Constructing a Novel Three-Dimensional Biomimetic Corneal Endothelium Graft by Culturing Corneal Endothelium Cells on Compressed Collagen Gels

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

Design of functional biomaterials as substrates for corneal endothelium tissue engineering

Corneal endothelium defects are one of the leading causes of blindness worldwide. The actual treatment is transplantation, which requires the use of human cadaveric donors, but it faces several problems, such as global shortage of donors. Therefore, new alternatives are being developed and, among them, cell therapy has gained interest in the last years due to its promising results in tissue regeneration. Nevertheless, the direct administration of cells may sometimes have limited success due to the immune response, hence requiring the combination with extracellular mimicking materials. In this review, we present different methods to obtain corneal endothelial cells from diverse cell sources such as pluripotent or multipotent stem cells. Moreover, we discuss different substrates in order to allow a correct implantation as a cell sheet and to promote an enhanced cell behavior. For this reason, natural or synthetic matrixes that mimic the native environment have been developed. These matrixes have been optimized in terms of their physicochemical properties, such as stiffness, topography, composition and transparency. To further enhance the matrixes properties, these can be tuned by incorporating certain molecules that can be delivered in a sustained manner in order to enhance biological behavior. Finally, we elucidate future directions for corneal endothelial regeneration, such as 3D printing, in order to obtain patient-specific substrates.

Comparative culture of human corneal endothelial cells following treatment with human platelet lysate/fibrin hydrogel versus Y-27632 ROCK inhibitor: in vitro and ex vivo study

PURPOSE

The advancement of tissue engineering and cell therapy research has resulted in innovative therapeutic options for patients with corneal endothelial diseases. The aim of this study was to compare the potential effect of using human platelet lysate (HPL)/Fibrin hydrogel versus using a Y-27632 ROCK inhibitor, on the culture of human corneal endothelial cells (HCECs) under in vitro and ex vivo conditions.

METHODS

HCECs were isolated from human donors and treated separately with HPL/Fibrin hydrogel, a Y-27632 ROCK inhibitor, and fetal bovine serum (FBS). MTT viability assay and cell counting were performed on the treated cells. Subsequently, we prepared ex vivo models of human corneal endothelial dysfunction and incubated them with DiI-labeled-HCECs. Specular and fluorescence microscopy were then performed on each of the ex vivo models.

RESULTS

In comparison, similar viability results were achieved in the cells treated with HPL/Fibrin hydrogel versus those treated with the Y-27632 ROCK inhibitor, but both treatments showed higher viability than the control group (FBS). More importantly, based on the specular and fluorescence microscopic results, the HPL/Fibrin hydrogel and the Y-27632 ROCK inhibitor treatments showed similar inducible effects on the attachment of the cells to the Descemet membranes of the ex vivo models.

CONCLUSION

HPL/Fibrin hydrogel and Y-27632 ROCK inhibitor have similar inducible effects on the viability and attachment of the HCECs. A definite advantage of treating cells with HPL/Fibrin hydrogel is that it serves as a xeno-free and biocompatible material which can have autologous applications in future usage by clinics.

Corneal supply and the use of technology to reduce its demand: A review

Recovery and access to end-of-life corneal tissue for corneal transplantation, training and research is globally maldistributed. The reasons for the maldistribution are complex and multifaceted, and not well defined or understood. Currently there are few solutions available to effectively address these issues. This review provides an overview of the system, key issues impacting recovery and allocation and emphasises how end-user ophthalmologists and researchers, with support from administrators and the wider sector, can assist in increasing access long-term through sustaining eye banks nationally and globally. We posit that prevention measures and improved surgical techniques, together with the development of novel therapies will play a significant role in reducing demand and enhance the equitable allocation of corneas.

Corneal endothelium tissue engineering: An evolution of signaling molecules, cells, and scaffolds toward 3D bioprinting and cell sheets

Cornea is an avascular and transparent tissue that focuses light on retina. Cornea is supported by the corneal-endothelial layer through regulation of hydration homeostasis. Restoring vision in patients afflicted with corneal endothelium dysfunction-mediated blindness most often requires corneal transplantation (CT), which faces considerable constrictions due to donor limitations. An emerging alternative to CT is corneal endothelium tissue engineering (CETE), which involves utilizing scaffold-based methods and scaffold-free strategies. The innovative scaffold-free method is cell sheet engineering, which typically generates cell layers surrounded by an intact extracellular matrix, exhibiting tunable release from the stimuli-responsive surface. In some studies, scaffold-based or scaffold-free technologies have been reported to achieve promising outcomes. However, yet some issues exist in translating CETE from bench to clinical practice. In this review, we compare different corneal endothelium regeneration methods and elaborate on the application of multiple cell types (stem cells, corneal endothelial cells, and endothelial precursors), signaling molecules (growth factors, cytokines, chemical compounds, and small RNAs), and natural and synthetic scaffolds for CETE. Furthermore, we discuss the importance of three-dimensional bioprinting strategies and simulation of Descemet's membrane by biomimetic topography. Finally, we dissected the recent advances, applications, and prospects of cell sheet engineering for CETE.

The future of keratoplasty: cell-based therapy, regenerative medicine, bioengineering keratoplasty, gene therapy

PURPOSE OF REVIEW

To provide an update on the state of development of novel therapeutic modalities for the treatment of corneal diseases.

RECENT FINDINGS

Novel corneal therapeutics may be broadly classified as cell therapy, regenerative medicine, bioengineered corneal grafts and gene therapy. Cell therapy encompasses cultivation of cells, such as corneal endothelial cells (CECs) and keratocytes to replenish the depleted native cell population. Regenerative medicine is mainly applicable to the corneal endothelium, and is dependent on the ability of native, healthy CECs to restore the corneal endothelium following trauma or descemetorhexis; this approach may be effective for the treatment of Peter's anomaly and Fuchs endothelial corneal dystrophy (FECD). Bioengineered corneal grafts are synthetic constructs designed to replace cadaveric corneal grafts; tissue-engineered endothelial-keratoplasty grafts and bioengineered stromal grafts have been experimented in animal models with favourable results. Gene therapy with antisense oligonucleotide and CRISPR endonucleases, including deactivated Cas9, may potentially be used to treat FECD and TGFBI-related corneal dystrophies.

SUMMARY

These novel therapeutic modalities may potentially supersede keratoplasty as the standard of care in the future.