Localization of thrombospondin-1 and myofibroblasts during corneal wound repair

TGF-β signaling in health, disease, and therapeutics

Transforming growth factor (TGF)-β is a multifunctional cytokine expressed by almost every tissue and cell type. The signal transduction of TGF-β can stimulate diverse cellular responses and is particularly critical to embryonic development, wound healing, tissue homeostasis, and immune homeostasis in health. The dysfunction of TGF-β can play key roles in many diseases, and numerous targeted therapies have been developed to rectify its pathogenic activity. In the past decades, a large number of studies on TGF-β signaling have been carried out, covering a broad spectrum of topics in health, disease, and therapeutics. Thus, a comprehensive overview of TGF-β signaling is required for a general picture of the studies in this field. In this review, we retrace the research history of TGF-β and introduce the molecular mechanisms regarding its biosynthesis, activation, and signal transduction. We also provide deep insights into the functions of TGF-β signaling in physiological conditions as well as in pathological processes. TGF-β-targeting therapies which have brought fresh hope to the treatment of relevant diseases are highlighted. Through the summary of previous knowledge and recent updates, this review aims to provide a systematic understanding of TGF-β signaling and to attract more attention and interest to this research area.

Prolactin-Induced Protein facilitates corneal wound healing

The purpose of the study was to investigate the role of Prolactin-Induced Protein (PIP) in corneal wound healing, in vivo and in vitro. In C57BL/6J mice, corneal epithelia was removed using an ocular burr. Phosphate buffered saline (PBS) or PIP (0.5 and 1.0 μg/mL) was applied topically or subconjunctivally injected. PIP accelerated wound closure as early as 24 h. PIP treatment promoted corneal wound healing and epithelial integrity and thickness. Integrin α6, integrin β4, Thrombospondin-1, and TGF-β1 expressions were all downregulated by PIP after wound closure. In vitro, scratch assays were performed using primary human epithelial cells (HCECs) and human corneal fibroblasts (HCFs), stimulated with PIP at various dosages. PIP treatment promoted both HCECs and HCFs migration. PIP upregulated expression of integrin α6, integrin β4, and Thrombospondin-1 in HCECs. Expression of TGF-β1 in HCECs and expression of smooth muscle actin (SMA) and Type III Collagen (Col III) in HCFs were significantly downregulated at 150 ng/mL PIP. PIP exhibits noteworthy anti-fibrotic potentiality. While the mechanism of how PIP is impactful on the corneal wound healing cascade is unknown, our findings are novel and further studies are warranted in order to unravel any therapeutic potential.

BMP3 inhibits TGFβ2-mediated myofibroblast differentiation during wound healing of the embryonic cornea

Often acute damage to the cornea initiates drastic tissue remodeling, resulting in fibrotic scarring that disrupts light transmission and precedes vision impairment. Very little is known about the factors that can mitigate fibrosis and promote scar-free cornea wound healing. We previously described transient myofibroblast differentiation during non-fibrotic repair in an embryonic cornea injury model. Here, we sought to elucidate the mechanistic regulation of myofibroblast differentiation during embryonic cornea wound healing. We found that alpha-smooth muscle actin (αSMA)-positive myofibroblasts are superficial and their presence inversely correlates with wound closure. Expression of TGFβ2 and nuclear localization of pSMAD2 were elevated during myofibroblast induction. BMP3 and BMP7 were localized in the corneal epithelium and corresponded with pSMAD1/5/8 activation and absence of myofibroblasts in the healing stroma. In vitro analyses with corneal fibroblasts revealed that BMP3 inhibits the persistence of TGFβ2-induced myofibroblasts by promoting disassembly of focal adhesions and αSMA fibers. This was confirmed by the expression of vinculin and pFAK. Together, these data highlight a mechanism to inhibit myofibroblast persistence during cornea wound repair.

Unravelling Novel Roles of Salivary Exosomes in the Regulation of Human Corneal Stromal Cell Migration and Wound Healing

Salivary exosomes have demonstrated vast therapeutic and diagnostic potential in numerous diseases. This study pioneers previously unexplored roles of SE in the context of corneal wound healing by utilizing primary corneal stromal cells from healthy (HCFs), type I diabetes mellitus (T1DMs), type II DM (T2DMs), and keratoconus (HKCs) subjects. Purified, healthy human SEs carrying tetraspanins CD9+, CD63+, and CD81+ were utilized. Scratch and cell migration assays were performed after 0, 6, 12, 24, and 48 h following SE stimulation (5 and 25 µg/mL). Significantly slower wound closure was observed at 6 and 12 h in HCFs with 5 μg/mL SE and T1DMs with 5 and 25 μg/mL SE. All wounds were closed by 24-hour, post-wounding. HKCs, T1DMs, and T2DMs with 25µg/mL SE exhibited a significant upregulation of cleaved vimentin compared to controls. Thrombospondin 1 was significantly upregulated in HCFs, HKCs, and T2DMs with 25 µg/mL SE. Lastly, HKCs, T1DMs, and T2DMs exhibited a significant downregulation of fibronectin with 25 μg/mL SE. Whether SEs can be utilized to clinical settings in restoring corneal defects is unknown. This is the first-ever study exploring the role of SEs in corneal wound healing. While the sample size was small, results are highly novel and provide a strong foundation for future studies.

Anterior pituitary, sex hormones, and keratoconus: Beyond traditional targets

"The Diseases of the Horny-coat of The Eye", known today as keratoconus, is a progressive, multifactorial, non-inflammatory ectatic corneal disorder that is characterized by steepening (bulging) and thinning of the cornea, irregular astigmatism, myopia, and scarring that can cause devastating vision loss. The significant socioeconomic impact of the disease is immeasurable, as patients with keratoconus can have difficulties securing certain jobs or even joining the military. Despite the introduction of corneal crosslinking and improvements in scleral contact lens designs, corneal transplants remain the main surgical intervention for treating keratoconus refractory to medical therapy and visual rehabilitation. To-date, the etiology and pathogenesis of keratoconus remains unclear. Research studies have increased exponentially over the years, highlighting the clinical significance and international interest in this disease. Hormonal imbalances have been linked to keratoconus, both clinically and experimentally, with both sexes affected. However, it is unclear how (molecular/cellular signaling) or when (age/disease stage(s)) those hormones affect the keratoconic cornea. Previous studies have categorized the human cornea as an extragonadal tissue, showing modulation of the gonadotropins, specifically luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Studies herein provide new data (both in vitro and in vivo) to further delineate the role of hormones/gonadotropins in the keratoconus pathobiology, and propose the existence of a new axis named the Hypothalamic-Pituitary-Adrenal-Corneal (HPAC) axis.

Understanding Drivers of Ocular Fibrosis: Current and Future Therapeutic Perspectives

Ocular fibrosis leads to severe visual impairment and blindness worldwide, being a major area of unmet need in ophthalmology and medicine. To date, the only available treatments are antimetabolite drugs that have significant potentially blinding side effects, such as tissue damage and infection. There is thus an urgent need to identify novel targets to prevent/treat scarring and postsurgical fibrosis in the eye. In this review, the latest progress in biological mechanisms underlying ocular fibrosis are discussed. We also summarize the current knowledge on preclinical studies based on viral and non-viral gene therapy, as well as chemical inhibitors, for targeting TGFβ or downstream effectors in fibrotic disorders of the eye. Moreover, the role of angiogenetic and biomechanical factors in ocular fibrosis is discussed, focusing on related preclinical treatment approaches. Moreover, we describe available evidence on clinical studies investigating the use of therapies targeting TGFβ-dependent pathways, angiogenetic factors, and biomechanical factors, alone or in combination with other strategies, in ocular tissue fibrosis. Finally, the recent progress in cell-based therapies for treating fibrotic eye disorders is discussed. The increasing knowledge of these disorders in the eye and the promising results from testing of novel targeted therapies could offer viable perspectives for translation into clinical use.

Therapeutic effects of placenta derived-, umbilical cord derived-, and adipose tissue derived-mesenchymal stem cells in chronic Helicobacter pylori infection: comparison and novel mechanisms

Supported with significant rejuvenating and regenerating actions of mesenchymal stem cells (MSCs) in various gastrointestinal diseases including Helicobacter pylori (H. pylori)-associated gastric diseases, we have compared these actions among placenta derived-MSCs (PD-MSCs), umbilical cord derived-MSCs (UC-MSCs), and adipose tissue derived-MSCs (AD-MSCs) and explored contributing genes implicated in rejuvenation of H. pylori-chronic atrophic gastritis (CAG) and tumorigenesis. In this study adopting H. pylori-initiated, high salt diet-promoted gastric carcinogenesis model, we have administered three kinds of MSCs around 15-18 weeks in H. pylori infected C57BL/6 mice and sacrificed at 24 and 48 weeks, respectively, in order to either assess the rejuvenating capability or anti-tumorigenesis. At 24 weeks, MSCs all led to significantly mitigated atrophic gastritis, for which significant inductions of autophagy, preservation of tumor suppressive 15-PGDH, attenuated apoptosis, and efficient efferocytosis was imposed with MSCs administration during atrophic gastritis. At 48 weeks, MSCs administered during H. pylori-associated atrophic gastritis afforded significant blocking the progression of CAG, as evidenced with statistically significant reduction in H. pylori-associated gastric tumor (p<0.05) accompanied with significant decreases in IL-1β, COX-2, STAT3, and NF-κB. Combined together with the changes of stanniocalcin-1 (STC-1), thrombospondin-1 (TSP-1), and IL-10 known as biomarkers reflecting stem cell activities at 48 weeks after H. pylori, PD-MSCs among MSCs afforded the best rejuvenating action against H. pylori-associated CAG via additional actions of efferocytosis, autophagy, and anti-apoptosis at 24 weeks. In conclusion, MSCs, especially PD-MSCs, exerted rejuvenating actions against H. pylori-associated CAG via anti-mutagenesis of IL-10, CD-36, ATG5 and cancer suppressive influences of STC-1, TSP-1, and 15-PGDH.

Thrombospondin 1-induced exosomal proteins attenuate hypoxia-induced paraptosis in corneal epithelial cells and promote wound healing

Thrombospondin-1 (TSP1) is involved in corneal wound healing caused by chemical injury. Herein, we examined the effects of TSP1 on hypoxia-induced damages and wound-healing activity in human corneal epithelial (HCE) cells. Exosomal protein expression was determined using liquid chromatography-tandem mass spectrometry, and HCE cell migration and motility were examined through wound-healing assay and time-lapse microscopy. Reestablishment of cell junctions by TSP1 was assessed through confocal microscopy and 3D image reconstruction. Our results show that CoCl₂ -induced hypoxia promoted HCE cell death by paraptosis. TSP1 protected these cells against paraptosis by attenuating mitochondrial membrane potential depletion, swelling and dilation of endoplasmic reticulum and mitochondria, and mitochondrial fission. Exosomes isolated from HCE cells treated with TSP1 contained wound healing-associated proteins that were taken up by HCE cells to promote tissue remodeling and repair. TSP1 protected HCE cells against hypoxia-induced damages and inhibited paraptosis progression by promoting cell migration, cell-cell adhesion, and extracellular matrix remodeling. These findings indicate that TSP1 ameliorates hypoxia-induced paraptosis in HCE cells and promotes wound healing and remodeling by regulating exosomal protein expression. TSP1 may, therefore, play important roles in the treatment of hypoxia-associated corneal diseases.

TGF beta -1, -2 and -3 in the modulation of fibrosis in the cornea and other organs

The TGF beta-1, -2 and -3 isoforms are transcribed from different genes but bind to the same receptors and signal through the same canonical and non-canonical signal transduction pathways. There are numerous regulatory mechanisms controlling the action of each isoform that include the organ-specific cells producing latent TGF beta growth factors, multiple effectors that activate the isoforms, ECM-associated SLRPs and basement membrane components that modulate the activity and localization of the isoforms, other interactive cytokine-growth factor receptor systems, such as PDGF and CTGF, TGF beta receptor expression on target cells, including myofibroblast precursors, receptor binding competition, positive and negative signal transduction effectors, and transcription and translational regulatory mechanisms. While there has long been the view that TGF beta-1and TGF beta-2 are pro-fibrotic, while TGF beta-3 is anti-fibrotic, this review suggests that view is too simplistic, at least in adult tissues, since TGF beta-3 shares far more similarities in its modulation of fibrotic gene expression with TGF beta-1 and TGF beta-2, than it does differences, and often the differences are subtle. Rather, TGF beta-3 should be seen as a fibro-modulatory partner to the other two isoforms that modulates a nuanced and better controlled response to injury. The complex interplay between the three isoforms and numerous interactive proteins, in the context of the cellular milieu, controls regenerative non-fibrotic vs. fibrotic healing in a response to injury in a particular organ, as well as the resolution of fibrosis, when that occurs.

Immunology and Pathology in Ocular Drug Development

Clear vision is dependent on features that protect the anatomical integrity of the eye (cornea and sclera) and those that contribute to internal ocular homeostasis by conferring hemangiogenic (avascular tissues and antiangiogenic factors), lymphangiogenic (lack of draining lymphatics), and immunologic (tight junctions that form blood-ocular barriers, immunosuppressive cells, and modulators) privileges. The later examples are necessary components that enable the eye to maintain an immunosuppressive environment that responds to foreign invaders in a deviated manner, minimizing destructive inflammation that would impair vision. These conditions allowed for the observations made by Medawar, in 1948, of delayed rejection of allogenic tissue grafts in the anterior chamber of mouse eye and permit the sequestration of foreign invaders (eg, Toxoplasma gondii) within the retina of healthy individuals. Yet successful development of intraocular drugs (biologics and delivery devices) has been stymied by adverse ocular pathology, much of which is driven by immune pathways. The eye can be intolerant of foreign protein irrespective of delivery route, and endogenous ocular cells have remarkable plasticity when recruited to preserve visual function. This article provides a review of current understanding of ocular immunology and the potential role of immune mechanisms in pathology observed with intraocular drug delivery.

TGFβ1 and TGFβ2 proteins in corneas with and without stromal fibrosis: Delayed regeneration of apical epithelial growth factor barrier and the epithelial basement membrane in corneas with stromal fibrosis

The purpose of this study was to investigate the expression and localization of transforming growth factor (TGF) β1 and TGFβ2 in rabbit corneas that healed with and without stromal fibrosis, and to further study defective perlecan incorporation in the epithelial basement membrane (EBM) in corneas with scarring fibrosis. A total of 120 female rabbits had no surgery, -4.5D PRK, or -9D PRK. Immunohistochemistry (IHC) was performed at time points from unwounded to eight weeks after surgery, with four corneas at each time point in each group. Multiplex IHC was performed for TGFβ1 or TGFβ2, with Image-J quantitation, and keratocan, vimentin, alpha-smooth muscle actin (SMA), perlecan, laminin-alpha 5, nidogen-1 or CD11b. Corneas at the four-week peak for myofibroblast and fibrosis development were evaluated using Imaris 3D analysis. Delayed regeneration of both an apical epithelial growth factor barrier and EBM barrier function, including defective EBM perlecan incorporation, was greater in high injury -9D PRK corneas compared to -4.5D PRK corneas without fibrosis. Defective apical epithelial growth factor barrier and EBM allowed epithelial and tear TGFβ1 and tear TGFβ2 to enter the corneal stroma to drive myofibroblast generation in the anterior stroma from vimentin-positive corneal fibroblasts, and likely fibrocytes. Vimentin-positive cells and unidentified vimentin-negative, CD11b-negative cells also produce TGFβ1 and/or TGFβ2 in the stroma in some corneas. TGFβ1 and TGFβ2 were at higher levels in the anterior stroma in the weeks preceding myofibroblast development in the -9D group. All -9D corneas (beginning two to three weeks after surgery), and four -4.5D PRK corneas developed significant SMA + myofibroblasts and stromal fibrosis. Both the apical epithelial growth factor barrier and/or EBM barrier functions tended to regenerate weeks earlier in -4.5D PRK corneas without fibrosis, compared to -4.5D or -9D PRK corneas with fibrosis. SMA-positive myofibroblasts were markedly reduced in most corneas by eight weeks after surgery. The apical epithelial growth factor barrier and EBM barrier limit TGFβ1 and TGFβ2 entry into the corneal stroma to modulate corneal fibroblast and myofibroblast development associated with scarring stromal fibrosis. Delayed regeneration of these barriers in corneas with more severe injuries promotes myofibroblast development, prolongs myofibroblast viability and triggers stromal scarring fibrosis.

Therapeutic Potential of Extracellular Vesicles for the Treatment of Corneal Injuries and Scars

Infection, trauma, and chemical exposure of the ocular surface can severely damage the cornea, resulting in visually significant stromal scars. Current medical treatments are ineffective in mitigating corneal scarring, and corneal transplantation is the only therapy able to restore vision in these eyes. However, because of a severe shortage of corneal tissues, risks of blinding complications associated with corneal transplants, and a higher rate of graft failure in these eyes, an effective and deliverable alternative therapy for the prevention and treatment of corneal scarring remains a significant unmet medical need globally. In recent years, the therapeutic potential of extracellular vesicles (EVs) secreted by cells to mediate cell-cell communication has been a topic of increasing interest. EVs derived from mesenchymal stem cells, in particular human corneal stromal stem cells, have antifibrotic, anti-inflammatory, and regenerative effects in injured corneas. The exact mechanism of action of these functional EVs are largely unknown. Therapeutic development of EVs is at an early stage and warrants further preclinical studies.

Corneal myofibroblasts and fibrosis

Myofibroblasts are alpha-smooth muscle actin (SMA)+ cells that have a critical role in the corneal stromal response to infections, injuries, and surgeries, and which produce corneal scarring fibrosis when they develop in excess. These contractile and opaque cells-produce large amounts of disordered extracellular matrix (ECM)-and develop from keratocyte-derived corneal fibroblasts or bone marrow-derived fibrocytes, and possibly other cell types, in response to TGFβ1, TGFβ2 and PDGF from the epithelium, tears, endothelium, and other stromal cells. Recent proteomic analyses have revealed that the myofibroblasts that develop from different progenitors aren't interchangeable, but have major differences in protein expression and functions. Absence or defective regeneration of the epithelial basement membrane (EBM) and/or Descemet's basement membrane (DBM) results in development and persistence of myofibroblasts in the corneal stroma. The functions of myofibroblasts in the cornea include production of volume-additive ECM, tissue contraction, production of various growth factors, cytokines and chemokines that regulate stromal cells, including other myofibroblasts, production of collagenases and metalloproteinases involved in tissue remodeling, and the expression of toll-like receptors that likely have critical roles in the clearance of bacteria and viruses causing corneal infections.

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

BMP-1 disrupts cell adhesion and enhances TGF-β activation through cleavage of the matricellular protein thrombospondin-1

Bone morphogenetic protein 1 (BMP-1) is an important metalloproteinase that synchronizes growth factor activation with extracellular matrix assembly during morphogenesis and tissue repair. The mechanisms by which BMP-1 exerts these effects are highly context dependent. Because BMP-1 overexpression induces marked phenotypic changes in two human cell lines (HT1080 and 293-EBNA cells), we investigated how BMP-1 simultaneously affects cell-matrix interactions and growth factor activity in these cells. Increasing BMP-1 led to a loss of cell adhesion that depended on the matricellular glycoprotein thrombospondin-1 (TSP-1). BMP-1 cleaved TSP-1 between the VWFC/procollagen-like domain and the type 1 repeats that mediate several key TSP-1 functions. This cleavage induced the release of TSP-1 C-terminal domains from the extracellular matrix and abolished its previously described multisite cooperative interactions with heparan sulfate proteoglycans and CD36 on HT1080 cells. In addition, BMP-1-dependent proteolysis potentiated the TSP-1-mediated activation of latent transforming growth factor-β (TGF-β), leading to increased signaling through the canonical SMAD pathway. In primary human corneal stromal cells (keratocytes), endogenous BMP-1 cleaved TSP-1, and the addition of exogenous BMP-1 enhanced cleavage, but this had no substantial effect on cell adhesion. Instead, processed TSP-1 promoted the differentiation of keratocytes into myofibroblasts and stimulated production of the myofibroblast marker α-SMA, consistent with the presence of processed TSP-1 in human corneal scars. Our results indicate that BMP-1 can both trigger the disruption of cell adhesion and stimulate TGF-β signaling in TSP-1-rich microenvironments, which has important potential consequences for wound healing and tumor progression.

An immune response to the avascular lens following wounding of the cornea involves ciliary zonule fibrils

The lens and central cornea are avascular. It was assumed that the adult lens had no source of immune cells and that the basement membrane capsule surrounding the lens was a barrier to immune cell migration. Yet, microfibril‐associated protein‐1 (MAGP1)‐rich ciliary zonules that originate from the vasculature‐rich ciliary body and extend along the surface of the lens capsule, form a potential conduit for immune cells to the lens. In response to cornea debridement wounding, we find increased expression of MAGP1 throughout the central corneal stroma. The immune cells that populate this typically avascular region after wounding closely associate with this MAGP1‐rich matrix. These results suggest that MAGP1‐rich microfibrils support immune cell migration post‐injury. Using this cornea wound model, we investigated whether there is an immune response to the lens following cornea injury involving the lens‐associated MAGP1‐rich ciliary zonules. Our results provide the first evidence that following corneal wounding immune cells are activated to travel along zonule fibers that extend anteriorly along the equatorial surface of the lens, from where they migrate across the anterior lens capsule. These results demonstrate that lens‐associated ciliary zonules are directly involved in the lens immune response and suggest the ciliary body as a source of immune cells to the avascular lens.

Biology of corneal fibrosis: soluble mediators, integrins, and extracellular vesicles

Corneal fibrosis develops in response to injury, infection, postsurgical complications, or underlying systemic disease that disrupts the homeostasis of the tissue leading to irregular extracellular matrix deposition within the stroma. The mechanisms that regulate corneal scarring are focused heavily on the canonical transforming growth factor-β pathway and relevant activators, and their role in promoting myofibroblast differentiation. In this paper, we discuss the biochemical pathways involved in corneal fibrosis in the context of different injury models-epithelial debridement, superficial keratectomy, and penetrating incision. We elaborate on the interplay of the major pro-fibrotic factors involved in corneal scar development (e.g., transforming growth factor-β1, thrombospondin-1, and ανβ6), and explore a novel role for extracellular vesicles secreted by the wounded epithelium and the importance of the basement membrane.

Thrombospondin-1 in ocular surface health and disease

Thrombospondin 1 (TSP-1) is an extracellular matrix protein that interacts with a wide array of ligands including cell receptors, growth factors, cytokines and proteases to regulate various physiological and pathological processes. Constitutively expressed by certain ocular surface tissues (e.g. corneal and conjunctival epithelium), TSP-1 expression is modulated during ocular surface inflammation. TSP-1 is an important activator of latent TGF-β, serving to promote the immunomodulatory and wound healing functions of TGF-β. Mounting research has deepened our understanding of how TSP-1 expression (and lack thereof) contributes to ocular surface homeostasis and disease. Here, we review current knowledge of the function of TSP-1 in dry eye disease, ocular allergy, angiogenesis/lymphangiogenesis, corneal transplantation, corneal wound healing and infectious keratitis.

Transcriptomic analysis of differential gene expression during chick periocular neural crest differentiation into corneal cells

BACKGROUND

Multipotent neural crest cells (NCC) contribute to the corneal endothelium and keratocytes during ocular development, but the molecular mechanisms that underlie this process remain poorly understood. We performed RNA-Seq analysis on periocular neural crest (pNC), corneal endothelium, and keratocytes and validated expression of candidate genes by in situ hybridization.

RESULTS

RNA-Seq profiling revealed enrichment of genes between pNC and neural crest-derived corneal cells, which correspond to pathways involved in focal adhesion, ECM-receptor interaction, cell adhesion, melanogenesis, and MAPK signaling. Comparisons of candidate NCC genes to ocular gene expression revealed that majority of the NCC genes are expressed in the pNC, but they are either differentially expressed or maintained during corneal development. Several genes involved in retinoic acid, transforming growth factor-β, and Wnt signaling pathways and their modulators are also differentially expressed. We identified differentially expressed transcription factors as potential downstream candidates that may instruct expression of genes involved in establishing corneal endothelium and keratocyte identities.

CONCLUSION

Combined, our data reveal novel changes in gene expression profiles as pNC differentiate into highly specialized corneal endothelial cells and keratocytes. These data serve as platform for further analyses of the molecular networks involved in NCC differentiation into corneal cells and provide insights into genes involved in corneal dysgenesis and adult diseases.

Initiation of fibrosis in the integrin Αvβ6 knockout mice

We previously demonstrated that β6 knockout mice showed impaired wound repair in corneal debridement and keratectomy wounds. In the current investigation, we continued our examination of integrin αvβ6 in order to determine if it was required for the initiation of wound healing in a corneal wound model that normally heals in a fibrotic manner. A full-thickness corneal incision was made in C57BL/6 J wild type (WT) and C57BL/6-Itgb6 KO (β6-/-) mice. The mice were observed at 3, 7, 14, and 28 days post-incision. The morphology of corneal restoration was observed in tissue sections stained with hemotoxilin and eosin (H&E). In addition, indirect-immunofluorescence (IF) was performed on sections and/or whole mounts to evaluate the immunolocalization of α-smooth muscle actin (SMA) and thrombospondin-1 (TSP-1). H&E staining revealed that the corneas in β6-/- mice healed slower than those in WT mice, with an obvious delay in the restoration of the stromal matrix and epithelium. In sections at 3 and 7 days, SMA and TSP-1 were greatly reduced in the β6-/- mice as compared to WT, but peaked at 28 days after incision. Whole mount SMA IF results were consistent with those from sections. Therefore, the initiation of fibrosis was inhibited by the lack of αvβ6; however, there appeared to be an alternate mechanism that initiated fibrosis 7-14 days later. Localization of TSP-1 correlated with expression of SMA whether wound healing was delayed or initiated immediately after wounding.

Soluble Fas ligand blocks destructive corneal inflammation in mouse models of corneal epithelial debridement and LPS induced keratitis

Neutrophil-mediated inflammation plays a critical role in corneal damage following injury or infection. Previous studies demonstrated that membrane-bound FasL (mFasL) induces neutrophil chemokine production. However, the extracellular domain of mFasL is normally cleaved by matrix metalloproteinases to release a soluble form of FasL (sFasL) and sFasL antagonizes mFasL-mediated chemokine production. Therefore, we hypothesized that sFasL could be used to prevent neutrophil-mediated corneal inflammation associated with injury and bacterial keratitis. To test this hypothesis, GFP-only, sFasL-GFP, or mFasL-GFP were expressed in the corneal stroma of C57BL/6 mice, using intra-stromal injections of plasmid DNA or adenoviral vectors (AV) and the role of mFasL and sFasL in corneal inflammation was examined in models of corneal injury and LPS-induced keratitis. Our work addresses an important area of disagreement in the field of FasL, with regard to the mechanism by which sFasL regulates ocular inflammation. Herein, we demonstrate that an intrastromal injection of GFP-only, sFasL-GFP, or mFasL-GFP plasmid DNA resulted in GFP expression throughout the corneal stroma for up to two weeks with little to no evidence of inflammation in the GFP-only and sFasL-GFP groups and mild corneal inflammation in the mFasL-GFP group. Similarly, following epithelial debridement, corneas expressing GFP-only or sFasL-GFP showed no significant signs of corneal inflammation, with clear corneas at 15 days post debridement. By contrast, epithelial debridement of corneas expressing mFasL-GFP triggered persistent corneal inflammation and the development of central corneal opacities that was blocked by sFasL. Similar to the mFasL-GFP plasmid DNA, intrastromal injection of mFasL-GFP AV triggered mild corneal inflammation, but it was transient and resolved by day 10 with corneas remaining clear out to 30 days post injection. Nevertheless, intrastromal expression of mFasL-GFP AV exacerbated LPS-induced keratitis, corneal opacity, and neovascularization, while sFasL-GFP AV expression prevented LPS-induced keratitis, resulting in a clear cornea. Histological analysis of corneas with LPS-induced keratitis revealed a robust infiltration of macrophages and neutrophils and sFasL expression specifically blocked the neutrophil influx. Overall, our data demonstrate that stromal expression of mFasL is inflammatory, while sFasL is non-inflammatory, and opposes the effects of mFasL in mouse models of epithelial debridement and LPS-induced keratitis. These data demonstrate that a delicate balance between sFasL and mFasL regulates ocular inflammation. This study further identifies sFasL as a potent inhibitor of neutrophil-mediated corneal damage, and supports the potential use of sFasL in the treatment of neutrophil-mediated keratitis. These results strongly support the hypothesis that, in the immune privileged environment of the eye, the isoform of FasL regulates immune privilege and determines the extent of inflammation: mFasL promotes inflammation and sFasL blocks inflammation.

Thrombospondin-1 Is Necessary for the Development and Repair of Corneal Nerves

Thrombospondin-1-deficient (TSP-1^-/-) mice are used as an animal model of Sjögren's Syndrome because they exhibit many of the symptoms associated with the autoimmune type of dry eye found in primary Sjögren's Syndrome. This type of dry eye is linked to the inflammation of the lacrimal gland, conjunctiva, and cornea, and is thought to involve dysfunction of the complex neuronal reflex arc that mediates tear production in response to noxious stimuli on the ocular surface. This study characterizes the structural and functional changes to the corneal nerves that are the afferent arm of this arc in young and older TSP-1^-/- and wild type (WT) mice. The structure and subtype of nerves were characterized by immunohistochemistry, in vivo confocal microscopy, and confocal microscopy. Cytokine expression analysis was determined by Q-PCR and the number of monocytes was measured by immunohistochemistry. We found that only the pro-inflammatory cytokine MIP-2 increased in young corneas of TSP-1^-/- compared to WT mice, but tumor necrosis factor-α (TNF-α), monocyte chemoattractant protein-1 (MCP-1), and macrophage inflammatory protein-2 (MIP-2) all increased in older TSP-1^-/- mouse corneas. In contrast, CD11b+ pro-inflammatory monocytes did not increase even in older mouse corneas. Calcitonin gene-related peptide (CGRP)-, but not Substance P (SubP)-containing corneal nerves decreased in older, but not younger TSP-1^-/- compared to WT mouse corneas. We conclude that CGRP-containing corneal sensory nerves exhibit distinct structural deficiencies as disease progresses in TSP-1^-/- mice, suggesting that: (1) TSP-1 is needed for the development or repair of these nerves and (2) impaired afferent corneal nerve structure and hence function may contribute to ocular surface dysfunction that develops as TSP-1^-/- mice age.

Nrf2, a Potential Therapeutic Target against Oxidative Stress in Corneal Diseases

Corneal diseases are one of the major causes of blindness worldwide. Conservative medical agents, which may prevent sight-threatening corneal disease progression, are urgently desired. Numerous evidences have revealed the involvement of oxidative stress in various corneal diseases, such as corneal wound healing and Fuchs endothelial corneal dystrophy (FECD). Nuclear factor (erythroid-derived 2)-like 2 (Nrf2)/Kelch-like erythroid-cell-derived protein with CNC homology- (ECH-) associated protein 1 (Keap1)/antioxidant response element (ARE) signaling is well known as one of the main antioxidative defense systems. To the best of our knowledge, this is the first review to elucidate the different expression profiles of Nrf2 signaling as well as the underlying mechanisms in corneal diseases, implicating that Nrf2 may serve as a potentially promising therapeutic target for corneal diseases.

[Influence of TGF-β cytokine and a number of other biochemical factors on regenerative process]

Scarring is one of the main causes of surgical failure in a number of eye diseases, dacryologic conditions in particular. The process of wound healing, including postoperative wound healing, goes through several stages mediated by various biochemical factors, such as growth factors and pro- and anti-inflammatory cytokines. The balance between the latter directly influences the wound healing. However, current data on the effect of these factors on postoperative outcomes are few and contradictory. Thus, in dacryology as well as in other areas of ophthalmology, the role of cytokines and growth factors in healing of surgical wounds is being intensively researched.

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.

Corneal stromal stem cells reduce corneal scarring by mediating neutrophil infiltration after wounding

Corneal scarring limits vision for millions of individuals worldwide. Corneal transplantation (keratoplasty) is the standard of care for corneal opacity; however, it bears the risk of graft rejection and infection and is not universally available. Stem cell therapy holds promise as an alternative to keratoplasty. Stem cells from human corneal stroma (CSSC) induce regeneration of transparent corneal tissue in a mouse wound-healing model. In this study we investigated the mechanism by which CSSC prevent deposition of fibrotic tissue. Infiltration by CD11b+/Ly6G+ neutrophils and myeloperoxidase expression were increased in corneas 24 hr after corneal wounding but were reduced in CSSC-treated wounds. Secretion of TSG-6, a protein known to regulate neutrophil migration, was up-regulated in CSSC in response to TNFα and as CSSC differentiate to keratocytes. In vivo, wounded mouse corneas treated with CSSC contained human TSG-6. Inhibition of neutrophil infiltration into cornea by CSSC was reversed when TSG-6 expression was knocked down using siRNA. Silencing of TSG-6 expression in CSSC reduced their ability to block scarring and the expression of mRNA for fibrosis-associated proteins collagen III, tenascin C, and smooth muscle actin in wounded corneas. Neutropenic mice exhibited a significant reduction in corneal scarring and fibrotic mRNA expression 2 weeks after wounding. These results support the conclusion that neutrophil infiltration is an essential event in the fibrotic response to corneal damage and that prevention of scarring by CSSC is mediated by secretion of TSG-6 by these cells.

TGF-β-target genes are differentially regulated in corneal epithelial cells and fibroblasts

PURPOSE

Transforming growth factor-beta (TGF-β) activates the canonical Smad pathway, which includes the Smad family of proteins and SARA (Smad Anchor for Receptor Activation) and other less understood pathways, including one involving p38^MAPK. The goal of the current research was to determine if corneal epithelial cells and fibroblasts used the classical or alternative TGF-β-signaling pathways. To examine this question, we made use of Trx-SARA, which inhibits native SARA, thus blocking the Smad pathway.

METHODS

A human corneal epithelial cell line (HCE-TJ), and stromal fibroblasts (HCF) were infected with retroviruses (RTV) containing either Trx-SARA or Trx-GA (a control plasmid). The effect of Trx-SARA on thrombospondin-1 (TSP-1) expression in both cell types, p15^ink4b expression in HCE-TJ, and cellular fibronectin (cFN) expression in HCF was determined. In addition, the effect of p38^MAPK inhibitor on TSP-1 and p15^ink4b were examined.

RESULTS

In HCE-TJ with TGF-β1, TSP-1-protein levels increased and peaked at 24 hours. Trx-SARA reduced TSP-1 expression in HCE-TJ, but had no effect on p15^ink4b. With HCF, Trx-SARA failed to reduce TSP-1 expression; however, cFN expression decreased and proliferation was inhibited. By blocking the p38^MAPK pathway, TSP-1 expression was reduced in HCF and p15^ink4b expression was decreased in HCE-TJ.

CONCLUSIONS

Surprisingly, TSP-1 was regulated through the Smad pathway in HCE-TJ and the p38^MAPK pathway in HCF. The p38^MAPK pathway also induced p15^ink4b in HCE-TJ. Our results indicate that not all TGF-β-target proteins require the Smad pathway, and it may be possible to block certain TGF-β-target proteins without blocking the expression of all the TGF-β-target proteins.

Gene Expression Profile of Extracellular Matrix and Adhesion Molecules in the Human Normal Corneal Stroma

PURPOSE

There is limited information on region-specific gene expression in the human corneal stroma. In this study, we aimed to investigate the expression profile of the extracellular matrix and adhesion molecules in the normal corneal stroma using laser capture microdissection (LCM) and molecular techniques.

METHODS

Frozen sections of human cornea without ocular disease were used to isolate the central and peripheral corneal stromal keratocytes by LCM. RNA was extracted from LCM-captured tissues and the RT² Profiler PCR Arrays were used to examine the expression profile of extracellular matrix and adhesion molecules in the central and peripheral stroma. Real-time quantitative PCR was used to quantify gene expression. Proteomic and western blotting (WB) analyses were performed to confirm gene expression at protein level. Function association network was generated via the web tools String and Cytoscape.

RESULTS

The gene expression profiling demonstrated that 35 out of the 84 extracellular matrix and adhesion molecules represented in the array were expressed in stromal keratocytes. Among them, 24 genes were not previously described in the corneal stroma. Two genes were found more abundantly expressed in the central stroma than in the periphery: TGFBI, COL6A2 (p < 0.05). ADAMTS13 was detected only in the central stroma. Proteomics and WB analysis confirmed the expression of 10 genes. Functional analysis revealed that most identified genes were presented in a core cluster that had multiple and strong associations with other genes.

CONCLUSION

This study identified genes not previously described in the corneal stroma, revealed regional differences in gene expression between central and peripheral stroma, and also detected some interesting candidate genes that may play important roles in corneal function. These observations serve as the foundation to further investigate the molecular and cellular mechanisms regulating the pathogenesis of regional corneal stromal disorders such as keratoconus.

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.

Molecular insights on the effect of TGF-β1/-β3 in human corneal fibroblasts

Transforming growth factor β (TGF-β) plays a critical role in wound healing and the pathogenesis of fibrosis (scarring). Three isoforms of TGF-β have been identified in mammals. Previous studies have shown that the addition of TGF-β1 (T1) or -β2 (T2) to human corneal fibroblasts (HCF) cultured in a 3-dimensional construct resulted in a fibrotic matrix, while the addition of TGF-β3 (T3) resulted in the production of enhanced non-fibrotic matrix as compared to control (Vitamin C [VitC] only). In the current investigation, we undertook the molecular comparison of fibrosis-related gene expression in T1 or T3-treated HCF to gain further insights into the regulation and roles of these two isoforms on the fibrotic response. HCF were cultured in 100 mm dishes in basic medium (Eagles minimum essential medium [EMEM] with 10% fetal bovine serum [FBS]). At 70-80% confluency, cells were exposed to basic medium with 0.5 mM 2-O-α-d-glucopyranosyl-l-ascorbic acid (VitC) ± 2 ng/ml of T1 or T3. After 4 h or 3 days, cells were harvested, and mRNA or protein was isolated. Fibrosis related mRNA levels were assayed using a commercial qRT-PCR Array. Selected proteins were examined using Western blotting (WB). Experiments were performed 6 times for the qRT-PCR and 4 times for WB for each condition. qRT-PCR results showed that most of the fibrosis-related genes were up or downregulated in HCF exposed to T1 or T3 as compared with VitC control. At 4 h, only Smad7 expression was significantly altered in T3-treated HCF, compared to T1, and at 3 days, five genes were altered. WB confirmed that T1 significantly decreased Smad7 expression compared to T3 and control, and that the expression of thrombospondin-1 in T3-stimulated HCF was enhanced compared to T1-treated cells. Finally, both T1 and T3 decreased Smad3 expression dramatically at both time points. At early time points, T1 and T3 have similar effects on expression of fibrosis related genes; however, with a longer exposure, an increasing number of genes were differentially expressed. Interestingly, most of the differentially expressed gene products are secreted by the cells and may be related to the modulation of extracellular matrix.

Scleral fibroblast response to experimental glaucoma in mice

PURPOSE

To study the detailed cellular and molecular changes in the mouse sclera subjected to experimental glaucoma.

METHODS

Three strains of mice underwent experimental bead-injection glaucoma and were euthanized at 3 days and 1, 3, and 6 weeks. Scleral protein expression was analyzed with liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) using (16)O/(18)O labeling for quantification in 1- and 6-week tissues. Sclera protein samples were also analyzed with immunoblotting with specific antibodies to selected proteins. The proportion of proliferating scleral fibroblasts was quantified with Ki67 and 4',6-diamidino-2-phenylindole (DAPI) labeling, and selected proteins were studied with immunohistochemistry.

RESULTS

Proteomic analysis showed increases in molecules involved in integrin-linked kinase signaling and actin cytoskeleton signaling pathways at 1 and 6 weeks after experimental glaucoma. The peripapillary scleral region had more fibroblasts than equatorial sclera (p=0.001, n=217, multivariable regression models). There was a sixfold increase in proliferating fibroblasts in the experimental glaucoma sclera at 1 week and a threefold rise at 3 and 6 weeks (p=0.0005, univariate regression). Immunoblots confirmed increases for myosin, spectrin, and actinin at 1 week after glaucoma. Thrombospondin-1 (TSP-1), HINT1, vimentin, actinin, and α-smooth muscle actin were increased according to immunohistochemistry.

CONCLUSIONS

Scleral fibroblasts in experimental mouse glaucoma show increases in actin cytoskeleton and integrin-related signaling, increases in cell division, and features compatible with myofibroblast transition.

Progress in corneal wound healing

Corneal wound healing is a complex process involving cell death, migration, proliferation, differentiation, and extracellular matrix remodeling. Many similarities are observed in the healing processes of corneal epithelial, stromal and endothelial cells, as well as cell-specific differences. Corneal epithelial healing largely depends on limbal stem cells and remodeling of the basement membrane. During stromal healing, keratocytes get transformed to motile and contractile myofibroblasts largely due to activation of transforming growth factor-β (TGF-β) system. Endothelial cells heal mostly by migration and spreading, with cell proliferation playing a secondary role. In the last decade, many aspects of wound healing process in different parts of the cornea have been elucidated, and some new therapeutic approaches have emerged. The concept of limbal stem cells received rigorous experimental corroboration, with new markers uncovered and new treatment options including gene and microRNA therapy tested in experimental systems. Transplantation of limbal stem cell-enriched cultures for efficient re-epithelialization in stem cell deficiency and corneal injuries has become reality in clinical setting. Mediators and course of events during stromal healing have been detailed, and new treatment regimens including gene (decorin) and stem cell therapy for excessive healing have been designed. This is a very important advance given the popularity of various refractive surgeries entailing stromal wound healing. Successful surgical ways of replacing the diseased endothelium have been clinically tested, and new approaches to accelerate endothelial healing and suppress endothelial-mesenchymal transformation have been proposed including Rho kinase (ROCK) inhibitor eye drops and gene therapy to activate TGF-β inhibitor SMAD7. Promising new technologies with potential for corneal wound healing manipulation including microRNA, induced pluripotent stem cells to generate corneal epithelium, and nanocarriers for corneal drug delivery are discussed. Attention is also paid to problems in wound healing understanding and treatment, such as lack of specific epithelial stem cell markers, reliable identification of stem cells, efficient prevention of haze and stromal scar formation, lack of data on wound regulating microRNAs in keratocytes and endothelial cells, as well as virtual lack of targeted systems for drug and gene delivery to select corneal cells.

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.

The contribution of the sclera and lamina cribrosa to the pathogenesis of glaucoma: Diagnostic and treatment implications

Glaucoma, the second most common cause of world blindness, results from loss of retinal ganglion cells (RGC). RGC die as a consequence of injury to their axons, as they pass through the transition between the environment within the eye and that of the retrobulbar optic nerve, as they course to central visual centers. At the optic nerve head (ONH), axonal transport becomes abnormal, at least in part due to the effect of strain induced by intraocular pressure (IOP) on the sclera and ONH. Animal glaucoma models provide the ability to study how alterations in ocular connective tissues affect this pathological process. New therapeutic interventions are being investigated to mitigate glaucoma blindness by modifying the remodeling of ocular tissues in glaucoma. Some genetically altered mice are resistant to glaucoma damage, while treatment of the sclera with cross-linking agents makes experimental mouse glaucoma damage worse. Inhibition of transforming growth factor β activity is strikingly protective. Treatments that alter the response of ocular connective tissues to IOP may be effective in protecting those with glaucoma from vision loss.

Modifications in stromal extracellular matrix of aged corneas can be induced by ultraviolet A irradiation

With age, structural and functional changes can be observed in human cornea. Some studies have shown a loss of corneal transparency and an increase in turbidity associated with aging. These changes are caused by modifications in the composition and arrangement of extracellular matrix in the corneal stroma. In human skin, it is well documented that exposure to solar radiation, and mainly to the UVA wavelengths, leads to phenotypes of photoaging characterized by alteration in extracellular matrix of the dermis. Although the cornea is also exposed to solar radiation, the extracellular matrix modifications observed in aging corneas have been mainly attributed to chronological aging and not to solar exposure. To ascertain the real implication of UVA exposure in extracellular matrix changes observed with age in human cornea, we have developed a model of photoaging by chronically exposing corneal stroma keratocytes with a precise UVA irradiation protocol. Using this model, we have analyzed UVA-induced transcriptomic and proteomic changes in corneal stroma. Our results show that cumulative UVA exposure causes changes in extracellular matrix that are found in corneal stromas of aged individuals, suggesting that solar exposure catalyzes corneal aging. Indeed, we observe a downregulation of collagen and proteoglycan gene expression and a reduction in proteoglycan production and secretion in response to cumulative UVA exposure. This study provides the first evidence that chronic ocular exposure to sunlight affects extracellular matrix composition and thus plays a role in corneal changes observed with age.

Thrombospondin-1 induces differential response in human corneal and conjunctival epithelial cells lines under in vitro inflammatory and apoptotic conditions

Recently, thrombospondin-1 (TSP-1) has been reported to be critical for maintaining a healthy ocular surface. The purpose of the study was to characterize the expression of TSP-1 and of its receptors CD36 and CD47 in corneal and conjunctival epithelial cells and determine the effect of exogenous TSP-1 treatment on these cells, following the induction of inflammation- and apoptosis-related changes. The expression of TSP-1, CD36 and CD47 by corneal and conjunctival cell lines was firstly characterized by ELISA, immunofluorescence analysis, Western blotting and reverse transcription polymerase chain reaction (RT-PCR). Benzalkonium chloride (BAC) exposure for 5 or 15 min was used as pro-inflammatory and pro-apoptotic stimulus for corneal or conjunctival epithelial cells, respectively. To analyze inflammation and apoptosis-related changes, IL-6 and TGF-β2 secretion determined by ELISA was used as inflammatory markers, while activated caspase-3/7 levels and cell viability, determined by CellEvent™ Caspase-3/7 Green Detection Reagent and XTT cytotoxicity assay, respectively, were used as apoptotic markers. Changes in CD36 and CD47 mRNA expression were quantified by real time RT-PCR. Corneal epithelial cells secreted and expressed higher protein levels of TSP-1 than conjunctival epithelial cells, although TSP-1 mRNA expression levels were similar and had lower CD36 and CD47, both at protein and mRNA levels. Both cell lines responded to exogenous TSP-1 treatment increasing CD36 at protein and mRNA levels. Blocking experiments revealed a predominance of TSP-1/CD47 rather than TSP-1/CD36 interactions to up-regulate CD36 levels in conjunctival epithelial cells, but not in corneal epithelial cells. BAC exposure increased IL-6 secretion and caspase-3/7 levels and decreased cell viability in both, corneal and conjunctival epithelial cells. Moreover, BAC exposure increased latent TGF-β2 levels in conjunctival epithelial cells. Interestingly, CD36 mRNA expression was down-regulated after BAC exposure in both cell lines. Exogenous TSP-1 treatment reduced TGF-β2 up-regulated levels by BAC exposure in conjunctival epithelial cells and less pronounced reduced IL-6 in BAC-exposed corneal epithelial cells. The effect on CD36 and CD47 regulation was less pronounced or even opposite depending on the inflammation- and apoptosis-related markers tested. Our results show evidence of the capacity of corneal and conjunctival epithelial cells to respond to TSP-1 via CD36 or CD47. Experimental simulation of inflammation- and apoptosis-related conditions changed the effects differentially elicited by TSP-1 on corneal and conjunctival epithelial cells, suggesting an unexpected and relevant contribution of TSP-1 on ocular surface homeostasis regulation.

Moesin expression in fibrosis in the mouse cornea after sterile mechanical trauma or infection

PURPOSE

The aim of this study was to compare the expression patterns of 3 important biochemical characteristics of fibrosis-moesin, transforming growth factor (TGF)-β1, and α-smooth muscle actin (SMA) in the mouse cornea with fibrosis induced by common etiologies-sterile mechanical injury and infection.

METHODS

Corneas of 8-week-old C57BL6 mice were either wounded after an anterior keratectomy or were infected by Pseudomonas aeruginosa, and the animals were killed on days 2 and 7, and on weeks 2 and 4 after the procedure. Western blot and immunofluorescence were used to analyze the expression of moesin and phospho-moesin, and the presence of myofibroblasts identified by the expression of α-SMA in the corneal stroma. The expression of TGF-β1 was analyzed by immunofluorescence.

RESULTS

By immunofluorescent analysis, TGF-β1, α-SMA, and phospho-moesin were not detected in the normal corneal stroma. However, after either treatment, TGF-β1 expression increased, along with phospho-moesin in the wounded corneal stroma until day 7, and decreased after week 2. No expression of TGF-β1 and phospho-moesin was found at postoperative week 4. Moesin expression increased until week 2. Myofibroblasts positive for α-SMA were detected on day 2 until week 4 and peaked at week 2. Western blot analysis confirmed the immunofluorescent data for moesin, phospho-moesin, and α-SMA.

CONCLUSIONS

The similar expression pattern of moesin, phospho-moesin, TGF-β1, and α-SMA in the mouse cornea with fibrosis caused by sterile mechanical injury or infection indicated a role for moesin signaling in corneal fibrosis. Interference with the action of moesin may be a potential approach for intervention strategies to avert fibrosis after infection or mechanical injury.

Matricellular protein thrombospondins: influence on ocular angiogenesis, wound healing and immuneregulation

Thrombospondins are a family of large multi-domain glycoproteins described as matricelluar proteins based on their ability to interact with a broad range of receptors, matrix molecules, growth factors or proteases, and to modulate array of cellular functions including intracellular signaling, proliferation and migration. Two members of the thrombospondin family, thrombospondin 1 (TSP-1) and thrombospondin 2 (TSP-2) are studied extensively to determine their structure and function. While expressed at low levels in normal adult tissues, their increased expression is seen predominantly in response to cellular perturbations. Despite structural similarities, a notable functional difference between TSP-1 and TSP-2 includes the ability of former to activate of latent TGF-β and its competitive inhibition by the latter. Both these thrombospondins are reported to play important roles in TGF-β rich ocular environment with most reports related to TSP-1. They are expressed by many ocular cell types and detectable in the aqueous and vitreous humor. TSP-1 and TSP-2 influence many cellular interactions in the eye such as angiogenesis, cell migration, wound healing, TGF-β activation and regulation of inflammatory immune responses. Together, these processes are known to contribute to the immune privilege status of the eye. Emerging roles of TSP-1 and TSP-2 in ocular functions and pathology are reviewed here.

Reversal of fibrosis by TGF-β3 in a 3D in vitro model

Corneal scarring following moderate to severe injury is inevitable. Despite significant advancements in the field, current treatments following these types of injuries are limited, and often, the visual recovery is poor. One of the problems and limitations is that corneal wound healing is a complex process, involving corneal cells, extracellular matrix components and growth factors. Therefore, further understanding is required, along with new treatments and techniques to reduce or prevent corneal scarring following injury. Two isoforms of transforming growth factor-beta (TGF-β), TGF-β1 and -β3 (T1 and T3, respectively), are associated with corneal wound healing. T1 has been shown to drive the corneal keratocytes to differentiate into myofibroblasts; whereas, T3 has been found to inhibit fibrotic markers. In the current study, we examined whether the fibrotic characteristics expressed by human corneal fibroblasts (HCF) in our 3-dimensional (3D) construct following T1 stimulation could be reversed by introducing T3 to the in vitro system. To do this, HCF were isolated and cultured in 10% serum, and when they reached confluence, the cells were stimulated with a stable Vitamin C (VitC) derivative for 4 weeks, which allowed them to secrete a self-assembled matrix. Three conditions were tested: (1) CONTROL: 10% serum (S) only, (2) T1: 10%S + T1, or (3) Rescue: 10%S + T1 for two weeks and then switched to 10%S + T3 for another two weeks. At the end of 4 weeks, the constructs were processed for analysis by indirect-immunofluorescence (IF) and transmission electron microscopy (TEM). Different collagens that are normally present in healthy corneas in vivo, such as Type I and V, as well as Type III, which is a fibrotic indicator, were examined. In addition, we examined smooth muscle actin (SMA), a marker of myofibroblasts, and thrombospondin-1 (TSP-1), a multifunctional matrix protein known to activate the latent complex of TGF-β and appear upon wounding in vivo. Our data showed high expression of collagens type I and V under all conditions throughout the 3D constructs; however, type III and SMA expression were higher in the constructs that were stimulated with T1 and reduced to almost nothing in the Rescue samples. A similar pattern was seen with TSP-1, where TSP-1 expression following "rescue" was decreased considerably. Overall, this data is in agreement with our previous observations that T3 has a significant non-fibrotic effect on HCFs, and presents a novel model for the "rescue" of both cellular and matrix fibrotic components with a single growth factor.

Polymorphism in THBS1 gene is associated with post-refractive surgery chronic ocular surface inflammation

PURPOSE

To determine the association of single nucleotide polymorphisms (SNPs) of the thrombospondin 1 (THBS1) gene with development of chronic ocular surface inflammation (keratoconjunctivitis) after refractive surgery.

DESIGN

Retrospective cohort study.

PARTICIPANTS

Active duty U.S. Army soldiers (n = 143) who opted for refractive surgery.

METHODS

Conjunctival impression cytology samples collected from participants before the surgery were used to harvest DNA for genotyping 5 THBS1 SNPs (rs1478604, rs2228262, rs2292305, rs2228262, and rs3743125) using the Sequenom iPLEX Gold platform (Sequenom, San Diego, CA). Samples collected after surgery were used to harvest RNA for gene expression analysis by real-time polymerase chain reaction (PCR). Participants were followed for 1 year after surgery to monitor the status of keratoconjunctivitis.

MAIN OUTCOME MEASURES

Genetic basis of the development of chronic keratoconjunctivitis after refractive surgery.

RESULTS

Carriers of minor alleles of 3 SNPs each were found to be more susceptible to developing chronic keratoconjunctivitis (rs1478604: odds ratio [OR], 2.5; 95% confidence interval [CI], 1.41-4.47; P = 2.5 × 10(-3); rs2228262 and rs2292305: OR, 1.9; 95% CI, 1.05-3.51; P = 4.8 × 10(-2)). Carriers of the rs1478604 minor allele expressed significantly reduced levels of thrombospondin 1 (TSP1) (P = 0.042) and increased levels of an inflammatory cytokine associated with keratoconjunctivitis, interleukin-1β (P = 0.025), in their ocular surface epithelial cells compared with homozygous major allele controls.

CONCLUSIONS

Genetic variation in the THBS1 gene that results in decreased expression of the encoded glycoprotein TSP1 in ocular surface epithelial cells significantly increases the susceptibility to develop chronic ocular surface inflammation after refractive surgery. Further investigation of THBS1 SNPs in a larger sample size is warranted.

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.

TGF-β3 stimulates stromal matrix assembly by human corneal keratocyte-like cells

PURPOSE

We have previously shown that TGF-β3 (T3) stimulates extracellular matrix (ECM) assembly while maintaining antifibrotic characteristics in a model using human corneal fibroblasts (HCFs). This model, however, requires non-physiological levels of serum. In the current study, we tested whether T3 could stimulate human corneal keratocytes (HCKs) in vitro to assemble a functional ECM, while maintaining their characteristics.

METHODS

Human corneal keratocytes and HCFs were isolated and cultured using 1% or 10% serum, respectively ±T3. The constructs were processed for indirect immunofluorescence (IF), transmission electron microscopy (TEM), and qRT-PCR, analyzing for keratocyte marker, keratocan, and ECM components, collagen (col) types I, III, and V.

RESULTS

Quantitative reverse transcriptase PCR data showed that keratocan, col I, and V were all upregulated in HCKs compared with HCFs, whereas col III was expressed at low levels in HCKs. Transforming growth factor beta 3 stimulation further enhanced the level of change. Without T3, HCK constructs were very thin, approximately 5 μm; however, as with HCFs, upon stimulation with T3, HCK constructs increased in thickness by approximately 5-fold. Cell counts and ECM production revealed that HCKs assembled more ECM per unit area compared with HCFs, and IF revealed downregulation of fibrotic markers, col III, and thrombospondin-1, with T3 stimulation. Transmission electron microscopy data revealed aligned ECM with long fibrils for all conditions except HCK Controls. Human corneal keratocytes+T3 also showed denser collagen fibrils with more consistent fibril diameter.

CONCLUSIONS

Overall, the data suggests that it is possible to stimulate matrix secretion and assembly by HCKs in vitro by using a single growth factor, T3.

Role of thrombospondin-1 in repair of penetrating corneal wounds

PURPOSE

Thrombospondin-1 (THBS1) has been suggested as a corneal wound-healing modulator. Therefore, we compromised the integrity of the cornea to elucidate the role of THBS1.

METHODS

Full-thickness penetrating corneal incisions (1.5 mm) were created in wild type (WT, 129S2/SvPas) and THBS1-deficient mice (Thbs1⁻/⁻), 129S2/SvPas-Thbs1(tm1Hyn)/Thbs1(tm1Hyn)), and allowed to heal up to 1 month, while being monitored by slit-lamp and intravital corneal examinations. Corneas also were examined by transmission electron microscopy and indirect immunofluorescence. To determine how THBS1 was involved in the healing process, we examined THBS1 and α-smooth muscle actin (SMA), a marker of myofibroblasts and myoepithelial cells.

RESULTS

In WT mice by 1 month, corneas appeared transparent with a thin scar, and endothelium and Descemet's membrane (DM) were restored. In contrast, Thbs1⁻/⁻ corneas exhibited chronic edema and persistent opacity after wounding. The DM and endothelium were not restored, and wound contraction was impaired. The THBS1 was localized in epithelial cells at early stages of the healing process, and in the stroma and endothelial cells during later stages. The SMA-positive epithelial cells and myofibroblasts were observed within the healing area at day 4, peaked at day 14, and disappeared at day 30. The SMA-positive cells were reduced greatly in Thbs1⁻/⁻ mice.

CONCLUSIONS

In the current study, we demonstrated that corneal restoration is strikingly compromised by a penetrating incision in Thbs1⁻/⁻ mice. The wound results in persistent edema and wound gaping. This appears to be the result of the lack of endothelial migration and DM restoration. In addition, myofibroblast formation is compromised, resulting in the lack of wound contraction.

Bevacizumab eye drops delay corneal epithelial wound healing and increase the stromal response to epithelial injury in rats

BACKGROUND

To evaluate the effects of bevacizumab eye drops on corneal epithelial wound healing and the stromal response after epithelial injury in rats.

METHODS

We divided 160 Sprague-Dawley male rats into two groups and de-epithelized corneas with a microblade. Five percent bevacizumab (Avastin) and antibiotic (Cravit) eyedrops were treated four times daily in the bevacizumab group and antibiotic eye drops only in the control group. Wound area evaluation, enzyme-linked immunosorbent assay, immunofluorescent staining, and real-time polymerase chain reaction were performed with rat corneas.

RESULTS

The percentage of wound healing in the bevacizumab group was lower than in the control group at 24, 48 and 72 hours after epithelial debridement (P = 0.02, 0.01 and 0.01). Corneal matrix metalloproteinase-2 (P = 0.02, 0.01 and 0.02), matrix metalloproteinase-9 (P = 0.03, 0.01 and 0.01) and transforming growth factor-β (P = 0.02, 0.02 and 0.01) proteins in the bevacizumab group were higher than control group at 24, 48, and 72 hours. Matrix metalloproteinase-2, matrix metalloproteinase-9, transforming growth factor-b and a-smooth muscle actin were strongly stained in the bevacizumab corneas compared with control corneas in immunofluorescent staining. Matrix metalloproteinase-2 (P = 0.04, 0.03 and 0.04), matrix metalloproteinase- 9 (P = 0.03, 0.01 and 0.02), transforming growth factor-b (P = 0.03, 0.03 and 0.03) and a-smooth muscle actin (P = 0.04, 0.01 and 0.04) messenger RNA levels in the bevacizumab group were also highly expressed compared with the control group at 24, 48, and 72 hours.

CONCLUSIONS

The bevacizumab eye drops delay the wound healing and increase stromal response after corneal epithelial injury in rats.

Development of ex vivo organ culture models to mimic human corneal scarring

PURPOSE

To develop ex vivo organ culture models of human corneal scarring suitable for pharmacological testing and the study of the molecular mechanisms leading to corneal haze after laser surgery or wounding.

METHODS

Corneas from human donors were cultured ex vivo for 30 days, either at the air-liquid interface (AL) or immersed (IM) in the culture medium. Histological features and immunofluorescence for fibronectin, tenascin C, thrombospondin-1, and α-smooth muscle actin were graded from 0 to 3 for control corneas and for corneas wounded with an excimer laser. The effects of adding 10 ng/ml transforming growth factor-β1 (TGF-β1) to the culture medium and of prior complete removal of the epithelium and limbus, thus preventing reepithelialization, were also analyzed on wounded corneas. Collagen III expression was detected with real-time PCR.

RESULTS

Wounding alone was sufficient to induce keratocyte activation and stromal disorganization, but it was only in the presence of added TGF-β1 that intense staining for fibronectin and tenascin C was found in the AL and IM models (as well as thrombospondin-1 in the AL model) and that α-smooth muscle actin became detectable. The scar-like appearance of the corneas was exacerbated when TGF-β1 was added and reepithelialization was prevented, resulting in the majority of corneas becoming opaque and marked upregulation of collagen III.

CONCLUSIONS

THE MAIN FEATURES OF CORNEAL SCARRING WERE REPRODUCED IN THESE TWO COMPLEMENTARY MODELS: the AL model preserved differentiation of the epithelium and permits the topical application of active molecules, while the IM model ensures better perfusion by soluble compounds.

uPA binding to PAI-1 induces corneal myofibroblast differentiation on vitronectin

PURPOSE

Vitronectin (VN) in provisional extracellular matrix (ECM) promotes cell migration. Fibrotic ECM also includes VN and, paradoxically, strongly adherent myofibroblasts (Mfs). Because fibrotic Mfs secrete elevated amounts of urokinase plasminogen activator (uPA), we tested whether increased extracellular uPA promotes the persistence of Mfs on VN.

METHODS

Primary human corneal fibroblasts (HCFs) were cultured in supplemented serum-free medium on VN or collagen (CL) with 1 ng/mL transforming growth factor β1 (TGFβ1). Adherent cells were quantified using crystal violet. Protein expression was measured by Western blotting and flow cytometry. Transfection of short interfering RNAs was performed by nucleofection. Mfs were identified by α-smooth muscle actin (α-SMA) stress fibers. Plasminogen activator inhibitor (PAI-1) levels were quantified by ELISA.

RESULTS

TGFβ1-treated HCFs secreted PAI-1 (0.5 uM) that bound to VN, competing with αvβ3/αvβ5 integrin/VN binding, thus promoting cell detachment from VN. However, addition of uPA to cells on VN increased Mf differentiation (9.7-fold), cell-adhesion (2.2-fold), and binding by the VN integrins αvβ3 and -β5 (2.2-fold). Plasmin activity was not involved in promoting these changes, as treatment with the plasmin inhibitor aprotinin had no effect. A dominant negative PAI-1 mutant (PAI-1R) that binds to VN but does not inhibit uPA prevented the increase in uPA-stimulated cell adhesion and reduced uPA-stimulated integrin αvβ3/αvβ5 binding to VN by 73%.

CONCLUSIONS

uPA induction of TGFβ1-dependent Mf differentiation on VN supports the hypothesis that elevated secretion of uPA in fibrotic tissue may promote cell adhesion and the persistence of Mfs. By blocking uPA-stimulated cell adhesion, PAI-1R may be a useful agent in combating corneal scarring.