational cohort study comprised 13 eyes of 13 patients who underwent vitrectomy for RRD and who had been subjected to PRK years earlier. The occurrence of LOCH was evaluated together with all the preoperative, intraoperative, and postoperative factors that could affect final corneal status. LOCH developed in 2 eyes. Both patients had undergone PRK for high myopia--one 3 years and the other 9 years prior to RRD. Both patients presented with RRD due to giant retinal tear and were subjected to scleral buckle, 20-gauge vitrectomy, and silicone oil tamponade. Three months after vitrectomy and 1 month after silicone oil removal they both developed LOCH. During vitreoretinal surgery neither of the 2 patients needed mechanical epithelial debridement. Intraoperative epithelial debridement was performed in 2 of the other patients of the series, who had undergone previous PRK for high myopia and had clear corneas at presentation; in 1 of them this manoeuvre hampered intraoperative visualization. Follow-up after retinal detachment surgery ranged from 6 to 156 months (mean, 37.5 months). Subepithelial corneal scarring may be reactivated many years after PRK. In our series this happened after vitrectomy.

Myofibroblasts

Myofibroblasts are activated in response to tissue injury with the primary task to repair lost or damaged extracellular matrix. Enhanced collagen secretion and subsequent contraction - scarring - are part of the normal wound healing response and crucial to restore tissue integrity. Due to myofibroblasts ability to repair but not regenerate, accumulation of scar tissue is always associated with reduced organ performance. This is a fair price to pay by the body for not falling apart. Whereas myofibroblasts typically vanish after successful repair, dysregulation of the normal repair process can lead to persistent myofibroblast activation, for instance by chronic inflammation or mechanical stress in the tissue. Excessive repair leads to the accumulation of stiff collagenous ECM contractures - fibrosis - with dramatic consequences for organ function. The clinical need to terminate detrimental myofibroblast activities has stimulated researchers to answer a number of essential questions: where do myofibroblasts come from, what are the factors leading to their activation, how do we discriminate myofibroblasts from other cells, what is the molecular basis for their contractile activity, and how can we stop or at least control them? This article reviews the current state of the myofibroblast literature by emphasizing their role in ocular repair and fibrosis. It appears that although the eye is quite an extraordinary organ, ocular myofibroblasts behave or misbehave just like their siblings in other organs.

Quercetin modulates keratoconus metabolism in vitro

Corneal scarring is the result of a disease, infection or injury. The resulting scars cause significant loss of vision or even blindness. To-date, the most successful treatment is corneal transplantation, but it does not come without side effects. One of the corneal dystrophies that are correlated with corneal scarring is keratoconus (KC). The onset of the disease is still unknown; however, altered cellular metabolism has been linked to promoting the fibrotic phenotype and therefore scarring. We have previously shown that human keratoconus cells (HKCs) have altered metabolic activity when compared to normal human corneal fibroblasts (HCFs). In our current study, we present evidence that quercetin, a natural flavonoid, is a strong candidate for regulating metabolic activity of both HCFs and HKCs in vitro and therefore a potential therapeutic to target the altered cellular metabolism characteristic of HKCs. Targeted mass spectrometry-based metabolomics was performed on HCFs and HKCs with and without quercetin treatment in order to identify variations in metabolite flux. Overall, our study reveals a novel therapeutic target OF Quercetin on corneal stromal cell metabolism in both healthy and diseased states. Clearly, further studies are necessary in order to dissect the mechanism of action of quercetin.

Inhibitory effects of PPARγ ligands on TGF-β1-induced CTGF expression in cat corneal fibroblasts

Ligands of Peroxisome Proliferator Activated Receptor gamma (PPARγ) possess strong anti-fibrotic properties in the cornea and several other body tissues. In the cornea, we recently showed this class of molecules to prevent stromal myofibroblast differentiation partially by blocking the actions of p38 mitogen-activated protein kinase (MAPK). However, given the important role assigned to connective tissue growth factor (CTGF) in mediating corneal fibrosis, here we asked whether PPARγ ligands also act by affecting transforming growth factor-β (TGF-β) 1-induced expression of CTGF in cultured corneal fibroblasts. Corneal keratocytes were isolated from young, adult cats and early passage cells were exposed to TGF-β1 with or without the PPARγ ligands Rosiglitazone, Troglitazone and 15d-PGJ2. Western blots were used to assay levels of CTGF and alpha smooth muscle actin (αSMA), a marker of myofibroblast differentiation. CTGF siRNA demonstrated a critical role for CTGF in TGF-β1-mediated myofibroblast differentiation, while exogenously applied CTGF potentiated the pro-fibrogenic effects of TGF-β1. TGF-β1-mediated increases in CTGF and αSMA expression were strongly inhibited by all three PPARγ ligands tested, and by a c-jun N-terminal kinase (JNK) inhibitor. However, while extracellular signal-regulated kinase (ERK) 1/2, protein kinase B (AKT) and p38 MAPK inhibitors also blocked TGF-β1-induced αSMA induction, they did not dampen TGF-β1-induced increases in levels of CTGF. Thus, we conclude that PPARγ ligands block TGF-β1-induced increases in CTGF levels in cat corneal fibroblasts. They appear to do this in addition to their anti-fibrotic effect on p38 MAPK, providing a second intracellular pathway by which PPARγ ligands block αSMA induction.

Corneal Regeneration After Photorefractive Keratectomy: A Review

Photorefractive keratectomy (PRK) remodels corneal stroma to compensate refractive errors. The removal of epithelium and the ablation of stroma provoke the disruption of corneal nerves and a release of several peptides from tears, epithelium, stroma and nerves. A myriad of cytokines, growth factors, and matrix metalloproteases participate in the process of corneal wound healing. Their balance will determine if reepithelization and stromal remodeling are appropriate. The final aim is to achieve corneal transparency for restoring corneal function, and a proper visual quality. Therefore, wound-healing response is critical for a successful refractive surgery. Our goal is to provide an overview into how corneal wounding develops following PRK. We will also review the influence of intraoperative application of mitomycin C, bandage contact lenses, anti-inflammatory and other drugs in preventing corneal haze and post-PRK pain.

Effect of HDAC Inhibitors on Corneal Keratocyte Mechanical Phenotypes in 3-D Collagen Matrices

PURPOSE

Histone deacetylase inhibitors (HDAC) have been shown to inhibit the TGFβ-induced myofibroblast transformation of corneal fibroblasts in 2-D culture. However, the effect of HDAC inhibitors on keratocyte spreading, contraction, and matrix remodeling in 3-D culture has not been directly assessed. The goal of this study was to investigate the effects of the HDAC inhibitors Trichostatin A (TSA) and Vorinostat (SAHA) on corneal keratocyte mechanical phenotypes in 3-D culture using defined serum-free culture conditions.

METHODS

Rabbit corneal keratocytes were plated within standard rat tail type I collagen matrices (2.5 mg/ml) or compressed collagen matrices (~100 mg/ml) and cultured for up to 4 days in serum-free media, PDGF BB, TGFβ1, and either 50 nM TSA, 10 μM SAHA, or vehicle (DMSO). F-actin, α-SM-actin, and collagen fibrils were imaged using confocal microscopy. Cell morphology and global matrix contraction were quantified digitally. The expression of α-SM-actin was assessed using western blotting.

RESULTS

Corneal keratocytes in 3-D matrices had a quiescent mechanical phenotype, as indicated by a dendritic morphology, a lack of stress fibers, and minimal cell-induced matrix remodeling. This phenotype was generally maintained following the addition of TSA or SAHA. TGFβ1 induced a contractile phenotype, as indicated by a loss of dendritic cell processes, the development of stress fibers, and significant matrix compaction. In contrast, cells cultured in TGFβ1 plus TSA or SAHA remained dendritic and did not form stress fibers or induce ECM compaction. Western blotting showed that the expression of α-SM actin after treatment with TGFβ1 was inhibited by TSA and SAHA. PDGF BB stimulated the elongation of keratocytes and the extension of dendritic processes within 3-D matrices without inducing stress fiber formation or collagen reorganization. This spreading response was maintained in the presence of TSA or SAHA.

CONCLUSIONS

Overall, HDAC inhibitors appear to mitigate the effects of TGFβ1 on the transformation of corneal keratocytes to a contractile, myofibroblast phenotype in both compliant and rigid 3-D matrices while preserving normal cell spreading and their ability to respond to the pro-migratory growth factor PDGF.

Platelet-Rich Plasma Prolongs Myofibroblast Accumulation in Corneal Stroma with Incisional Wound

PURPOSE

The purpose of this study was to determine whether platelet-rich plasma (PRP) has an effect on corneal stromal cells in a rat model of wound healing following corneal incision.

MATERIALS AND METHODS

The effect of PRP on corneal wound healing in vivo was investigated in a corneal incision wound model in rats. 40 rats were wounded by deep corneal incision, and treated with either topically administered PRP (20 rats) or sodium chloride (20 rats). At 4 h and 1, 3, and 5 days after incision, α-smooth muscle actin (α SMA), SMAD2 and SMAD3 expression and apoptosis in stromal cells were evaluated by immunohistochemistry, and IL-1β mRNA expression was evaluated by real time PCR.

RESULTS

PRP-treated corneas exhibited reduced stromal cell apoptosis at day 3 and day 5 (p = 0.038, and <0.001, respectively) relative to controls. Interleukin-1β mRNA expression, however, was unchanged in PRP-treated corneas relative to controls. Topical PRP treatment resulted in a higher proportion of αSMA-positive myofibroblasts recruited to the wound site relative to control corneas. PRP did not affect activation of SMAD2 but activation of SMAD3 was significantly reduced at day 1 (p = 0.001) and dramatically increased at day 5 (p = 0.032).

CONCLUSIONS

PRP treatment resulted in suppressed stromal cell apoptosis followed by SMAD3 activation and a greater proportion of myofibroblasts present at the wound site. Suppression of stromal cell apoptosis after corneal wounding by use of a growth factor-rich formulation may lead to myofibroblast accumulation by modulation of the TGF-β pathway.

Mechanical interactions and crosstalk between corneal keratocytes and the extracellular matrix

The generation of cellular forces and the application of these physical forces to the ECM play a central role in mediating matrix patterning and remodeling during fundamental processes such as developmental morphogenesis and wound healing. In addition to growth factors and other biochemical factors that can modulate the keratocyte mechanical phenotype, another key player in the regulation of cell-induced ECM patterning is the mechanical state of the ECM itself. In this review we provide an overview of the biochemical and biophysical factors regulating the mechanical interactions between corneal keratocytes and the stromal ECM at the cellular level. We first provide an overview of how Rho GTPases regulate the sub-cellular pattern of force generation by corneal keratocytes, and the impact these forces have on the surrounding ECM. We next review how feedback from local matrix structural and mechanical properties can modulate keratocyte phenotype and mechanical activity. Throughout this review, we provide examples of how these biophysical interactions may contribute to clinical outcomes, with a focus on corneal wound healing.

Epithelial basement membrane proteins perlecan and nidogen-2 are up-regulated in stromal cells after epithelial injury in human corneas

The epithelial basement membrane (BM) is a specialized extracellular matrix that has been shown to have a critical role in corneal development, wound healing, and disease. Although the epithelial BM contributes to corneal homeostasis, relatively little is know about non-epithelial production of its components that may be important in defective regeneration of the epithelial basement membrane associated with opacity after photorefractive keratectomy. The purpose of the current study was to investigate stromal production of corneal epithelial BM proteins in wounded human corneas using immunohistochemistry. A total of five unwounded control eyes and five 30-min epithelial-wounded corneas were obtained from fresh corneoscleral buttons removed from human eyes enucleated due to choroidal melanoma with normal anterior segments. In the wounded corneas, an eight mm patch of central corneal epithelium and epithelial BM was removed with a Beaver blade when the patient was under general anesthesia. Immunohistochemical analyses were performed to detect perlecan and nidogen-2 proteins-important components of the epithelial BM lamina lucida and lamina densa zones. Perlecan and nidogen-2 proteins were detected in the BM itself and at low levels in keratocytes in all unwounded corneas. After epithelial injury, both perlecan and nidogen-2 were expressed at high levels in stromal keratocytes, including superficial keratocytes in the early phases of apoptosis. Thus, after epithelial and epithelial BM injury, stromal keratocytes contribute important perlecan and nidogen-2 components to the regenerating epithelial BM.

Plasma rich in growth factors (PRGF) eye drops stimulates scarless regeneration compared to autologous serum in the ocular surface stromal fibroblasts

Autologous serum (AS) eye drops was the first blood-derived product used for the treatment of corneal pathologies but nowadays PRGF arises as a novel interesting alternative to this type of diseases. The purpose of this study was to evaluate and compare the biological outcomes of autologous serum eye drops or Plasma rich in growth factors (PRGF) eye drops on corneal stromal keratocytes (HK) and conjunctival fibroblasts (HConF). To address this, blood from healthy donors was collected and processed to obtain autologous serum (AS) eye drops and plasma rich in growth factors (PRGF) eye drops. Blood-derivates were aliquoted and stored at -80°C until use. PDGF-AB, VEGF, EGF, FGFb and TGF-β1 were quantified. The potential of PRGF and AS in promoting wound healing was evaluated by means of proliferation and migration assays in HK and HConF. Fibroblast cells were induced to myofibroblast differentiation after treatment with 2.5ng/mL of TGF-β1. The capability of PRGF and AS to prevent and inhibit TGF-β1-induced differentiation was evaluated. Results showed significant higher levels of all growth factors analyzed in PRGF eye drops compared to AS. Moreover, PRGF eye drops enhanced significantly the biological outcomes of both HK and HConF, and reduced TGF-β1-induced myofibroblast differentiation in contrast to autologous serum eye drops (AS). In summary, these results suggest that PRGF exerts enhanced biological outcomes than AS. PRGF may improve the treatment of ocular surface wound healing minimizing the scar formation compared to AS. Results obtained herein suggest that PRGF protects and reverses the myofibroblast phenotype while promotes cell proliferation and migration.

Stromal-epithelial interaction study: The effect of corneal epithelial cells on growth factor expression in stromal cells using organotypic culture model

Interactions between stromal and epithelial cells play important roles in the development, homeostasis, and pathological conditions of the cornea. Soluble cytokines are critical factors in stromal-epithelial interactions, and growth factors secreted from corneal stromal cells contribute to the regulation of proliferation and differentiation of corneal epithelial cells (CECs). However, the manner in which the expression of growth factors is regulated in stromal cells has not been completely determined. To study stromal-epithelial cell interactions, we used an organotypic culture model. Human or rabbit CECs (HCECs or RCECs) were cultured on amniotic membranes placed on human corneal fibroblasts (HCFs) embedded in a collagen gel. The properties of the organotypic culture were examined by hematoxylin-eosin staining and immunofluorescence. In the organotypic culture, HCECs or RCECs were stratified into two-three layers after five days and five-seven layers after nine days. However, stratification was not observed when the HCECs were seeded on a collagen gel without fibroblasts. K3/K12 were expressed on day 9. The HCF-embedded collagen gels were collected on days 3, 5, or 9 after seeding the RCECs, and mRNA expression of growth factors FGF7, HGF, NGF, EGF, TGF-α, SCF, TGF-β1, TGF-β2, and TGF-β3 were quantified by real-time PCR. mRNA expression of the growth factors in HCFs cultured with RCECs were compared with those cultured without RCECs, as well as in monolayer cultures. mRNA expression of TGF-α was markedly increased in HCFs cultured with RCECs. However, mRNA expression of the TGF-β family was suppressed in HCFs cultured with RCECs. Principal component analysis revealed that mRNA expression of the growth factors in HCFs were generally similar when they were cultured with RCECs. In organotypic cultures, the morphological changes in the CECs and the expression patterns of the growth factors in the stromal cells clearly demonstrated stromal-epithelial cell interactions, and the results suggest that stromal cells and epithelial cells may act in concert in the cornea.

Matrix Metalloproteinase-13 as a Target for Suppressing Corneal Ulceration Caused by Pseudomonas aeruginosa Infection

PURPOSE

Pseudomonas aeruginosa keratitis is characterized by severe corneal ulceration. This study investigated whether matrix metalloproteinase-13 (MMP13) is involved in P. aeruginosa-induced corneal ulceration and whether it therefore can be targeted for preventing P. aeruginosa keratitis.

METHODS

MMP13 expression in P. aeruginosa-infected C57BL/6 mouse corneas was assessed by quantitative polymerase chain reaction and immunohistochemistry analyses. An MMP13-inhibitor (MMP13i) was either injected subconjunctivally prior to or coapplied topically with gatifloxacin 16 hours after infection. Disease severity was assessed by corneal imaging, clinical scoring, bacterial burden, neutrophil infiltration, and CXCL2 expression. Corneal damage and infiltration were also determined by immunohistochemistry analysis and whole-mount confocal microscopy.

RESULTS

P. aeruginosa infection induced an increased expression of MMP13 in mouse corneas from 6 to 24 hours after infection in a Toll-liked receptor 5-dependent manner. Subconjunctival injection of MMP13i prior to P. aeruginosa inoculation significantly decreased keratitis severity, as evidenced by preserved epithelium integrity and intact basement membrane, leading to reduced bacterial dissemination to the stroma. Furthermore, topical coapplication of MMP13i with gatifloxacin greatly improved disease outcomes, including accelerated opacity dissolution; decreased inflammation, cellular infiltration, and collagen disorganization; and basement membrane preservation.

CONCLUSIONS

Elevated MMP13 activity may contribute to P. aeruginosa keratitis through basement membrane degradation, and its inhibition could potentially be used as an adjunctive therapy to treat microbial keratitis and other mucosal infections.

Resolvin E1 analog RX-10045 0.1% reduces corneal stromal haze in rabbits when applied topically after PRK

PURPOSE

To perform a masked study to determine whether resolvin E1 (RvE1), a lipid-derived immunomodulator, could regulate the development of corneal haze and opacity-related myofibroblasts after opacity-generating high correction photorefractive keratectomy (PRK) in rabbits.

METHODS

Three groups of eight rabbits each were included in the study. Nine diopter (D) PRK for myopia was performed in each test cornea, and the eyes were treated with 30 µl of topical solution every 4 h (six times a day) for 5 days starting immediately after PRK. Group 1 was treated with 0.1% RX-10045, a prodrug of an RvE1 analog; group 2 was treated with 0.01% RX-10045; and group 3 was treated with vehicle control solution. At 1 month after PRK, haze was graded at the slit-lamp by a masked observer. Immunohistochemistry for α-smooth muscle actin (SMA) was performed on the central cornea of each test eye to determine the anterior stromal myofibroblast density.

RESULTS

Corneal opacity was significantly lower in the 0.1% RX-10045 group, but not the 0.01% RX-10045 group, compared to the vehicle control group (p=0.029), at 1 month after -9.0D PRK. At 1 month after -9.0D PRK, SMA+ myofibroblast densities in the anterior stroma were not statistically significantly different among the three groups, although a trend toward lower myofibroblast generation was noted in the 0.1% RX-10045 group.

CONCLUSIONS

Topical 0.1% RX-10045, a prodrug of an RvE1 analog, reduces corneal opacity after haze-generating PRK in rabbits. Further studies are needed to determine the precise points at which RvE1 decreases corneal opacity after injury.

Limbal Fibroblasts Maintain Normal Phenotype in 3D RAFT Tissue Equivalents Suggesting Potential for Safe Clinical Use in Treatment of Ocular Surface Failure

Limbal epithelial stem cell deficiency can cause blindness, but transplantation of these cells on a carrier such as human amniotic membrane can restore vision. Unfortunately, clinical graft manufacture using amnion can be inconsistent. Therefore, we have developed an alternative substrate, Real Architecture for 3D Tissue (RAFT), which supports human limbal epithelial cells (hLE) expansion. Epithelial organization is improved when human limbal fibroblasts (hLF) are incorporated into RAFT tissue equivalent (TE). However, hLF have the potential to transdifferentiate into a pro-scarring cell type, which would be incompatible with therapeutic transplantation. The aim of this work was to assess the scarring phenotype of hLF in RAFT TEs in hLE+ and hLE- RAFT TEs and in nonairlifted and airlifted RAFT TEs. Diseased fibroblasts (dFib) isolated from the fibrotic conjunctivae of ocular mucous membrane pemphigoid (Oc-MMP) patients were used as a pro-scarring positive control against which hLF were compared using surrogate scarring parameters: matrix metalloproteinase (MMP) activity, de novo collagen synthesis, α-smooth muscle actin (α-SMA) expression, and transforming growth factor-β (TGF-β) secretion. Normal hLF and dFib maintained different phenotypes in RAFT TE. MMP-2 and -9 activity, de novo collagen synthesis, and α-SMA expression were all increased in dFib cf. normal hLF RAFT TEs, although TGF-β1 secretion did not differ between normal hLF and dFib RAFT TEs. Normal hLF do not progress toward a scarring-like phenotype during culture in RAFT TEs and, therefore, may be safe to include in therapeutic RAFT TE, where they can support hLE, although in vivo work is required to confirm this. dFib RAFT TEs (used in this study as a positive control) may be useful toward the development of an ex vivo disease model of Oc-MMP.

The Antifibrosis Effects of Peroxisome Proliferator-Activated Receptor δ on Rat Corneal Wound Healing after Excimer Laser Keratectomy

Corneal stromal fibrosis characterized by myofibroblasts and abnormal extracellular matrix (ECM) is usually the result of inappropriate wound healing. The present study tested the hypothesis that the ligand activation of peroxisome proliferator-activated receptor (PPAR) δ had antifibrosis effects in a rat model of corneal damage. Adult Sprague-Dawley rats underwent bilateral phototherapeutic keratectomy (PTK). The eyes were randomized into four groups: PBS, GW501516 (a selective agonist of PPARδ), GSK3787 (a selective antagonist of PPARδ), or GW501516 combined with GSK3787. The agents were subconjunctivally administered twice a week until sacrifice. The cellular aspects of corneal wound healing were evaluated with in vivo confocal imaging and postmortem histology. A myofibroblast marker (α-smooth muscle actin) and ECM production (fibronectin, collagen type III and collagen type I) were examined by immunohistochemistry and RT-PCR. At the early stages of wound healing, GW501516 inhibited reepithelialization and promoted angiogenesis. During the remodeling phase of wound healing, GW501516 attenuated the activation and proliferation of keratocytes, which could be reversed by GSK3787. GW501516 decreased transdifferentiation from keratocytes into myofibroblasts, ECM synthesis, and corneal haze. These results demonstrate that GW501516 controls corneal fibrosis and suggest that PPARδ may potentially serve as a therapeutic target for treating corneal scars.

Cellular and extracellular matrix modulation of corneal stromal opacity

Stromal transparency is a critical factor contributing to normal function of the visual system. Corneal injury, surgery, disease and infection elicit complex wound healing responses that serve to protect against insults and maintain the integrity of the cornea, and subsequently to restore corneal structure and transparency. However, in some cases these processes result in prolonged loss of corneal transparency and resulting diminished vision. Corneal opacity is mediated by the complex actions of many cytokines, growth factors, and chemokines produced by the epithelial cells, stromal cells, bone marrow-derived cells, lacrimal tissues, and nerves. Myofibroblasts, and the disorganized extracellular matrix produced by these cells, are critical determinants of the level and persistence of stromal opacity after corneal injury. Decreases in corneal crystallins in myofibroblasts and corneal fibroblasts contribute to cellular opacity in the stroma. Regeneration of a fully functional epithelial basement membrane (BM) appears to have a critical role in the maintenance of corneal stromal transparency after mild injuries and recovery of transparency when opacity is generated after severe injuries. The epithelial BM likely has a regulatory function whereby it modulates epithelium-derived growth factors such as transforming growth factor (TGF) β and platelet-derived growth factor (PDGF) that drive the development and persistence of myofibroblasts from precursor cells. The purpose of this article is to review the factors involved in the maintenance of corneal transparency and to highlight the mechanisms involved in the appearance, persistency and regression of corneal opacity after stromal injury.

Transforming growth factor β and insulin signal changes in stromal fibroblasts of individual keratoconus patients

Keratoconus (KC) is a complex thinning disease of the cornea that often requires transplantation. The underlying pathogenic molecular changes in this disease are poorly understood. Earlier studies reported oxidative stress, metabolic dysfunctions and accelerated death of stromal keratocytes in keratoconus (KC) patients. Utilizing mass spectrometry we found reduced stromal extracellular matrix (ECM) proteins in KC, suggesting ECM-regulatory changes that may be due to altered TGFβ signals. Here we investigated properties of stromal cells from donor (DN) and KC corneas grown as fibroblasts in serum containing DMEM: F12 or in serum-free medium containing insulin, transferrin, selenium (ITS). Phosphorylation of SMAD2/3 of the canonical TGFβ pathway, was high in serum-starved DN and KC fibroblast protein extracts, but pSMAD1/5/8 low at base line, was induced within 30 minutes of TGFβ1 stimulation, more so in KC than DN, suggesting a novel TGFβ1-SMAD1/5/8 axis in the cornea, that may be altered in KC. The serine/threonine kinases AKT, known to regulate proliferation, survival and biosynthetic activities of cells, were poorly activated in KC fibroblasts in high glucose media. Concordantly, alcohol dehydrogenase 1 (ADH1), an indicator of increased glucose uptake and metabolism, was reduced in KC compared to DN fibroblasts. By contrast, in low glucose (5.5 mM, normoglycemic) serum-free DMEM and ITS, cell survival and pAKT levels were comparable in KC and DN cells. Therefore, high glucose combined with serum-deprivation presents some cellular stress difficult to overcome by the KC stromal cells. Our study provides molecular insights into AKT and TGFβ signal changes in KC, and a mechanism for functional studies of stromal cells from KC corneas.

Effects of the loss of conjunctival Muc16 on corneal epithelium and stroma in mice

PURPOSE

To examine the role of conjunctival Muc16 in the homeostasis of the ocular surface epithelium and stroma using Muc16-null knockout (KO) mice.

METHODS

We used KO mice (n = 58) and C57/BL6 (WT) mice (n = 58). Histology and immunohistochemistry were employed to analyze the phenotypes in the ocular surface epithelium. The expression of phospho-Stat3, AP-1 components, interleukin 6 (IL-6), and tumor necrosis factor-α (TNFα) in the cornea and conjunctiva was examined. The shape of the nuclei of corneal epithelial cells was examined to evaluate intraepithelial cell differentiation. Epithelial cell proliferation was studied using bromo-deoxyuridine labeling. Finally, the wound healing of a round defect (2-mm diameter) in the corneal epithelium was measured. The keratocyte phenotype and macrophage invasion in the stroma were evaluated after epithelial repair.

RESULTS

The loss of Muc16 activated Stat3 signal, affected JunB signal, and upregulated the expression of IL-6 in the conjunctiva. Basal-like cells were observed in the suprabasal layer of the corneal epithelium with an increase in proliferation. The loss of Muc16 accelerated the wound healing of the corneal epithelium. The incidence of myofibroblast appearance and macrophage invasion were more marked in KO stroma than in WT stroma after epithelial repair.

CONCLUSIONS

The loss of Muc16 in the conjunctiva affected the homeostasis of the corneal epithelium and stroma. The mechanism might include the upregulation of the inflammatory signaling cascade (i.e., Stat3 signal, and IL-6 expression in the KO conjunctiva). Current data provides insight into the research of the pathophysiology of dry eye syndrome.

Effect of Surgical Technique on Corneal Implant Performance

PURPOSE

Our aim was to determine the effect of a surgical technique on biomaterial implant performance, specifically graft retention.

METHODS

Twelve mini pigs were implanted with cell-free, 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC)/N-hydroxysuccinimide (NHS) cross-linked recombinant human collagen type III (RHCIII) hydrogels as substitutes for donor corneal allografts using overlying sutures with or without human amniotic membrane (HAM) versus interrupted sutures with HAM. The effects of the retention method were compared as well as the effects of collagen concentration (13.7% to 15% RHCIII).

RESULTS

All implanted corneas showed initial haze that cleared with time, resulting in corneas with optical clarity matching those of untreated controls. Biochemical analysis showed that by 12 months post operation, the initial RHCIII implants had been completely remodeled, as type I collagen, was the major collagenous protein detected, whereas no RHCIII could be detected. Histological analysis showed all implanted corneas exhibited regeneration of epithelial and stromal layers as well as nerves, along with touch sensitivity and tear production. Most neovascularization was seen in corneas stabilized by interrupted sutures.

CONCLUSIONS

This showed that the surgical technique used does have a significant effect on the overall performance of corneal implants, overlying sutures caused less vascularization than interrupted sutures.

TRANSLATIONAL RELEVANCE

Understanding the significance of the suturing technique can aid the selection of the most appropriate procedure when implanting artificial corneal substitutes. The same degree of regeneration, despite a higher collagen content indicates that future material development can progress toward stronger, more resistant implants.

Inhibitory effects of PPARγ ligands on TGF-β1-induced corneal myofibroblast transformation

Corneal scarring, whether caused by trauma, laser refractive surgery, or infection, remains a significant problem for humans. Certain ligands for peroxisome proliferator-activated receptor gamma (PPARγ) have shown promise as antiscarring agents in a variety of body tissues. In the cornea, their relative effectiveness and mechanisms of action are still poorly understood. Here, we contrasted the antifibrotic effects of three different PPARγ ligands (15-deoxy-Δ12,14-prostaglandin J2, troglitazone, and rosiglitazone) in cat corneal fibroblasts. Western blot analyses revealed that all three compounds reduced transforming growth factor (TGF)-β1-driven myofibroblast differentiation and up-regulation of α-smooth muscle actin, type I collagen, and fibronectin expression. Because these effects were independent of PPARγ, we ascertained whether they occurred by altering phosphorylation of Smads 2/3, p38 mitogen-activated protein kinase, stress-activated protein kinase, protein kinase B, extracellular signal-regulated kinase, and/or myosin light chain 2. Only p38 mitogen-activated protein kinase phosphorylation was significantly inhibited by all three PPARγ ligands. Finally, we tested the antifibrotic potential of troglitazone in a cat model of photorefractive keratectomy-induced corneal injury. Topical application of troglitazone significantly reduced α-smooth muscle actin expression and haze in the stromal ablation zone. Thus, the PPARγ ligands tested here showed great promise as antifibrotics, both in vitro and in vivo. Our results also provided new evidence for the signaling pathways that may underlie these antifibrotic actions in corneal fibroblasts.

The integrin needle in the stromal haystack: emerging role in corneal physiology and pathology

Several studies have established the role of activated corneal keratocytes in the fibrosis of the cornea. However, the role of keratocytes in maintaining the structural integrity of a normal cornea is less appreciated. We focus on the probable functions of integrins in the eye and of the importance of integrin-mediated keratocyte interactions with stromal matrix in the maintenance of corneal integrity. We point out that further understanding of how keratocytes interact with their matrix could establish a novel direction in preventing corneal pathology including loss of structural integrity as in keratoconus or as in fibrosis of the corneal stroma.

TGFβ and PDGF-B signaling blockade inhibits myofibroblast development from both bone marrow-derived and keratocyte-derived precursor cells in vivo

Myofibroblasts, the primary cells associated with corneal stromal haze (opacity), can be derived from both cornea-derived and bone marrow-derived precursor cells. In the present study, the role of TGFβ or PDGF blockage on bone marrow-derived myofibroblast development was investigated using a green fluorescent protein (GFP) chimeric bone marrow mouse model and plasmid vectors that blocked TGFβ or PDGF signaling. At the peak of corneal haze one month after irregular phototherapeutic keratectomy the central stroma had significantly less alpha-smooth muscle actin (α-SMA)-positive cells derived from GFP+ bone marrow-derived cells or GFP- keratocyte/corneal fibroblast-derived cells when corneas were treated with the TGFβ blocking vector pGFPC1.TGFRBKDEL or the PDGF blocking vector pCMV.PDGFRB.23KDEL compared with the corresponding empty vector treated or untreated control groups. In individual animals, 30-60% of myofibroblasts were derived from bone marrow-derived precursor cells and 40-70% of myofibroblasts were derived from keratocyte-derived precursor cells. TGFβ and PDGF regulate corneal myofibroblast development from bone marrow-derived precursor cells and keratocyte/corneal fibroblast-derived precursor cells.