Methods for Investigating Corneal Cell Interactions and Extracellular Vesicles In Vitro

Corneal epithelium models for safety assessment in drug development: Present and future directions

The human corneal epithelial barrier plays a crucial role in drug testing studies, including drug absorption, distribution, metabolism, and excretion (ADME), as well as toxicity testing during the preclinical stages of drug development. However, despite the valuable insights gained from animal and current in vitro models, there remains a significant discrepancy between preclinical drug predictions and actual clinical outcomes. Additionally, there is a growing emphasis on adhering to the 3R principles (refine, reduce, replace) to minimize the use of animals in testing. To tackle these challenges, there is a rising demand for alternative in vitro models that closely mimic the human corneal epithelium. Recently, remarkable advancements have been made in two key areas: microphysiological systems (MPS) or organs-on-chips (OoCs), and stem cell-derived organoids. These cutting-edge platforms integrate four major disciplines: stem cells, microfluidics, bioprinting, and biosensing technologies. This integration holds great promise in developing powerful and biomimetic models of the human cornea.

Corneal epithelial basement membrane assembly is mediated by epithelial cells in coordination with corneal fibroblasts during wound healing

Purpose

To understand which cell types, either alone or in combination, contribute to the assembly of the epithelial basement membrane (BM) during corneal wound healing.

Methods

A 3D corneal organotypic model and an in situ rabbit photorefractive keratectomy (PRK) model were used in this study. The 3D corneal organotypic model was established by culturing the rabbit corneal epithelial cells with either corneal fibroblasts or myofibroblasts embedded in collagen type I for 18 days. Corneal fibroblasts were isolated from fresh rabbit corneas, and the myofibroblasts were derived either directly from bone marrow or differentiated from corneal fibroblasts. Immunocytochemistry for alpha-smooth muscle actin (SMA), vimentin, desmin, and vinculin markers confirmed well-differentiated myofibroblasts. Immunohistochemistry was performed in cryofixed sections for BM markers, including laminin alpha-5, laminin beta-3, perlecan, nidogen-1, and collagen type IV. Specimens were also examined with transmission electron microscopy (TEM). Corneas were collected from rabbits after -3 diopter (D) PRK at different time points after surgery, with four corneas at each time point in each group. Cryofixed corneal sections were stained for vimentin, alpha-SMA, and nidogen-1.

Results

The formation of an epithelial BM with expression of laminin alpha-5, laminin beta-3, perlecan, nidogen-1, and collagen IV was observed at the interface between the corneal epithelial cells and corneal fibroblasts. TEM images further confirmed the presence of epithelial BM in organotypic cultures of epithelial cells and corneal fibroblasts. No epithelial BM was observed in cultures of corneal epithelial cells and myofibroblasts (cornea or bone marrow derived), corneal epithelial cells alone, or corneal fibroblasts alone. In rabbit corneas after -3D PRK, a strong association was observed between the regenerating epithelial BM and the presence of corneal fibroblasts at the site of epithelial BM generation.

Conclusions

The corneal epithelial BM assembly is mediated by epithelial cells in coordination with corneal fibroblasts during wound healing.

Quercetin Decreases Corneal Haze In Vivo and Influences Gene Expression of TGF-Beta Mediators In Vitro

We have previously reported the flavonoid, quercetin, as a metabolic regulator and inhibitor of myofibroblast differentiation in vitro. Our current study evaluated the effects of topical application of quercetin on corneal scar development using two different animal models followed by RNA analysis in vitro. Wild-type C57BL/6J mice were anesthetized and the corneal epithelium and stroma were manually debrided, followed by quercetin (0.5, 1, 5, or 50 mM) or vehicle application. Corneal scarring was assessed for 3 weeks by slit lamp imaging and clinically scored. In a separate animal study, six New Zealand White rabbits underwent lamellar keratectomy surgery, followed by treatment with 5 mM quercetin or vehicle twice daily for three days. Stromal backscattering was assessed at week 3 by in vivo confocal microscopy. In mice, a single dose of 5 mM quercetin reduced corneal scar formation. In rabbits, stromal backscattering was substantially lower in two out of three animals in the quercetin-treated group. In vitro studies of human corneal fibroblasts showed that quercetin modulated select factors of the transforming growth factor-β (TGF-β) signaling pathway. These results provide evidence that quercetin may inhibit corneal scarring. Further studies in a larger cohort are required to validate the efficacy and safety of quercetin for clinical applications.

Arginine Supplementation Promotes Extracellular Matrix and Metabolic Changes in Keratoconus

Keratoconus (KC) is a common corneal ectatic disease that affects 1:500–1:2000 people worldwide and is associated with a progressive thinning of the corneal stroma that may lead to severe astigmatism and visual deficits. Riboflavin-mediated collagen crosslinking currently remains the only approved treatment to halt progressive corneal thinning associated with KC by improving the biomechanical properties of the stroma. Treatments designed to increase collagen deposition by resident corneal stromal keratocytes remain elusive. In this study, we evaluated the effects of arginine supplementation on steady-state levels of arginine and arginine-related metabolites (e.g., ornithine, proline, hydroxyproline, spermidine, and putrescine) and collagen protein expression by primary human corneal fibroblasts isolated from KC and non-KC (healthy) corneas and cultured in an established 3D in vitro model. We identified lower cytoplasmic arginine and spermidine levels in KC-derived constructs compared to healthy controls, which corresponded with overall higher gene expression of arginase. Arginine supplementation led to a robust increase in cytoplasmic arginine, ornithine, and spermidine levels in controls only and a significant increase in collagen type I secretion in KC-derived constructs. Further studies evaluating safety and efficacy of arginine supplementation are required to elucidate the potential therapeutic applications of modulating collagen deposition in the context of KC.

Extracellular Vesicles in the Cornea: Insights from Other Tissues

Extracellular vesicles (EVs) are phospholipid bilayer-bound particles secreted by cells that have been found to be important in mediating cell-cell communication, signal transduction, and extracellular matrix remodeling. Their role in both physiological and pathological processes has been established in different tissues throughout the human body. The human cornea functions as a transparent and refractive barrier that protects the intraocular elements from the external environment. Injury, infection, or disease may cause the loss of corneal clarity by altering extracellular matrix organization within the stroma that may lead to detrimental effects on visual acuity. Over the years, numerous studies have identified many of the growth factors (e.g., transforming growth factor-β1, thrombospondin-1, and platelet-derived growth factor) important in corneal wound healing and scarring. However, the functional role of bound factors encapsulated in EVs in the context of corneal biology is less defined. In this review, we describe the discovery and characterization of EVs in the cornea. We focus on EV-matrix interactions, potential functions during corneal wound healing, and the bioactivity of mesenchymal stem cell-derived EVs. We also discuss the development of EVs as stable, drug-loaded therapeutics for ocular applications.