Effect of isolation method on human corneal stromal cell behaviour.

Current research on healthy corneal stromal cells will typically use primary cells as they are the most representative of in vivo behaviour. Primary cells are normally isolated from the limbus of discarded donor peripheral corneal tissue left over from transplantation (due to its relative abundance). Therefore, the central part of the cornea is less used in research as this tissue is usually used for transplantation. In some cases, although rare, the whole cornea, can become available for research. It is important to keep in mind that these corneas often have longer storage time, but the use of the central tissue for research is even more interesting, as knowing what cells are being transplanted into recipients would be highly relevant. To this end, stromal cells were extracted from both the limbus and central button of healthy corneas donated for research. This allowed for important comparison between central and limbal cells in culture. Of interest here was the extraction method of stromal cells from the donor tissue. The two most common methods of extraction are enzyme digestion and explant migration. However, no work has been done to understand how each method relatively affects the extracted cells. The extraction method and location from which stromal cells are harvested seems to have a significant effect on the cell adherence, survival, and gene expression of the stromal cells in culture. Enzyme digested cells showed that limbal and central cells had different gene expressions prior to culture, with gene such as ALDH3A1 being much more expressed in limbal cells. Enzyme digesting the limbal ring seems to yield the hardiest populations of stromal cells, a desirable trait in the culture of primary cells.

Keratocyte mechanobiology.

In vivo, corneal keratocytes reside within a complex 3D extracellular matrix (ECM) consisting of highly aligned collagen lamellae, growth factors, and other extracellular matrix components, and are subjected to various mechanical stimuli during developmental morphogenesis, fluctuations in intraocular pressure, and wound healing. The process by which keratocytes convert changes in mechanical stimuli (e.g. local topography, applied force, ECM stiffness) into biochemical signaling is known as mechanotransduction. Activation of the various mechanotransductive pathways can produce changes in cell migration, proliferation, and differentiation. Here we review how corneal keratocytes respond to and integrate different biochemical and biophysical factors. We first highlight how growth factors and other cytokines regulate the activity of Rho GTPases, cytoskeletal remodeling, and ultimately the mechanical phenotype of keratocytes. We then discuss how changes in the mechanical properties of the ECM have been shown to regulate keratocyte behavior in sophisticated 2D and 3D experimental models of the corneal microenvironment. Finally, we discuss how ECM topography and protein composition can modulate cell phenotypes, and review the different methods of fabricating in vitro mimics of corneal ECM topography, novel approaches for examining topographical effects in vivo, and the impact of different ECM glycoproteins and proteoglycans on keratocyte behavior.

Preparation of collagen/cellulose nanocrystals composite films and their potential applications in corneal repair.

As the main component of the natural cornea, collagen (COL) has been widely applied to the construction of corneal repair materials. However, the applications of collagen are limited due to its poor mechanical properties. Cellulose nanocrystals (CNCs) possess excellent mechanical properties, optical transparency and good biocompatibility. Therefore, in this study, we attempted to introduce cellulose nanocrystals into collagen-based films to obtain corneal repair materials with a high strength. CNCs were incorporated at 1, 3, 5, 7 and 10 wt%. The physical properties of these composite films were characterized, and in vitro cell-based analyses were also performed. The COL/CNC films possessed better mechanic properties, and the introduction of CNCs did not affect the water content and light transmittance. The COL/CNC films demonstrated good biocompatibility toward rabbit corneal epithelial cells and keratocytes in vitro. Moreover, the collagen films with appropriate ration of CNCs effectively induced the migration of corneal epithelial cells and inhibited the myofibroblast differentiation of keratocytes. A collagen film with 7 wt% CNCs displayed the best combination of physical properties and biological performance in vitro among all the films. This study describes a nonchemical cross-linking method to enhance the mechanical properties of collagen for use in corneal repair materials and highlights potential application in corneal tissue engineering.

Keratocyte biology.

The study of corneal stromal keratocytes is motivated by its strong association with corneal health and visual function. They play a dominant role in the maintenance of corneal homeostasis and transparency through the production of collagens, proteoglycans and corneal crystallins. Trauma-induced apoptosis of keratocytes and replacement by fibroblasts and myofibroblasts disrupt the stromal matrix organization, resulting in corneal haze formation and vision loss. It is, therefore, important to understand the biology and behaviours of keratocytes and the associated stromal cell types (like fibroblasts, myofibroblasts, stromal stem cells) in wound healing, corneal pathologies (including keratoconus, keratitis, endothelial disorders) as well as different ophthalmic situations (such as collagen crosslinking/photodynamic treatment, keratoplasty and refractive surgery, and topical medications). The recent development of ex vivo propagation of keratocytes and stromal stem cells, and their translational applications, either via stromal injection or incorporated in bioscaffold, have been shown to restore the corneal transparency and regenerate native stromal tissue in animal models of corneal haze and other disorders.

Collagen XII Is a Regulator of Corneal Stroma Structure and Function.

Purpose: The aim of this study was to determine the roles of collagen XII in the regulation of stromal hierarchical organization, keratocyte organization, and corneal mechanics. Methods: The temporal and spatial expression of collagen XII at postnatal days 4, 10, 30, 90, and 150 were evaluated in wild-type (WT) mice. The role of collagen XII in hierarchical organization was analyzed by measuring fibril diameter and density, as well as stromal lamellar structure, within ultrastructural micrographs obtained from WT and collagen XII-deficient mice (Col12a1-/-). Keratocyte morphology and networks were assessed using actin staining with phalloidin and in vivo confocal microscopy. The effects of collagen XII on corneal biomechanics were evaluated with atomic force microscopy. Results: Collagen XII was localized homogeneously in the stroma from postnatal day 4 to day 150, and protein accumulation was shown to increase during this period using semiquantitative immunoblots. Higher fibril density (P < 0.001) and disruption of lamellar organization were found in the collagen XII null mice stroma when compared to WT mice. Keratocyte networks and organization were altered in the absence of collagen XII, as demonstrated using fluorescent microscopy after phalloidin staining and in vivo confocal microscopy. Corneal stiffness was increased in the absence of collagen XII. Young's modulus was 16.2 ± 5.6 kPa in WT and 32.8 ± 6.4 kPa in Col12a1-/- corneas. The difference between these two groups was significant (P < 0.001, t-test). Conclusions: Collagen XII plays a major role in establishing and maintaining stromal structure and function. In the absence of collagen XII, the corneal stroma showed significant abnormalities, including decreased interfibrillar space, disrupted lamellar organization, abnormal keratocyte organization, and increased corneal stiffness.

Substrate mediated interaction between pairs of keratocytes: Multipole traction force models describe their migratory behavior.

A series of traction force microscopy experiments involving pairs of keratocytes migrating on compliant substrates were analyzed. We observed several instances where keratocytes that are about to collide turn before they touch. We term this phenomenon collision avoidance behavior and we propose that the turning is caused by the substrate mediated elastic interactions between the cells. A multipole analysis of the cell traction reveals that the left-right symmetry of the keratocyte traction pattern is broken during collision avoidance events. The analysis further shows that the cell migration direction reorients before the principal traction dipoles as the cells turn. Linear elasticity theory is used to derive the cell-cell interaction energy between pairs of keratocytes. The traction force applied by each cell is modeled as a two points (dipole) or three points (tripod) force model. We show that both models predict that cells that are about to collide in a head-on manner will turn before touching. The tripod model is further able to account for the quadrupole components of the traction force profile that we observed experimentally. Also, the tripod model proposes a mechanism that may explain why cells tend to scatter with a finite angle after a collision avoidance event. A relationship between the scattering angle and the traction force quadrupole moment is also established. Dynamical simulations of migrating model cells are further used to explain the emergence of other cell pair trajectories that we observed experimentally.

Decorin antagonizes corneal fibroblast migration via caveolae-mediated endocytosis of epidermal growth factor receptor.

Decorin (Dcn), a small leucine-rich proteoglycan, is involved in the regulation of corneal wound healing. Epidermal growth factor receptor (EGFR) plays a critical role in corneal fibroblasts proliferation, migration and extracellular matrix (ECM) modulation upon injury or infection. The present study aimed to investigate the mechanistic role of Dcn in EGFR internalization to the regulation of corneal stromal fibroblasts (CSFs) migration, a key step in the corneal wound healing. Human corneal stromal fibroblasts (hCSF) cultures were generated from donor corneas. At 70% confluence, cells were switched to serum-free conditions for 48 h and then treated with decorin (250 nM) in the presence or absence of EGF (100 ng/ml) for various time points (10-60 min). Cell lysates were subjected to proteome array analysis screening for 42 different phosphorylated human receptor tyrosine kinases (RTKs), immunocytochemistry, and western blots to analyze EGFR phosphorylation. The scratch-wound assay was performed to evaluate the effects of decorin on EGF-mediated hCSF migration. Dcn caused a rapid EGFR phosphorylation within 10 min of exposure in RTK blot defining its role as a biological ligand for EGFR in hCSFs. Prolonged exposure to Dcn caused complete disappearance of EGFR and inhibition of the hCSF migration in the scratch wound assay suggesting Dcn binding to EGFR causes EGFR down-regulation. Immunostaining studies indicated that Dcn-treatment to hCSFs internalizes Dcn-EGFR complex, which does not require tyrosine kinase activity when treated with the AG1478 inhibitor and co-localizes the complex to the perinuclear region. Next, we found that Dcn-EGFR complex does not follow canonical early endosome internalization as revealed by the EEA1 antibody instead binds to the CD63 antibody directed for degradation by the late endosome. We also found that Dcn regulates the EGFR recycling by preventing its binding to Rab11, a specific antibody for recycling endosome. Further, hCSFs-pretreated with pharmacological inhibitors, methyl-ß-cyclodextrin and chlorpromazine and supplemented with Dcn suggested EGFR trafficking via the caveolae-mediated pathway. These results suggest that Dcn acts as a biological ligand for EGFR and modulates hCSF migration via EGFR down-regulation, thus playing a vital role in corneal wound healing.

YAP and TAZ are distinct effectors of corneal myofibroblast transformation.

PURPOSE: Transforming growth factor ß1 (TGFß1) is elevated in wounds after injury and promotes the transdifferentiation of quiescent cells in the stroma (keratocytes, to activated fibroblasts and subsequently myofibroblasts-KFM transformation). Coactivators of transcription, YAP (Yes-associated protein) and TAZ (Transcriptional coactivator with PDZ-binding motif), are mechanotransducers that intersect with the TGFß pathway via interactions with Smad proteins. Here, we examined the distinct role of YAP and TAZ on TGFß1 induced myofibroblast transformation of primary human corneal fibroblasts (HCFs). METHODS: A knockdown approach was used to silence YAP and TAZ individually in HCFs. Forty-eight hours post siRNA transfection, cells were cultured in the presence or absence of 2 ng/ml TGFß1 for 24h. The cells were subjected to nuclear and cytoplasmic fractionation. The expression of α-smooth muscle actin (αSMA), Smad 2, 3 and 4, CTGF and phospho-Smad2, 3, and 4 were assessed by qPCR and Western blotting. RESULTS: TGFß1 stimulation resulted in the decreased phosphorylation of YAP in the cytosol, and increased levels of phosphorylated TAZ and Smad2/3/4 in the nucleus. Knockdown of TAZ resulted in elevated YAP expression but not vice versa. Additionally, knockdown of TAZ but not YAP resulted in upregulation of αSMA expression in the presence and absence of TGFß1. In the presence of TGFß1 YAP knockdown increased Smad2/3/4 expression and Smad4 phosphorylation, while TAZ knockdown had no effect on Smad2/3/4 expression and phosphorylation. YAP knockdown inhibited CTGF expression while TAZ knockdown resulted in its increased expression. Finally, simultaneous knockdown of YAP and TAZ resulted in cell death. CONCLUSION: Our findings suggest that YAP and TAZ function as distinct modulators of TGFß1 induced myofibroblast transformation and have different roles in signalling. Specifically, TAZ limits YAP's ability to mediate KFM transformation via Smad proteins. The data also suggest that while having distinct effects, YAP and TAZ have redundant or combinatorial functions critical to cell survival. These results suggest that a loss of TAZ may help drive corneal haze and fibrosis and that the balance between YAP/TAZ is essential in controlling myofibroblast differentiation.

Modulation of human corneal stromal cell differentiation by hepatocyte growth factor and substratum compliance.

Corneal wound healing is a complex process that consists of cellular integration of multiple soluble biochemical cues and cellular responses to biophysical attributes associated with the matrix of the wound space. Upon corneal stromal wounding, the transformation of corneal fibroblasts to myofibroblasts is promoted by transforming growth factor-ß (TGFß). This process is critical for wound healing; however, excessive persistence of myofibroblasts in the wound space has been associated with corneal fibrosis resulting in severe vision loss. The objective of this study was to determine the effect of hepatocyte growth factor (HGF), which can modulate TGFß signaling, on corneal myofibroblast transformation by analyzing the expression of α-smooth muscle actin (αSMA) as a marker of myofibroblast phenotype particularly as it relates to biomechanical cues. Human corneal fibroblasts were cultured on tissue culture plastic (>1 GPa) or hydrogel substrates mimicking human normal or wounded corneal stiffness (25 and 75 kPa) in media containing TGFß1 ±â€¯HGF. The expression of αSMA was analyzed by quantitative PCR, Western blot and immunocytochemistry. Cellular stiffness, which is correlated with cellular phenotype, was measured by atomic force microscopy (AFM). In primary human corneal fibroblasts, the mRNA expression of αSMA showed a clear dose response to TGFß1. The expression was significantly suppressed when cells were incubated with 20 ng/ml HGF in the presence of 2 ng/ml of TGFß1. The protein expression of αSMA induced by 5 ng/ml TGFß1 was also decreased by 20 ng/ml of HGF. Cells cultured on hydrogels mimicking human normal (25 kPa) and fibrotic (75 kPa) cornea also showed an inhibitory effect of HGF on αSMA expression in the presence or absence of TGFß1. Cellular stiffness was decreased by HGF in the presence of TGFß1 as measured by AFM. In this study, we have demonstrated that HGF can suppress the myofibroblast phenotype promoted by TGFß1 in human corneal stromal cells. These data suggest that HGF holds the potential as a therapeutic agent to improve wound healing outcomes by minimizing corneal fibrosis.

Safety and Feasibility of Intrastromal Injection of Cultivated Human Corneal Stromal Keratocytes as Cell-Based Therapy for Corneal Opacities.

Purpose: To evaluate the safety and feasibility of intrastromal injection of human corneal stromal keratocytes (CSKs) and its therapeutic effect on a rodent early corneal opacity model. Methods: Twelve research-grade donor corneas were used in primary culture to generate quiescent CSKs and activated stromal fibroblasts (SFs). Single and repeated intrastromal injections of 2 to 4 × 104 cells to rat normal corneas (n = 52) or corneas with early opacities induced by irregular phototherapeutic keratectomy (n = 16) were performed, followed by weekly examination of corneal response under slit-lamp biomicroscopy and in vivo confocal microscopy with evaluation of haze level and stromal reflectivity, and corneal thickness using anterior segment optical coherence tomography (AS-OCT). Time-lapse tracing of Molday ION-labelled cells was conducted using Spectralis OCT and label intensity was measured. Corneas were collected at time intervals for marker expression by immunofluorescence, cell viability, and apoptosis assays. Results: Injected CSKs showed proper marker expression with negligible SF-related features and inflammation, hence maintaining corneal clarity and stability. The time-dependent loss of injected cells was recovered by repeated injection, achieving an extended expression of human proteoglycans inside rat stroma. In the early corneal opacity model, intrastromal CSK injection reduced stromal reflectivity and thickness, resulting in recovery of corneal clarity, whereas noninjected corneas were thicker and had haze progression. Conclusions: We demonstrated the safety, feasibility, and therapeutic efficacy of intrastromal CSK injection. The cultivated CSKs can be a reliable cell source for potential cell-based therapy for corneal opacities.

Hypoxia modulates the development of a corneal stromal matrix model.

Deposition of matrix proteins during development and repair is critical to the transparency of the cornea. While many cells respond to a hypoxic state that can occur in a tumor, the cornea is exposed to hypoxia during development prior to eyelid opening and during the diurnal sleep cycle where oxygen levels can drop from 21% to 8%. In this study, we used 2 three-dimensional (3-D) models to examine how stromal cells respond to periods of acute hypoxic states. The first model, a stromal construct model, is a 3-D stroma-like construct that consists of human corneal fibroblasts (HCFs) stimulated by a stable form of ascorbate for 1, 2, and 4 weeks to self-assemble their own extracellular matrix. The second model, a corneal organ culture model, is a corneal wound-healing model, which consists of wounded adult rat corneas that were removed and placed in culture to heal. Both models were exposed to either normoxic or hypoxic conditions for varying time periods, and the expression and/or localization of matrix proteins was assessed. No significant changes were detected in Type V collagen, which is associated with Type I collagen fibrils; however, significant changes were detected in the expression of both the small leucine-rich repeating proteoglycans and the larger heparan sulfate proteoglycan, perlecan. Also, hypoxia decreased both the number of Cuprolinic blue-positive glycosaminoglycan chains along collagen fibrils and Sulfatase 1, which modulates the effect of heparan sulfate by removing the 6-O-sulfate groups. In the stromal construct model, alterations were seen in fibronectin, similar to those that occur in development and after injury. These changes in fibronectin after injury were accompanied by changes in proteoglycans. Together these findings indicate that acute hypoxic changes alter the physiology of the cornea, and these models will allow us to manipulate the conditions in the extracellular environment in order to study corneal development and trauma.

Latrunculin B and substratum stiffness regulate corneal fibroblast to myofibroblast transformation.

The transformation of keratocytes and fibroblasts to myofibroblasts is important to corneal wound healing as well as formation of stromal haze. The purpose of this study was to determine the effect of latrunculin B, an actin cytoskeleton disruptor in conjunction with a fundamental biophysical cue, substrate stiffness, on myofibroblast transformation in vitro and in vivo. Rabbit corneal fibroblasts were cultured on substrates of differing compliance (1.5, 22, and 71 kPa) and tissue culture plastic (TCP; > 1 GPa) in media containing 0 or 10 ng/ml TGFß1 for 72 h. Cells were treated with 0.4 µM Lat-B or DMSO for 30 min every 24 h for 72 h. RNA was collected from cells and expression of alpha-smooth muscle actin (α-SMA), keratocan, and ALDH1A1 determined using qPCR; immunocytochemistry was used to assess α-SMA protein expression. A rabbit phototherapeutic keratectomy (PTK) model was used to assess the impact of 0.1% Lat-B (n = 3) or 25% DMSO (vehicle control, n = 3) on corneal wound healing by assessment of epithelial wound size with fluorescein stain and semi-quantitative stromal haze scoring by an observer masked to treatment group as well as Fourier-domain optical coherence tomography (FD-OCT) at set time points. Statistical analysis was completed using one-way or two-way analysis of variance. Treatment with Lat-B versus DMSO resulted in significantly less αSMA mRNA (P ≤ 0.007) for RCF cells grown on 22 and 71 kPa substrates as well as TCP without or with TGFß1, and significantly decreased α-SMA protein expression in RCFs cultured on the intermediate (22 kPa) stiffness in the absence (P = 0.028) or presence (P = 0.018) of TGFß1. Treatment with Lat-B versus DMSO but did not significantly alter expression of keratocan or ALDH1A1 mRNA in RCFs (P > 0.05) in the absence or presence of TGFß1, but RCFs grown on stiff hydrogels (71 kPa) had significantly more keratocan mRNA expression versus the 22 kPa hydrogel or TCP (P < 0.001) without TGFß1. Administration of topical Lat-B BID was well tolerated by rabbits post-PTK but did not significantly alter epithelial wound closure, stromal haze score, stromal haze thickness as measured by FD-OCT in comparison to DMSO-treated rabbits. When corneal stromal cells are cultured on substrates possessing biologically relevant substratum stiffnesses, Lat-B modulates mRNA and protein expression of α-SMA and thus modulates myofibroblast transformation. At a dose and dose-frequency that reduced IOP in human glaucoma patients, Lat-B treatment did not substantially impact corneal epithelial or stromal wound healing in a rabbit PTK model. While a significant impact on wound healing was observed at the concentration and dose frequency reported here was not found, encouraging in vitro data support further investigations of topically applied Lat-B to determine if this compound can reduce stromal fibrosis.

Surface Modifications of the PMMA Optic of a Keratoprosthesis to Improve Biointegration.

Biointegration of a keratoprosthesis (KPro) is critical for the mitigation of various long-term postoperative complications. Biointegration of a KPro occurs between the haptic skirt (corneal graft) and the central optic [poly(methyl methacrylate) (PMMA)]. Various studies have highlighted common problems associated with poor bonding and biointegration between these 2 incompatible biomaterials. Resolution of these issues could be achieved by surface modification of the inert material (PMMA). A calcium phosphate (CaP) coating deposited on dopamine-activated PMMA sheets by simulated body fluid incubation (d-CaP coating) was shown to improve adhesion to collagen type I (main component of corneal stroma) compared with untreated PMMA and PMMA with other surface modifications. However, the d-CaP coating could easily undergo delamination, thereby reducing its potential for modification of KPro optical cylinders. In addition, the coating did not resemble the Ca and P composition of hydroxyapatite (HAp). A novel dip-coating method that involves the creation of cavities to trap and immobilize HAp nanoparticles on the PMMA surface was introduced to address the problems associated with the d-CaP coating. The newly obtained coating offered high hydrophilicity, resistance to delamination, and preservation of the Ca and P composition of HAp. These advantages resulted in improved adhesion strength by more than 1 order of magnitude compared with untreated PMMA. With respect to biointegration, human corneal stromal fibroblasts were able to adhere strongly and proliferate on HAp-coated PMMA. Furthermore, the new coating technique could be extended to immobilization of HAp nanoparticles on 3-mm-diameter PMMA cylinders, bringing it closer to clinical application.

3D Functional Corneal Stromal Tissue Equivalent Based on Corneal Stromal Stem Cells and Multi-Layered Silk Film Architecture.

The worldwide need for human cornea equivalents continues to grow. Few clinical options are limited to allogenic and synthetic material replacements. We hypothesized that tissue engineered human cornea systems based on mechanically robust, patterned, porous, thin, optically clear silk protein films, in combination with human corneal stromal stem cells (hCSSCs), would generate 3D functional corneal stroma tissue equivalents, in comparison to previously developed 2D approaches. Silk film contact guidance was used to control the alignment and distribution of hCSSCs on RGD-treated single porous silk films, which were then stacked in an orthogonally, multi-layered architecture and cultured for 9 weeks. These systems were compared similar systems generated with human corneal fibroblasts (hCFs). Both cell types were viable and preferentially aligned along the biomaterial patterns for up to 9 weeks in culture. H&E histological sections showed that the systems seeded with the hCSSCs displayed ECM production throughout the entire thickness of the constructs. In addition, the ECM proteins tested positive for keratocyte-specific tissue markers, including keratan sulfate, lumican, and keratocan. The quantification of hCSSC gene expression of keratocyte-tissue markers, including keratocan, lumican, human aldehyde dehydrogenase 3A1 (ALDH3A1), prostaglandin D2 synthase (PTDGS), and pyruvate dehydrogenase kinase, isozyme 4 (PDK4), within the 3D tissue systems demonstrated upregulation when compared to 2D single silk films and to the systems generated with the hCFs. Furthermore, the production of ECM from the hCSSC seeded systems and subsequent remodeling of the initial matrix significantly improved cohesiveness and mechanical performance of the constructs, while maintaining transparency after 9 weeks.

Matrix-Based Regenerating Agent for Corneal Wound Healing After Collagen Cross-Linking.

PURPOSE: Corneal collagen cross-linking (CXL) is associated with massive cellular damage, the long-term clinical consequences of which have still to be elucidated. In this study, we seek to determine whether matrix-based regenerating agent (RGTA) therapy has a positive influence on stromal regeneration after CXL. METHODS: Twelve New Zealand white rabbits were randomly divided into 2 groups of 6. CXL of the right cornea was performed in both groups, followed by topical corneal application of one drop of vehicle or one drop of RGTA. In vivo confocal laser-scanning microscopy was used to monitor corneal changes over time. At the end of the experiment, 2 weeks postoperatively, the corneas were excised and processed for histology. RESULTS: Epithelial healing was noted in both groups 3 days after CXL. One week after treatment, in vivo confocal laser-scanning microscopy in both groups revealed a significantly thicker acellular stromal zone in the vehicle group compared with the RGTA group (P = 0.0165). This difference persisted up to 2 weeks postoperatively, when acellular zone thickness was 96 ± 10 µm in the RGTA group and 146 ± 34 µm in the vehicle group (P = 0.0058). These findings were confirmed by histology, which also revealed earlier cell repopulation in the RGTA group. CONCLUSIONS: The anterior stroma of cross-linked corneas exhibited cell loss after CXL. Cell repopulation in the anterior corneal stroma occurred earlier when CXL was followed by topical RGTA treatment. Further studies are required to examine the impact of this ophthalmic RGTA on corneal nerve regeneration after CXL.

Corneal Wound Healing Requires IKB kinase ß Signaling in Keratocytes.

IkB kinase ß (IKKß) 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 IKKß in the cornea using Ikkß(ΔCS) mice in which the Ikkß gene was specifically deleted in the corneal stromal keratocytes. The Ikkß(ΔCS) corneas had normal morphology, transparency and thickness; however, they did not heal well from mild alkali burn injury. In contrast to the Ikkß(F/F) corneas that restored transparency in 2 weeks after injury, over 50% of the Ikkß(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 ß-Gal activity and scar tissue formation at the late stage of wound healing. In addition, the Ikkß(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 IKKß in keratocytes is required to repress oxidative stress and attenuate fibrogenesis and senescence in corneal wound healing.

Dental stem cells: a future asset of ocular cell therapy.

Regenerative medicine using patient's own stem cells (SCs) to repair dysfunctional tissues is an attractive approach to complement surgical and pharmacological treatments for aging and degenerative disorders. Recently, dental SCs have drawn much attention owing to their accessibility, plasticity and applicability for regenerative use not only for dental, but also other body tissues. In ophthalmology, there has been increasing interest to differentiate dental pulp SC and periodontal ligament SC (PDLSC) towards ocular lineage. Both can commit to retinal fate expressing eye field transcription factors and generate rhodopsin-positive photoreceptor-like cells. This proposes a novel therapeutic alternative for retinal degeneration diseases. Moreover, as PDLSC shares similar cranial neural crest origin and proteoglycan secretion with corneal stromal keratoctyes and corneal endothelial cells, this offers the possibility of differentiating PDLSC to these corneal cell types. The advance could lead to a shift in the medical management of corneal opacities and endothelial disorders from highly invasive corneal transplantation using limited donor tissue to cell therapy utilizing autologous cells. This article provides an overview of dental SC research and the perspective of utilizing dental SCs for ocular regenerative medicine.

Application of retinoic acid improves form and function of tissue engineered corneal construct.

Retinoic acid has recently been shown to control the phenotype and extracellular matrix composition of corneal stromal cells cultured in vitro as monolayers. This study set out to investigate the effects of retinoic acid on human corneal keratocytes within a 3D environment. Human corneal keratocytes were encapsulated in collagen gels, which were subsequently compressed under load, and cultured in serum-free media supplemented with 10 µM retinoic acid or DMSO vehicle for 30 days. Cell proliferation was quantified on selected days, while the expression of several important keratocytes markers was evaluated at day 30 using RT-PCR and immunoblotting. The weight and size of the collagen constructs were measured before and after hydration and contraction analyses. Retinoic acid enhanced keratocyte proliferation until day 30, whereas cells in control culture conditions showed reduced numbers after day 21. Both gene and protein expressions of keratocyte-characteristic proteoglycans (keratocan, lumican and decorin), corneal crystallins and collagen type I and V were significantly increased following retinoic acid supplementation. Retinoic acid also significantly reduced the expression of matrix metalloproteases 1, 3 and 9 while not increasing α-smooth muscle actin and fibronectin expression. Furthermore, these effects were also correlated with the ability of retinoic acid to significantly inhibit the contractility of keratocytes while allowing the build-up of corneal stromal extracellular matrix within the 3D constructs. Thus, retinoic acid supplementation represents a promising strategy to improve the phenotype of 3D-cultured keratocytes, and their usefulness as a model of corneal stroma for corneal biology and regenerative medicine applications.

Measuring the Refractive Index of Bovine Corneal Stromal Cells Using Quantitative Phase Imaging.

The cornea is the primary refractive lens in the eye and transmits >90% of incident visible light. It has been suggested that the development of postoperative corneal haze could be due to an increase in light scattering from activated corneal stromal cells. Quiescent keratocytes are thought to produce crystallins that match the refractive index of their cytoplasm to the surrounding extracellular material, reducing the amount of light scattering. To test this, we measured the refractive index (RI) of bovine corneal stromal cells, using quantitative phase imaging of live cells in vitro, together with confocal microscopy. The RI of quiescent keratocytes (RI = 1.381 ± 0.004) matched the surrounding matrix, thus supporting the hypothesis that keratocyte cytoplasm does not scatter light in the normal cornea. We also observed that the RI drops after keratocyte activation (RI = 1.365 ± 0.003), leading to a mismatch with the surrounding intercellular matrix. Theoretical scattering models showed that this mismatch would reduce light transmission in the cornea. We conclude that corneal transparency depends on the matching of refractive indices between quiescent keratocytes and the surrounding tissue, and that after surgery or wounding, the resulting RI mismatch between the activated cells and their surrounds significantly contributes to light scattering.