Generation of Functional Immortalized Human Corneal Stromal Stem Cells.

In addition to their therapeutic potential in regenerative medicine, human corneal stromal stem cells (CSSCs) could serve as a powerful tool for drug discovery and development. Variations from different donors, their isolation method, and their limited life span in culture hinder the utility of primary human CSSCs. To address these limitations, this study aims to establish and characterize immortalized CSSC lines (imCSSC) generated from primary human CSSCs. Primary CSSCs (pCSSC), isolated from human adult corneoscleral tissue, were transduced with ectopic expression of hTERT, c-MYC, or the large T antigen of the Simian virus 40 (SV40T) to generate imCSSC. Cellular morphology, proliferation capacity, and expression of CSSCs specific surface markers were investigated in all cell lines, including TNFAIP6 gene expression levels in vitro, a known biomarker of in vivo anti-inflammatory efficacy. SV40T-overexpressing imCSSC successfully extended the lifespan of pCSSC while retaining a similar morphology, proliferative capacity, multilineage differentiation potential, and anti-inflammatory properties. The current study serves as a proof-of-concept that immortalization of CSSCs could enable a large-scale source of CSSC for use in regenerative medicine.

A novel transgenic mouse model for corneal scar visualization.

Corneal opacities affect vision for millions of individuals worldwide. Fibrotic scar tissues accumulate in reaction to inflammatory responses and remain permanently in corneal stroma, and conventionally correctable only by donor corneal transplantation. Numerous studies have explored innovative approaches to reverse corneal scarring through non-surgical means; however, existing mouse models limit these studies, due to the lack of visibility of scar tissue in mouse corneas with steep curvature. Here, we reported that corneal scarring was modelled using a transgenic mouse line, Tg(Col3a1-EGFP)DJ124Gsat, in which enhanced green fluorescence protein (EGFP) reporter expression was driven by the promoter of collagen 3a1 (COL3a1), a stromal fibrosis gene. Similar to wildtype, Col3a1-EGFP transgenic corneas developed opacities after wounding by alkali burn and mechanical ablation, respectively, as examined under stereomicroscopy and Spectral Domain optical coherent tomography. The time course induction of EGFP was aligned with Col3a1 upregulation and matched with the elevated expression of other fibrosis genes (α-smooth muscle actin, fibronectin and tenascin C). Measured by flow cytometry and enzyme-linked immunosorbent assay, increased number of EGFP expressing cells and fluorescent intensities were correlated to corneal thickening and scar volume. After treatment with human corneal stromal stem cells or their exosomes, EGFP expression was downregulated together with the reduction of scar volume and fibrosis gene expression. These results have demonstrated that the transgenic mouse line, Tg(Col3a1-EGFP)DJ124Gsat, can be a valuable tool for the detection of corneal fibrosis and scarring in vivo, and will be useful in monitoring the changes of corneal fibrosis over time.

Corneal stromal stem cells versus corneal fibroblasts in generating structurally appropriate corneal stromal tissue.

Recapitulation of human corneal stromal tissue is believed to be among the most challenging steps in engineering human corneal tissue because of the difficulty in reproducing its highly-ordered hierarchical ultrastructure, which imparts its robust biomechanical properties and optical transparency. In this study, we compared the feasibility of utilizing human corneal stromal stem cells (hCSSCs) and human corneal fibroblasts (hCFs) in the generation of human corneal stromal tissue on a highly-aligned fibrous substrate made from poly(ester urethane) urea. In the serum-free keratocyte differentiation medium supplemented with FGF-2 (10 ng/mL) and TGF-ß3 (0.1 ng/mL), hCSSCs successfully differentiated into keratocytes and secreted multilayered lamellae with orthogonally-oriented collagen fibrils, in a pattern mimicking human corneal stromal tissue. The constructs were 60-70 µm thick and abundant in cornea-specific extracellular matrix (ECM) components, including keratan sulfate, lumican, and keratocan. Under the identical conditions, hCFs tended to differentiate into myofibroblasts and deposited a less-organized collagen-fibrillar construct in a pattern with similarities to corneal scar tissue due to a lack of cornea-specific ECM components. These observations demonstrated that hCSSCs showed a much greater potential, under proper substrate and growth factor guidance, to facilitate the generation of a biological human cornea equivalent. Unlike hCSSCs, hCFs were less responsive to these environmental cues and under identical culture conditions generated an ECM that poorly mimicked the native, functional tissue structure and composition.

Concise review: Stem cells in the corneal stroma.

The cornea is a tough transparent tissue admitting and focusing light in the eye. More than 90% of the cornea is stroma, a highly organized, transparent connective tissue maintained by keratocytes, quiescent mesenchymal cells of neural crest origin. A small population of cells in the mammalian stroma displays properties of mesenchymal stem cells, including clonal growth, multipotent differentiation, and expression of an array of stem cell-specific markers. Unlike keratocytes, the corneal stromal stem cells (CSSCs) undergo extensive expansion in vitro without loss of the ability to adopt a keratocyte phenotype. Several lines of evidence suggest CSSCs to be of neural crest lineage and not from bone marrow. CSSCs are localized in the anterior peripheral (limbal) stroma near to stem cells of the corneal epithelium. CSSCs may function to support potency of the epithelial stem cells in their unique limbal niche. On the other hand, little information is available documenting a role for CSSCs in vivo in stromal wound healing or regeneration. In vitro CSSCs reproduce the highly organized connective tissue of the stroma, demonstrating a potential use of these cells in tissue bioengineering. Direct introduction of CSSCs into the corneal stroma generated transparent tissue in a mouse model of corneal opacity. Human CSSCs injected into mice corneas did not elicit immune rejection over an extended period of time. The CSSCs therefore appear offer an opportunity to develop cell- and tissue-based therapies for irreversible corneal blindness, conditions affecting more than 10 million individuals worldwide.

Comparison of proliferative capacity of genetically-engineered pig and human corneal endothelial cells.

PURPOSE: The possibility of providing cultured corneal endothelial cells (CECs) for clinical transplantation has gained much attention. However, the worldwide need for human (h) donor corneas far exceeds supply. The pig (p) might provide an alternative source. The aim of this study was to compare the proliferative capacity of CECs from wild-type (WT) pigs, genetically-engineered (GE) pigs, and humans. METHODS: The following CECs were cultured: hCECs from donors (i) ≤36 years (young), (ii) ≥49 years (old), and WT pCECs from (iii) neonatal (<5 days), (iv) young (<2 months), and (v) old (>20 months) pigs, and CECs from young (vi) GE pigs (GTKO/CD46 and GTKO/CD46/CD55). Proliferative capacity of CECs was assessed by direct cell counting over 15 days of culture and by BrdU assay. Cell viability during culture was assessed by annexin V staining. The MTT assay assessed cell metabolic activity. RESULTS: There was significantly lower proliferative capacity of old CECs than of young CECs (p < 0.01) in both pigs and humans. There was no significant difference in proliferative capacity/metabolic activity between young pCECs and young hCECs. However, there was a significantly higher percentage of cell death in hCECs compared to pCECs during culture (p < 0.01). Young GE pCECs showed similar proliferative capacity/cell viability/metabolic activity to young WT pCECs. CONCLUSIONS: Because of the greater availability of young pigs and the excellent proliferative capacity of cultured GE pCECs, GE pigs could provide a source of CECs for clinical transplantation.

Human corneal stromal stem cells express anti-fibrotic microRNA-29a and 381-5p - A robust cell selection tool for stem cell therapy of corneal scarring.

INTRODUCTION: Corneal blindness due to scarring is treated with corneal transplantation. However, a global problem is the donor material shortage. Preclinical and clinical studies have shown that cell-based therapy using corneal stromal stem cells (CSSCs) suppresses corneal scarring, potentially mediated by specific microRNAs transported in extracellular vesicles (EVs). However, not every CSSC batch from donors achieves similar anti-scarring effects. OBJECTIVES: To examine miRNA profiles in EVs from human CSSCs showing "healing" versus "non-healing" effects on corneal scarring and to design a tool to select CSSCs with strong healing potency for clinical applications. METHODS: Small RNAs from CSSC-EVs were extracted for Nanostring nCounter Human miRNA v3 assay. MicroRNAs expressed > 20 folds in "healing" EVs (P < 0.05) were subject to enriched gene ontology (GO) term analysis. MiRNA groups with predictive regulation on inflammatory and fibrotic signalling were studied by mimic transfection to (1) mouse macrophages (RAW264.7) for M1 phenotype assay; (2) human corneal keratocytes for cytokine-induced fibrosis, and (3) human CSSCs for corneal scar prevention in vivo. The expression of miR-29a was screened in additional CSSC batches and the anti-scarring effect of cells was validated in mouse corneal wounds. RESULTS: Twenty-one miRNAs were significantly expressed in "healing" CSSC-EVs and 9 miRNA groups were predicted to associate with inflammatory and fibrotic responses, and tissue regeneration (P <10-6). Overexpression of miR-29a and 381-5p significantly prevented M1 phenotype transition in RAW264.7 cells after lipopolysaccharide treatment, suppressed transforming growth factor ß1-induced fibrosis marker expression in keratocytes, and reduced scarring after corneal injury. High miR-29a expression in EV fractions distinguished human CSSCs with strong healing potency, which inhibited corneal scarring in vivo. CONCLUSION: We characterized the anti-inflammatory and fibrotic roles of miR-29a and 381-5p in CSSCs, contributing to scar prevention. MiR-29a expression in EVs distinguished CSSCs with anti-scarring quality, identifying good quality cells for a scarless corneal healing.

Lumican is required for neutrophil extravasation following corneal injury and wound healing.

An important aspect of wound healing is the recruitment of neutrophils to the site of infection or tissue injury. Lumican, an extracellular matrix component belonging to the small leucine rich proteoglycan (SLRP) family, is one of the major keratan sulfate proteoglycans (KSPGs) within the corneal stroma. Increasing evidence indicates that lumican can serve as a regulatory molecule for several cellular processes, including cell proliferation and migration. In the present study, we addressed the role of lumican in the process of extravasation of polymorphonuclear leukocytes (PMNs) during the early inflammatory phase present in the healing of the corneal epithelium following debridement. We used Lum(-/-) mice and a novel transgenic mouse, Lum(-/-),Kera-Lum, which expresses lumican only in the corneal stroma, to assess the role of lumican in PMN extravasation into injured corneas. Our results showed that PMNs did not readily invade injured corneas of Lum(-/-) mice and this defect was rescued by the expression of lumican in the corneas of Lum(-/-),Kera-Lum mice. The presence of lumican in situ facilitates PMN infiltration into the peritoneal cavity in casein-induced inflammation. Our findings are consistent with the notion that in addition to regulating the collagen fibril architecture, lumican acts to aid neutrophil recruitment and invasion following corneal damage and inflammation.

Hyaluronan synthesis mediates the fibrotic response of keratocytes to transforming growth factor beta.

TGFß induces fibrosis in healing corneal wounds, and in vitro corneal keratocytes up-regulate expression of several fibrosis-related genes in response to TGFß. Hyaluronan (HA) accumulates in healing corneas, and HA synthesis is induced by TGFß by up-regulation of HA synthase 2. This study tested the hypothesis that HA acts as an extracellular messenger, enhancing specific fibrotic responses of keratocytes to TGFß. HA synthesis inhibitor 4-methylumbelliferone (4MU) blocked TGFß induction of HA synthesis in a concentration-dependent manner. 4MU also inhibited TGFß-induced up-regulation of α-smooth muscle actin, collagen type III, and extra domain A-fibronectin. Chemical analogs of 4MU also inhibited fibrogenic responses in proportion to their inhibition of HA synthesis. 4MU, however, showed no effect on TGFß induction of luciferase by the 3TP-Lux reporter plasmid. Inhibition of HA using siRNA to HA synthase 2 reduced TGFß up-regulation of smooth muscle actin, fibronectin, and cell division. Similarly, brief treatment of keratocytes with hyaluronidase reduced TGFß responses. These results suggest that newly synthesized cell-associated HA acts as an extracellular enhancer of wound healing and fibrosis in keratocytes by augmenting a limited subset of the cellular responses to TGFß.

Adipose-derived stem cells differentiate to keratocytes in vitro.

PURPOSE: Adipose-derived stem cells (ADSC) are an abundant population of adult stem cells with the potential to differentiate into several specialized tissue types, including neural and neural crest-derived cells. This study sought to determine if ADSC express keratocyte-specific phenotypic markers when cultured under conditions inducing differentiation of corneal stromal stem cells to keratocytes. METHODS: Human subcutaneous adipose tissue was obtained by lipoaspiration. ADSC were isolated by collagenase digestion and differential centrifugation. Side population cells in ADSC were demonstrated using fluorescence-activated cell sorting after staining with Hoechst 33342. Differentiation to keratocyte phenotype was induced in fibrin gels or as pellet cultures with serum-free or reduced-serum media containing ascorbate. Keratocyte-specific gene expression was characterized using western blotting, quantitative RT-PCR, and immunostaining. RESULTS: ADSC contained a side population and exhibited differentiation to adipocytes and chondrocytes indicating adult stem-cell potential. Culture of ADSC in fibrin gels or as pellets in reduced-serum medium with ascorbate and insulin induced expression of keratocan, keratan sulfate, and aldehyde dehydrogenase 3 family, member A1 (ALDH3A1), products highly expressed by differentiated keratocytes. Expression of differentiation markers was quantitatively similar to corneal stromal stem cells and occurred in both serum-free and serum containing media. CONCLUSIONS: ADSC cultured under keratocyte-differentiation conditions express corneal-specific matrix components. Expression of these unique keratocyte products suggests that ADSC can adopt a keratocyte phenotype and therefore have potential for use in corneal cell therapy and tissue engineering.

Stem cell therapy restores transparency to defective murine corneas.

Corneal scarring from trauma and inflammation disrupts vision for millions worldwide, but corneal transplantation, the primary therapy for corneal blindness, is unavailable to many affected individuals. In this study, stem cells isolated from adult human corneal stroma were examined for the ability to correct stromal opacity in a murine model by direct injection of cells into the corneal stroma. In wild-type mice, injected human stem cells remained viable for months without fusing with host cells or eliciting an immune T-cell response. Human corneal-specific extracellular matrix, including the proteoglycans lumican and keratocan, accumulated in the treated corneas. Lumican-null mice have corneal opacity similar to that of scar tissue as a result of disruption of stromal collagen organization. After injection with human stromal stem cells, stromal thickness and collagen fibril defects in these mice were restored to that of normal mice. Corneal transparency in the treated mice was indistinguishable from that of wild-type mice. These results support the immune privilege of adult stem cells and the ability of stem cell therapy to regenerate tissue in a manner analogous to organogenesis and clearly different from that of normal wound healing. The results suggest that cell-based therapy can be an effective approach to treatment of human corneal blindness.

The anti-scarring effect of corneal stromal stem cell therapy is mediated by transforming growth factor ß3.

BACKGROUND: Corneal stromal stem cells (CSSC) reduce corneal inflammation, prevent fibrotic scarring, and regenerate transparent stromal tissue in injured corneas. These effects rely on factors produced by CSSC to block the fibrotic gene expression. This study investigated the mechanism of the scar-free regeneration effect. METHODS: Primary human CSSC (hCSSC) from donor corneal rims were cultivated to passage 3 and co-cultured with mouse macrophage RAW264.7 cells induced to M1 pro-inflammatory phenotype by treatment with interferon-γ and lipopolysaccharides, or to M2 anti-inflammatory phenotype by interleukin-4, in a Transwell system. The time-course expression of human transforming growth factor ß3 (hTGFß3) and hTGFß1 were examined by immunofluorescence and qPCR. TGFß3 knockdown for > 70% in hCSSC [hCSSC-TGFß3(si)] was achieved by small interfering RNA transfection. Naïve CSSC and hCSSC-TGFß3(si) were transplanted in a fibrin gel to mouse corneas, respectively, after wounding by stromal ablation. Corneal clarity and the expression of mouse inflammatory and fibrosis genes were examined. RESULTS: hTGFß3 was upregulated by hCSSC when co-cultured with RAW cells under M1 condition. Transplantation of hCSSC to wounded mouse corneas showed significant upregulation of hTGFß3 at days 1 and 3 post-injury, along with the reduced expression of mouse inflammatory genes (CD80, C-X-C motif chemokine ligand 5, lipocalin 2, plasminogen activator urokinase receptor, pro-platelet basic protein, and secreted phosphoprotein 1). By day 14, hCSSC treatment significantly reduced the expression of fibrotic and scar tissue genes (fibronectin, hyaluronan synthase 2, Secreted protein acidic and cysteine rich, tenascin C, collagen 3a1 and α-smooth muscle actin), and the injured corneas remained clear. However, hCSSC-TGFß3(si) lost these anti-inflammatory and anti-scarring functions, and the wounded corneas showed intense scarring. CONCLUSION: This study has demonstrated that the corneal regenerative effect of hCSSC is mediated by TGFß3, inducing a scar-free tissue response.

Impact on the corneal endothelium of mitomycin C during photorefractive keratectomy.

PURPOSE: This brief review examines both basic science and clinical studies to evaluate the potential impact on the health of the corneal endothelium of mitomycin C (MMC) usage during photorefractive keratectomy (PRK). METHODS: The mechanism of action and consequences of MMC are reviewed within the context of in vitro, animal, and clinical studies and a hypothesis of how this vital cell layer responds to MMC at both the cellular and clinical levels is formed. RESULTS: Seven basic science studies were reviewed demonstrating significant MMC toxicity to corneal endothelial cells. Of the five clinical studies reviewed, three demonstrated no effect on corneal endothelial density, whereas two studies found significant cell loss after MMC usage. CONCLUSIONS: Although all of the basic science studies reviewed highlight the toxicity of MMC on the corneal endothelium, current clinical studies are less conclusive. Given the corneal penetration of MMC and the fragile nature of the corneal endothelium, additional follow-up studies are needed to determine the long-term impact of MMC usage during PRK on the corneal endothelium.

Differentiation Capacity of Human Mesenchymal Stem Cells into Keratocyte Lineage.

Purpose: Mesenchymal stem cells (MSCs) have been extensively studied for their capacity to enhance wound healing and represent a promising research field for generating cell therapies for corneal scars. In the present study, we investigated MSCs from different tissues and their potential to differentiate toward corneal keratocytes. Methods: Adipose-derived stem cells, bone marrow MSCs, umbilical cord stem cells, and corneal stromal stem cells (CSSCs) were characterized by their expression of surface markers CD105, CD90, and CD73, and their multilineage differentiation capacity into adipocytes, osteoblasts, and chondrocytes. MSCs were also evaluated for their potential to differentiate toward keratocytes, and for upregulation of the anti-inflammatory protein TNFα-stimulated gene-6 (TNFAIP6) after simulation by IFN-γ and TNF-α. Results: Keratocyte lineage induction was achieved in all MSCs as indicated by the upregulated expression of keratocyte markers, including keratocan, lumican, and carbohydrate sulfotransferase. TNFAIP6 response to inflammatory stimulation was observed only in CSSCs; increasing by 3-fold compared with the control (P < 0.05). Conclusions: Based on our findings, CSSCs appeared to have the greatest differentiation potential toward the keratocyte lineage and the greatest anti-inflammatory properties in vitro.

Mesenchymal Stem Cells Reduce Corneal Fibrosis and Inflammation via Extracellular Vesicle-Mediated Delivery of miRNA.

Mesenchymal stem cells from corneal stromal stem cells (CSSC) prevent fibrotic scarring and stimulate regeneration of transparent stromal tissue after corneal wounding in mice. These effects rely on the ability of CSSC to block neutrophil infiltration into the damaged cornea. The current study investigated the hypothesis that tissue regeneration by CSSC is mediated by secreted extracellular vesicles (EVs). CSSC produced EVs 130-150 nm in diameter with surface proteins that include CD63, CD81, and CD9. EVs from CSSC reduced visual scarring in murine corneal wounds as effectively as did live cells, but EVs from human embryonic kidney (HEK)293T cells had no regenerative properties. CSSC EV treatment of wounds decreased expression of fibrotic genes Col3a1 and Acta2, blocked neutrophil infiltration, and restored normal tissue morphology. CSSC EVs labeled with carboxyfluorescein succinimidyl ester dye, rapidly fused with corneal epithelial and stromal cells in culture, transferring microRNA (miRNA) to the target cells. Knockdown of mRNA for Alix, a component of the endosomal sorting complex required for transport, using siRNA, resulted in an 85% reduction of miRNA in the secreted EVs. The EVs with reduced miRNA were ineffective at blocking corneal scarring. Furthermore, CSSC with reduced Alix expression also lost their regenerative function, suggesting EVs as an obligate component in the delivery of miRNA. The results of these studies support an essential role for extracellular vesicles in the process by which CSSC cells block scarring and initiate regeneration of transparent corneal tissue after wounding. EVs appear to serve as a delivery vehicle for miRNA, which affects the regenerative action. Stem Cells Translational Medicine 2019;8:1192-1201.

Non-enzymatic glycation of type I collagen diminishes collagen-proteoglycan binding and weakens cell adhesion.

Non-enzymatic glycation of type I collagen occurs in aging and diabetes, and may affect collagen solubility, charge, polymerization, and intermolecular interactions. Proteoglycans(1) (PGs) bind type I collagen and are proposed to regulate fibril assembly, function, and cell-collagen interactions. Moreover, on the collagen fibril a keratan sulfate (KS) PG binding region overlaps with preferred collagen glycation sites. Thus, we examined the effect of collagen modified by simple glycation on PG-collagen interactions. By affinity coelectrophoresis (ACE), we found reduced affinities of heparin and KSPGs for glycated but not normal collagen, whereas the dermatan sulfate (DS)PGs decorin and biglycan bound similarly to both, and that the affinity of heparin for normal collagen decreased with increasing pH. Circular dichroism (CD) spectroscopy revealed normal and glycated collagens to assume triple helical conformations, but heparin addition caused precipitation and decreased triple helical content-effects that were more marked with glycated collagen. A spectrophotometric assay revealed slower polymerization of glycated collagen. However, ultrastructural analyses indicated that fibrils assembled from normal and glycated collagen exhibited normal periodicity, and had similar structures and comparable diameter distributions. B-cells expressing the cell surface heparan sulfate PG syndecan-1 adhered well to normal but not glycated collagen, and endothelial cell migration was delayed on glycated collagen. We speculate that glycation diminishes the electrostatic interactions between type I collagen and PGs, and may interfere with core protein-collagen associations for KSPGs but not DSPGs. Therefore in vivo, collagen glycation may weaken PG-collagen interactions, thereby disrupting matrix integrity and cell-collagen interactions, adhesion, and migration.

Keratocyte phenotype is enhanced in the absence of attachment to the substratum.

PURPOSE: Keratocytes, mesenchymal cells populating the corneal stroma, secrete the unique transparent connective tissue of the cornea as well as opaque scar tissue after injury. Previous studies identified factors mediating keratocyte phenotype in vitro, particularly the expression of the keratan sulfate proteoglycans, which are essential for vision. Whereas earlier work emphasized effects of cytokines, the current study examines the effects of substratum attachment on keratocyte phenotype. METHODS: Primary keratocytes from collagenase digestion of bovine corneas were cultured on tissue-culture plastic or on poly (2-hydroxyethylmethacrylate)(polyHEMA)-coated, non-adhesive surfaces. Secreted proteoglycans from culture media and cell-associated proteins were characterized using western blotting or isotopic labeling. Gene expression was characterized with quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR). Secreted matrix was examined with immunostaining. RESULTS: We observed that virtually all primary keratocytes participate in the formation of spheroidal aggregates, remaining viable for at least four weeks in vitro. Spheroid keratocytes secrete more keratan sulfate and keratocan than attached cells in the same culture medium. In spheroids, keratocytes accumulate substantial matrix in intercellular spaces, including keratan sulfate, lumican, keratocan, and collagens V and VI. The unattached cells undergo limited cell division and do not differentiate into myofibroblasts in response to transforming growth factor beta (TGFbeta), which is based on the expression of extra domain A (EDA) fibronectin and alpha-smooth muscle actin. Similarly, the platelet derived growth factor, a cytokine initiating the fibroblastic phenotype in attached keratocytes, had a limited effect on the spheroid-associated keratocytes. Ascorbate-2-phosphate was the only agent stimulating keratan sulfate secretion in the spheroid keratocytes. CONCLUSIONS: These results provide a new paradigm for understanding signals that regulate extracellular matrix secretion. For primary keratocytes, the alteration of the cellular environment in terms of cell-cell and cell-matrix interactions mediates and can override signals from soluble cytokines in influencing matrix expression and also in adopting other aspects of the fibroblastic and myofibroblastic phenotypes found in healing wounds.

Compressed Collagen Enhances Stem Cell Therapy for Corneal Scarring.

Stem cells from human corneal stroma (CSSC) suppress corneal stromal scarring in a mouse wound-healing model and promote regeneration of native transparent tissue (PMID:25504883). This study investigated efficacy of compressed collagen gel (CCG) as a vehicle to deliver CSSC for corneal therapy. CSSC isolated from limbal stroma of human donor corneas were embedded in soluble rat-tendon collagen, gelled at 37°C, and partially dehydrated to a thickness of 100 µm by passive absorption. The CCG disks were dimensionally stable, easy to handle, and could be adhered securely to de-epithelialized mouse cornea with fibrin-based adhesive. CSSC in CCG maintained >80% viability for >1 week in culture media and could be cryopreserved in 20% fetal bovine serum-10%DMSO in liquid nitrogen. CCG containing as few as 500 CSSC effectively prevented visible scarring and suppressed expression of fibrotic Col3a1 mRNA. CSSC in CCG were more effective at blocking scarring on a per-cell basis than CSSC delivered directly in a fibrin gel as previously described. Collagen-embedded cells retained the ability to suppress corneal scarring after conventional cryopreservation. This study demonstrates use of a common biomaterial that can facilitate storage and handling of stem cells in a manner that may provide off-the-shelf delivery of stem cells as a therapy for corneal scarring. Stem Cells Translational Medicine 2018;7:487-494.

Multi-layered silk film coculture system for human corneal epithelial and stromal stem cells.

With insufficient options to meet the clinical demand for cornea transplants, one emerging area of emphasis is on cornea tissue engineering. In the present study, the goal was to combine the corneal stroma and epithelium into one coculture system, to monitor both human corneal stromal stem cell (hCSSC) and human corneal epithelial cell (hCE) growth and differentiation into keratocytes and differentiated epithelium in these three-dimensional tissue systems in vitro. Coculture conditions were first optimized, including the medium, air-liquid interface culture, and surface topography and chemistry of biomaterial scaffold films based on silk protein. The silk was used as scaffolding for both stromal and epithelial tissue layers because it is cell compatible, can be surface patterned, and is optically clear. Next, the effects of proliferating and differentiating hCEs and hCSSCs were studied in this in vitro system, including the effects on cell proliferation, matrix formation by immunochemistry, and gene expression by quantitative reverse transcription-polymerase chain reaction. The incorporation of both cell types into the coculture system demonstrated more complete differentiation and growth for both cell types compared to the corneal stromal cells and corneal epithelial cells alone. Silk films for corneal epithelial culture were optimized to combine a 4.0-µm-scale surface pattern with bulk-loaded collagen type IV. Differentiation of each cell type was in evidence based on increased expression of corneal stroma and epithelial proteins and transcript levels after 6 weeks in coculture on the optimized silk scaffolds.

Scaffold-free tissue engineering of functional corneal stromal tissue.

Blinding corneal scarring is predominately treated with allogeneic graft tissue; however, there is a worldwide shortage of donor tissue leaving millions in need of therapy. Human corneal stromal stem cells (CSSC) have been shown produce corneal tissue when cultured on nanofibre scaffolding, but this tissue cannot be readily separated from the scaffold. In this study, scaffold-free tissue engineering methods were used to generate biomimetic corneal stromal tissue constructs that can be transplanted in vivo without introducing the additional variables associated with exogenous scaffolding. CSSC were cultured on substrates with aligned microgrooves, which directed parallel cell alignment and matrix organization, similar to the organization of native corneal stromal lamella. CSSC produced sufficient matrix to allow manual separation of a tissue sheet from the grooved substrate. These constructs were cellular and collagenous tissue sheets, approximately 4 µm thick and contained extracellular matrix molecules typical of corneal tissue including collagen types I and V and keratocan. Similar to the native corneal stroma, the engineered corneal tissues contained long parallel collagen fibrils with uniform diameter. After being transplanted into mouse corneal stromal pockets, the engineered corneal stromal tissues became transparent, and the human CSSCs continued to express human corneal stromal matrix molecules. Both in vitro and in vivo, these scaffold-free engineered constructs emulated stromal lamellae of native corneal stromal tissues. Scaffold-free engineered corneal stromal constructs represent a novel, potentially autologous, cell-generated, biomaterial with the potential for treating corneal blindness. Copyright © 2016 John Wiley & Sons, Ltd.

Modeling Diabetic Corneal Neuropathy in a 3D In Vitro Cornea System.

Diabetes mellitus is a disease caused by innate or acquired insulin deficiency, resulting in altered glucose metabolism and high blood glucose levels. Chronic hyperglycemia is linked to development of several ocular pathologies affecting the anterior segment, including diabetic corneal neuropathy and keratopathy, neovascular glaucoma, edema, and cataracts leading to significant visual defects. Due to increasing disease prevalence, related medical care costs, and visual impairment resulting from diabetes, a need has arisen to devise alternative systems to study molecular mechanisms involved in disease onset and progression. In our current study, we applied a novel 3D in vitro model of the human cornea comprising of epithelial, stromal, and neuronal components cultured in silk scaffolds to study the pathological effects of hyperglycemia on development of diabetic corneal neuropathy. Specifically, exposure to sustained levels of high glucose, ranging from 35 mM to 45 mM, were applied to determine concentration-dependent effects on nerve morphology, length and density of axons, and expression of metabolic enzymes involved in glucose metabolism. By comparing these metrics to in vivo studies, we have developed a functional 3D in vitro model for diabetic corneal neuropathy as a means to investigate corneal pathophysiology resulting from prolonged exposure to hyperglycemia.