Experimental models for investigating intra-stromal migration of corneal keratocytes, fibroblasts and myofibroblasts

Towards a More Objective and High-throughput Spheroid Invasion Assay Quantification Method

Multicellular spheroids embedded in 3D hydrogels are prominent in vitro models for 3D cell invasion. Yet, quantification methods for spheroid cell invasion that are high throughput, objective and accessible are still lacking. Variations in spheroid sizes and the shapes of the cells within render it difficult to objectively assess invasion extent. The goal of this work is to develop a high-throughput quantification method of cell invasion into 3D matrices that minimizes sensitivity to initial spheroid size and cell spreading and provides precise integrative directionally-dependent metrics of invasion. By analyzing images of fluorescent cell nuclei, invasion metrics are automatically calculated at the pixel level. The initial spheroid boundary is segmented and automated calculations of the nuclear pixel distances from the initial boundary are used to compute common invasion metrics (i.e., the change in invasion area, mean distance) for the same spheroid at a later timepoint. We also introduce the area moment of inertia as an integrative metric of cell invasion that considers the invasion area as well as the pixel distances from the initial spheroid boundary. Further, we show that principal component analysis can be used to quantify the directional influence of a stimuli to invasion (e.g., due to a chemotactic gradient or contact guidance). To demonstrate the power of the analysis for cell types with different invasive potentials and the utility of this method for a variety of biological applications, the method is used to analyze the invasiveness of five different cell types. In all, implementation of this high throughput quantification method results in consistent and objective analysis of 3D multicellular spheroid invasion. We provide the analysis code in both MATLAB and Python languages as well as a GUI for ease of use for researchers with a range of computer programming skills and for applications in a variety of biological research areas such as wound healing and cancer metastasis.

Correction: Petroll et al. Experimental Models for Investigating Intra-Stromal Migration of Corneal Keratocytes, Fibroblasts and Myofibroblasts. J. Funct. Biomater. 2012, 3, 183-198

In the original publication [...].

The impact of UV cross-linking on corneal stromal cell migration, differentiation and patterning

Previous studies have demonstrated that UV cross-linking (CXL) increases stromal stiffness and produces alterations in extracellular matrix (ECM) microstructure. In order to investigate how CXL impacts both keratocyte differentiation and patterning within the stroma, and fibroblast migration and myofibroblast differentiation on top of the stroma, we combined CXL with superficial phototherapeutic keratectomy (PTK) in a rabbit model. Twenty-six rabbits underwent a 6 mm diameter, 70 μm deep phototherapeutic keratectomy (PTK) with an excimer laser to remove the epithelium and anterior basement membrane. In 14 rabbits, standard CXL was performed in the same eye immediately after PTK. Contralateral eyes served as controls. In vivo confocal microscopy through focusing (CMTF) was used to analyze corneal epithelial and stromal thickness, as well as stromal keratocyte activation and corneal haze. CMTF scans were collected pre-operatively, and from 7 to 120 days after the procedure. A subset of rabbits was sacrificed at each time point, and corneas were fixed and labeled in situ for multiphoton fluorescence microscopy and second harmonic generation imaging. In vivo and in situ imaging demonstrated that haze after PTK was primarily derived from a layer of myofibroblasts that formed on top of the native stroma. Over time, this fibrotic layer was remodeled into more transparent stromal lamellae, and quiescent cells replaced myofibroblasts. Migrating cells within the native stroma underneath the photoablated area were elongated, co-aligned with collagen, and lacked stress fibers. In contrast, following PTK + CXL, haze was derived primarily from highly reflective necrotic "ghost cells" in the anterior stroma, and fibrosis on top of the photoablated stroma was not observed at any time point evaluated. Cells formed clusters as they migrated into the cross-linked stromal tissue and expressed stress fibers; some cells at the edge of the CXL area also expressed α-SM actin, suggesting myofibroblast transformation. Stromal thickness increased significantly between 21 and 90 days after PTK + CXL (P < 0.001) and was over 35 μm higher than baseline at Day 90 (P < 0.05). Overall, these data suggest that cross-linking inhibits interlamellar cell movement, and that these changes lead to a disruption of normal keratocyte patterning and increased activation during stromal repopulation. Interestingly, CXL also prevents PTK-induced fibrosis on top of the stroma, and results in long term increases in stromal thickness in the rabbit model.

Hypoxia-induced mesenchymal stem cells inhibit corneal fibroblast proliferation by regulating the WWP2/Notch1 axis

Aim: This study aimed to explore the role of hypoxic mesenchymal stem cells (MSCs) in corneal alkali burns and the underlying mechanism. Materials & methods: Rat corneal fibroblasts were incubated with IL-6, followed by treatment with hypoxic MSC supernatant. A rat corneal alkali burn model was implemented and processed with hypoxic MSCs. The associated factors were detected by corresponding methods. Results: Hypoxic MSCs reduced the Notch1 level and the proliferation of rat corneal fibroblasts. Hypoxic MSCs or WWP2 overexpression in MSCs enhanced ubiquitination of Notch1. WWP2 interacted with Notch1, and WWP2 silencing reversed the effects of the hypoxic MSCs. Hypoxic MSC treatment in vivo decreased the corneal neovascularization scores and opacity scores. Conclusion: Hypoxic MSCs inhibited inflammation and alleviated corneal injury in alkali burns via the WWP2/Notch1 axis.

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.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Anti-Inflammatory and Anti-Fibrotic Effects of Human Amniotic Membrane Mesenchymal Stem Cells and Their Potential in Corneal Repair

Acute ocular chemical burns are ophthalmic emergencies requiring immediate diagnosis and treatment as they may lead to permanent impairment of vision. The clinical manifestations of such burns are produced by exacerbated innate immune response via the infiltration of inflammatory cells and activation of stromal fibroblasts. New therapies are emerging that are dedicated to repair mechanisms that improve the ocular surface after damage; for example, transplantation of stem cells (SC) has been successfully reported for this purpose. The pursuit of easily accessible, noninvasive procedures to obtain SC has led researchers to focus on human tissues such as amniotic membrane. Human amniotic mesenchymal SC (hAM-MSC) inhibits proinflammatory and fibrotic processes in different diseases. hAM-MSC expresses low levels of classical MHC-I and they do not express MHC-II, making them suitable for regenerative medicine. The aim of this study was to evaluate the effect of intracameral injection of hAM-MSC on the clinical manifestations, the infiltration of inflammatory cells, and the activation of stromal fibroblasts in a corneal alkali-burn model. We also determined the in vitro effect of hAM-MSC conditioned medium (CM) on α-SMA⁺ human limbal myofibroblast (HLM) frequency and on release of neutrophil extracellular traps (NETs). Our results show that intracameral hAM-MSC injection reduces neovascularization, opacity, stromal inflammatory cell infiltrate, and stromal α-SMA⁺ cells in our model. Moreover, in in vitro assays, CM from hAM-MSC decreased the quantity of α-SMA⁺ HLM and the release of NETs. These results suggest that intracameral hAM-MSC injection induces an anti-inflammatory and anti-fibrotic environment that promotes corneal wound healing. Stem Cells Translational Medicine 2018;7:906-917.

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

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

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

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

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

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

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

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

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

Platelet recruitment promotes keratocyte repopulation following corneal epithelial abrasion in the mouse

Corneal abrasion not only damages the epithelium but also induces stromal keratocyte death at the site of injury. While a coordinated cascade of inflammatory cell recruitment facilitates epithelial restoration, it is unclear if this cascade is necessary for keratocyte recovery. Since platelet and neutrophil (PMN) recruitment after corneal abrasion is beneficial to epithelial wound healing, we wanted to determine if these cells play a role in regulating keratocyte repopulation after epithelial abrasion. A 2 mm diameter central epithelial region was removed from the corneas of C57BL/6 wildtype (WT), P-selectin deficient (P-sel^-/-), and CD18 hypomorphic (CD18^hypo) mice using the Algerbrush II. Corneas were studied at 6h intervals out to 48h post-injury to evaluate platelet and PMN cell numbers; additional corneas were studied at 1, 4, 14, and 28 days post injury to evaluate keratocyte numbers. In WT mice, epithelial abrasion induced a loss of anterior central keratocytes and keratocyte recovery was rapid and incomplete, reaching ~70% of uninjured baseline values by 4 days post-injury but no further improvement at 28 days post-injury. Consistent with a beneficial role for platelets and PMNs in wound healing, keratocyte recovery was significantly depressed at 4 days post-injury (~30% of uninjured baseline) in P-sel^-/- mice, which are known to have impaired platelet and PMN recruitment after corneal abrasion. Passive transfer of platelets from WT, but not P-sel^-/-, into P-sel^-/- mice prior to injury restored anterior central keratocyte numbers at 4 days post-injury to P-sel^-/- uninjured baseline levels. While PMN infiltration in injured CD18^hypo mice was similar to injured WT mice, platelet recruitment was markedly decreased and anterior central keratocyte recovery was significantly reduced (~50% of baseline) at 4–28 days post-injury. Collectively, the data suggest platelets and platelet P-selectin are critical for efficient keratocyte recovery after corneal epithelial abrasion.

Mechanical boundary conditions bias fibroblast invasion in a collagen-fibrin wound model

Because fibroblasts deposit the collagen matrix that determines the mechanical integrity of scar tissue, altering fibroblast invasion could alter wound healing outcomes. Anisotropic mechanical boundary conditions (restraint, stretch, or tension) could affect the rate of fibroblast invasion, but their importance relative to the prototypical drivers of fibroblast infiltration during wound healing--cell and chemokine concentration gradients--is unknown. We tested whether anisotropic mechanical boundary conditions affected the directionality and speed of fibroblasts migrating into a three-dimensional model wound, which could simultaneously expose fibroblasts to mechanical, structural, steric, and chemical guidance cues. We created fibrin-filled slits in fibroblast-populated collagen gels and applied uniaxial mechanical restraint along the short or long axis of the fibrin wounds. Anisotropic mechanical conditions increased the efficiency of fibroblast invasion by guiding fibroblasts without increasing their migration speed. The migration behavior could be modeled as a biased random walk, where the bias due to multiple guidance cues was accounted for in the shape of a displacement orientation probability distribution. Taken together, modeling and experiments suggested an effect of strain anisotropy, rather than strain-induced fiber alignment, on fibroblast invasion.