Construction of a human corneal stromal equivalent with non-transfected human corneal stromal cells and acellular porcine corneal stromata

Human SMILE-Derived Stromal Lenticule Scaffold for Regenerative Therapy: Review and Perspectives

A transparent cornea is paramount for vision. Corneal opacity is one of the leading causes of blindness. Although conventional corneal transplantation has been successful in recovering patients’ vision, the outcomes are challenged by a global lack of donor tissue availability. Bioengineered corneal tissues are gaining momentum as a new source for corneal wound healing and scar management. Extracellular matrix (ECM)-scaffold-based engineering offers a new perspective on corneal regenerative medicine. Ultrathin stromal laminar tissues obtained from lenticule-based refractive correction procedures, such as SMall Incision Lenticule Extraction (SMILE), are an accessible and novel source of collagen-rich ECM scaffolds with high mechanical strength, biocompatibility, and transparency. After customization (including decellularization), these lenticules can serve as an acellular scaffold niche to repopulate cells, including stromal keratocytes and stem cells, with functional phenotypes. The intrastromal transplantation of these cell/tissue composites can regenerate native-like corneal stromal tissue and restore corneal transparency. This review highlights the current status of ECM-scaffold-based engineering with cells, along with the development of drug and growth factor delivery systems, and elucidates the potential uses of stromal lenticule scaffolds in regenerative therapeutics.

Tissue-Engineered Corneal Endothelial Sheets Using Ultrathin Acellular Porcine Corneal Stroma Substrates for Endothelial Keratoplasty

Tissue-engineered cornea endothelial sheets (TECES), created using a biocompatible thin and transparent carrier with corneal endothelial cells, could alleviate the shortage of donor corneas and provide abundant functional endothelial cells. In our previous clinical trials, the effectiveness and safety of the acellular porcine corneal stroma (APCS) applied in lamellar keratoplasty have been confirmed. In this study, we optimized the method to cut APCS into multiple 20 μm ultrathin lamellae by a cryostat microtome and investigated the feasibility of TECES by seeding rabbit corneal endothelial cells (RCECs) on ultrathin APCS. Cell adhesion, proliferation, and functional gene expression of RCECs on tissue-culture plastic and APCS of different thicknesses were compared. The results indicated that ultrathin lamellae were superior in increasing cell viability and maintaining cell functions. Analyzing with histology, electron microscopy, and immunofluorescence, we found that RCECs cultured on 20 μm ultrathin APCS for 5 days grew into a confluent monolayer with a density of 3726 ± 223 cells/mm² and expressed functional biomarkers Na⁺/K⁺-ATPase and zonula occludens. After 14 days, RCECs formed an early stage of Descemet's membrane-like structure by synthesizing collagen IV and laminin. Human corneal endothelial cells were also used to further validate the supportive effect of ultrathin APCS on cells. The resulting constructs were flexible and tough enough to implant into rabbits' anterior chambers through small incisions. TECES adhered to the posterior corneal stroma, and the thickness of cornea gradually reduced to normal after grafting. These results indicate that the ultrathin APCS can serve as a tissue engineering carrier and might be a suitable alternative for endothelial cells expansion in endothelial keratoplasty.

The effect of prior long-term recellularization with keratocytes of decellularized porcine corneas implanted in a rabbit anterior lamellar keratoplasty model

Decellularized porcine corneal scaffolds are a potential alternative to human cornea for keratoplasty. Although clinical trials have reported promising results, there can be corneal haze or scar tissue. Here, we examined if recellularizing the scaffolds with human keratocytes would result in a better outcome. Scaffolds were prepared that retained little DNA (14.89 ± 5.56 ng/mg) and demonstrated a lack of cytotoxicity by in vitro. The scaffolds were recellularized using human corneal stromal cells and cultured for between 14 in serum-supplemented media followed by a further 14 days in either serum free or serum-supplemented media. All groups showed full-depth cell penetration after 14 days. When serum was present, staining for ALDH3A1 remained weak but after serum-free culture, staining was brighter and the keratocytes adopted a native dendritic morphology with an increase (p < 0.05) of keratocan, decorin, lumican and CD34 gene expression. A rabbit anterior lamellar keratoplasty model was used to compare implanting a 250 μm thick decellularized lenticule against one that had been recellularized with human stromal cells after serum-free culture. In both groups, host rabbit epithelium covered the implants, but transparency was not restored after 3 months. Post-mortem histology showed under the epithelium, a less-compact collagen layer, which appeared to be a regenerating zone with some α-SMA staining, indicating fibrotic cells. In the posterior scaffold, ALDH1A1 staining was present in all the acellular scaffold, but in only one of the recellularized lenticules. Since there was little difference between acellular and cell-seeded scaffolds in our in vivo study, future scaffold development should use acellular controls to determine if cells are necessary.

Decellularization and recellularization of cornea: Progress towards a donor alternative

The global shortage of donor corneas for transplantation has led to corneal bioengineering being investigated as a method to generate transplantable tissues. Decellularized corneas are among the most promising materials for engineering corneal tissue since they replicate the complex structure and composition of real corneas. Decellularization is a process that aims to remove cells from organs or tissues resulting in a cell-free scaffold consisting of the tissues extracellular matrix. Here different decellularization techniques are described, including physical, chemical and biological methods. Analytical techniques to confirm decellularization efficiency are also discussed. Different cell sources for the recellularization of the three layers of the cornea, recellularization methods used in the literature and techniques used to assess the outcome of the implantation of such scaffolds are examined. Studies involving the application of decellularized corneas in animal models and human clinical studies are discussed. Finally, challenges for this technology are explored involving scalability, automatization and regulatory affairs.

Natural Biomaterials for Corneal Tissue Engineering, Repair, and Regeneration

Corneal blindness is a major cause of vision loss, estimated to affect over 10 million people worldwide. Once impaired through clouding or shape change, the best treatment option for restoring vision is corneal transplantation using full or partial thickness cadaveric grafts. However, donor corneas are globally limited and face rejection and graft failure, similar to other transplanted organs. Thus, there is a need for viable alternatives to donor corneas in order to increase supply, reduce rejection, and to minimize variability in tissue quality. To address this, researchers have developed new materials and strategies to tissue engineer full or partial thickness cornea grafts in order to repair, regenerate, or replace the diseased cornea. This progress report first reviews the anatomy and physiology of the cornea to frame the biological requirements and discuss the injuries and diseases that necessitate the need fortransplantation, as well as the requirements for a suitable donor tissue alternative. This is followed by recent progress using naturally derived biomaterials including silk, collagen, amniotic membranes, and decellularized corneas. Finally, remaining challenges in the field as they relate to the biomaterials discussed are identified, and the future research directions that should result in further advances in restoring corneal vision are highlighted.

Tetracaine induces apoptosis through a mitochondrion-dependent pathway in human corneal stromal cells in vitro

PURPOSE

Tetracaine is a local anesthetic widely used in ocular diagnosis and ophthalmic surgery and may lead to some adverse effects and complications at a clinical dose. To assess the cytotoxicity and molecular toxicity mechanisms of tetracaine, we used human corneal stromal (HCS) cells as an in vitro model to study the effects of tetracaine on HCS cells.

MATERIALS AND METHODS

The cytotoxicity of tetracaine on HCS cells was investigated by examining the changes of cell growth, morphology, viability and cell cycle progressing when HCS cells were treated with tetracaine at concentrations from 10 g/L to 0.078125 g/L. To prove the hypothesis that the cytotoxicity of tetracaine on HCS cells was related with apoptosis induction, we further detected multiple changes in HCS cells, including plasma membrane (PM) permeability, phosphatidylserine (PS) orientation, genomic DNA integrality, and cell ultrastrcuture after treated with tetracaine. Furthermore, the pro-apoptotic signalling pathway induced by tetracaine was explored through detecting the activation of various caspases, the changes of mitochondrial transmembrane potential (MTP), the expression level of Bcl-2 family proteins and the amount of mitochondria-released apoptosis regulating proteins in cytoplasm.

RESULTS

Tetracaine at concentrations above 0.15625 g/L had a dose- and time-dependent cytotoxicity to HCS cells, which resulted cell growth inhibition, proliferation retardation, morphological abnormalities and decreased viability. Meanwhile, we found that the HCS cells treated with tetracaine had typical features associated with apoptosis, including an increase in PM permeability, PS externalization, DNA fragmentation and apoptotic body formation. Tetracaine not only resulted in caspase-3, caspase-8 and caspase-9 activation and disruption of MTP but also downregulated Bcl-2 and Bcl-xL and upregulated Bad and Bax, along with the upregulation of cytoplasmic cytochrome c (Cyt. c) and apoptosis-inducing factor (AIF).

CONCLUSIONS

These results suggested that tetracaine-induced apoptosis might be triggered through Fas death receptors and mediated by Bcl-2 family proteins in the mitochondria-dependent pathway. Our findings identified the cytotoxicity and molecular mechanisms of tetracaine, which could provide a reference value for the safety of this medication and prospective therapeutic interventions in eye clinic.

Construction of Anterior Hemi-Corneal Equivalents Using Nontransfected Human Corneal Cells and Transplantation in Dog Models

Tissue-engineered human anterior hemi-cornea (TE-aHC) is a promising equivalent for treating anterior lamellar keratopathy to surmount the severe shortage of donated corneas. This study was intended to construct a functional TE-aHC with nontransfected human corneal stromal (ntHCS) and epithelial (ntHCEP) cells using acellular porcine corneal stromata (aPCS) as a carrier scaffold, and evaluate its biological functions in a dog model. To construct a TE-aHC, ntHCS cells were injected into an aPCS scaffold and cultured for 3 days; then, ntHCEP cells were inoculated onto the Bowman's membrane of the scaffold and cultured for 5 days under air-liquid interface condition. After its morphology and histological structure were characterized, the constructed TE-aHC was transplanted into dog eyes via lamellar keratoplasty. The corneal transparency, thickness, intraocular pressure, epithelial integrity, and corneal regeneration were monitored in vivo, and the histological structure and histochemical property were examined ex vivo 360 days after surgery, respectively. The results showed that the constructed TE-aHC was highly transparent and composed of a corneal epithelium of 7-8 layer ntHCEP cells and a corneal stroma of regularly aligned collagen fibers and well-preserved glycosaminoglycans with sparsely distributed ntHCS cells, mimicking a normal anterior hemi-cornea (aHC). Moreover, both ntHCEP and ntHCS cells maintained positive expression of their marker and functional proteins. After transplantation into dog eyes, the constructed TE-aHC acted naturally in terms of morphology, structure and inherent property, and functioned well in maintaining corneal clarity, thickness, normal histological structure, and composition in dog models by reconstructing a normal aHC, which could be used as a promising aHC equivalent in corneal regenerative medicine and aHC disorder therapy.

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

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

Proparacaine induces cytotoxicity and mitochondria-dependent apoptosis in corneal stromal cells both in vitro and in vivo

Proparacaine (PPC) is a widely used topical anaesthetic in the eye clinic; its abuse may damage the cornea and result in impairment of vision. Although PPC has been reported to be cytotoxic to human keratocytes, there is no scientific report about its toxic mechanisms in human corneal stroma. Here, we evaluated the cytotoxicity of PPC to corneal stroma in an in vitro model of human corneal stromal (HCS) cells and an in vivo model of cat corneas. To postulate the cellular and molecular mechanisms involved in PPC toxicity, changes in the hallmarks of apoptosis as well as in pro-apoptotic signaling pathways were investigated. Our results showed that PPC at concentrations varying from 5.0 to 0.15625 g L^-1 induced dose- and time-dependent cell atrophy, vacuolation, cytopathic effects, and viability decline in vitro. Moreover, PPC induced G₁ phase arrest, plasma membrane permeability elevation, phosphatidylserine externalization, DNA fragmentation, chromatin condensation, and apoptotic body formation of HCS cells. Furthermore, PPC could induce caspase-2, -3 and -9 activation, and mitochondrial transmembrane potential disruption. Expression of Bcl-xL and Bax were downregulated and upregulated, respectively, and cytoplasmic cytochrome c and apoptosis inducing factor were upregulated remarkably after PPC treatment. The cytotoxicity and pro-apoptotic effects of PPC were also proven by induced corneal edema, apoptotic-like ultrastructural alterations and DNA fragmentation of keratocytes in cat corneas in vivo. These results suggest that PPC above 1/32 of its clinical dosage has remarkable cytotoxicity to corneal stromal cells, which is achieved by inducing death receptor-mediated mitochondria-dependent apoptosis of HCS cells.

Acellular ostrich corneal stroma used as scaffold for construction of tissue-engineered cornea

AIM

To assess acellular ostrich corneal matrix used as a scaffold to reconstruct a damaged cornea.

METHODS

A hypertonic saline solution combined with a digestion method was used to decellularize the ostrich cornea. The microstructure of the acellular corneal matrix was observed by transmission electron microscopy (TEM) and hematoxylin and eosin (H&E) staining. The mechanical properties were detected by a rheometer and a tension machine. The acellular corneal matrix was also transplanted into a rabbit cornea and cytokeratin 3 was used to check the immune phenotype.

RESULTS

The microstructure and mechanical properties of the ostrich cornea were well preserved after the decellularization process. In vitro, the methyl thiazolyl tetrazolium results revealed that extracts of the acellular ostrich corneas (AOCs) had no inhibitory effects on the proliferation of the corneal epithelial or endothelial cells or on the keratocytes. The rabbit lamellar keratoplasty showed that the transplanted AOCs were transparent and completely incorporated into the host cornea while corneal turbidity and graft dissolution occurred in the acellular porcine cornea (APC) transplantation. The phenotype of the reconstructed cornea was similar to a normal rabbit cornea with a high expression of cytokeratin 3 in the superficial epithelial cell layer.

CONCLUSION

We first used AOCs as scaffolds to reconstruct damaged corneas. Compared with porcine corneas, the anatomical structures of ostrich corneas are closer to those of human corneas. In accordance with the principle that structure determines function, a xenograft lamellar keratoplasty also confirmed that the AOC transplantation generated a superior outcome compared to that of the APC graft.

Bioengineered multilayered human corneas from discarded human corneal tissue

Corneal transplantation has become a common procedure to improve visual acuity by replacing the opaque or distorted host tissue with clear healthy donor corneal tissue. However, globally its wide spread clinical utility is limited due to a lack of supply of high quality corneas. Bioengineered neo-corneas using discarded human corneas to isolate corneal endothelial and epithelial cells, as well as corneal stroma as a scaffolding material, could help address this shortage. The objective of this study was to fabricate multilayered corneal equivalents that could be suitable for full thickness cornea transplantation. To achieve this goal human corneal endothelial cells (hCEC) and human limbal epithelial cells (hLEC) were isolated from discarded human corneas and expanded in vitro, maintaining their phenotype for at least 3 passages. We used our previously described process of human cornea decellularization to create corneal scaffolds that preserve the native extracellular matrix of the corneal stroma. The corneal scaffolds were seeded with hCEC and hLEC, using a special apparatus that enabled seeding both sides of the scaffold. The cell-seeded corneal constructs supported hCEC and hLEC growth and multi-cellular organization for 2 weeks in vitro. Immunohistochemical analysis showed expression of typical hCEC and hLEC markers on their corresponding sides. Importantly, the cell-seeded corneal constructs were more transparent than non-seeded corneal scaffolds. Taken together, this study demonstrates the feasibility of creating multilayered cornea equivalents, exclusively from human donor-derived materials. These constructs may be suitable for corneal transplantation, and as a short-term application, may serve for ophthalmological drug testing purposes.