In vitro simulation of corneal epithelium microenvironment induces a corneal epithelial-like cell phenotype from human adipose tissue mesenchymal stem cells

Unlocking the versatile potential: Adipose-derived mesenchymal stem cells in ocular surface reconstruction and oculoplastics

This review comprehensively explores the versatile potential of mesenchymal stem cells (MSCs) with a specific focus on adipose-derived MSCs. Ophthalmic and oculoplastic surgery, encompassing diverse procedures for ocular and periocular enhancement, demands advanced solutions for tissue restoration, functional and aesthetic refinement, and aging. Investigating immunomodulatory, regenerative, and healing capacities of MSCs, this review underscores the potential use of adipose-derived MSCs as a cost-effective alternative from bench to bedside, addressing common unmet needs in the field of reconstructive and regenerative surgery.

Seeing the Future: A Review of Ocular Therapy

Ocular diseases present a unique challenge and opportunity for therapeutic development. The eye has distinct advantages as a therapy target given its accessibility, compartmentalization, immune privilege, and size. Various methodologies for therapeutic delivery in ocular diseases are under investigation that impact long-term efficacy, toxicity, invasiveness, and delivery range. While gene, cell, and antibody therapy and nanoparticle delivery directly treat regions that have been damaged by disease, they can be limited in the duration of the therapeutic delivery and have a focal effect. In contrast, contact lenses and ocular implants can more effectively achieve sustained and widespread delivery of therapies; however, they can increase dilution of therapeutics, which may result in reduced effectiveness. Current therapies either offer a sustained release or a broad therapeutic effect, and future directions should aim toward achieving both. This review discusses current ocular therapy delivery systems and their applications, mechanisms for delivering therapeutic products to ocular tissues, advantages and challenges associated with each delivery system, current approved therapies, and clinical trials. Future directions for the improvement in existing ocular therapies include combination therapies, such as combined cell and gene therapies, as well as AI-driven devices, such as cortical implants that directly transmit visual information to the cortex.

Advances in Cell Transplantation Therapy for Limbal Stem Cell Deficiency

BACKGROUND

Limbal stem cells (LSCs) are essential for maintaining corneal transparency and ocular surface integrity. Many external factors or genetic diseases can lead to corneal limbal stem cell deficiency (LSCD), resulting in the loss of barrier and corneal epithelial cell renewal functions. Stem cell transplantation is one of the primary treatments for LSCD, including limbal transplantation and cultivated limbal epithelial transplantation. In addition, a variety of non-limbal stem cell lines have been experimented with for LSCD treatment. Biological scaffolds are also used to support in vitro stem cell culture and transplantation. Here, we review the mechanisms of corneal maintenance by LSCs, the clinical stage and surgical treatment of LSCD, the source of stem cells, and the biological scaffolds required for in vitro culture.

METHODS

This study is a narrative retrospective study aimed at collecting available information on various aspects of surgical treatments for LSCD. Relevant literature was searched in a range of online databases, including Web of Science, Scopus, and PubMed from 2005 to March, 2023.

RESULTS

A total of 397 relevant articles were found, and 49 articles with strong relevance to the studies in this paper were obtained and analyzed. Moreover, 11 of these articles were on the concept of LSCD and the mechanism of LESCs maintaining the corneal epithelium, 3 articles on the staging and grading of LSCD, 17 articles on cell transplantation methods and donor cell sources, and 18 articles on scaffolds for delivering stem cells. We also summarized the advantages and disadvantages of different cell transplantation methods and the benefits and limitations of scaffolds based on the above literature.

CONCLUSION

The treatment of LSCD is determined by the clinical stage and whether it involves monocular or binocular eyes. Appropriate surgical techniques should be taken for LSCD patients in order to reconstruct the ocular surface, relieve symptoms, and restore visual function. Meanwhile, biological scaffolds assist in the ex vivo culture and implantation of stem cells.

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.

Mesenchymal Stem Cells and Exosomes: A Novel Therapeutic Approach for Corneal Diseases

The cornea, with its delicate structure, is vulnerable to damage from physical, chemical, and genetic factors. Corneal transplantation, including penetrating and lamellar keratoplasties, can restore the functions of the cornea in cases of severe damage. However, the process of corneal transplantation presents considerable obstacles, including a shortage of available donors, the risk of severe graft rejection, and potentially life-threatening complications. Over the past few decades, mesenchymal stem cell (MSC) therapy has become a novel alternative approach to corneal regeneration. Numerous studies have demonstrated the potential of MSCs to differentiate into different corneal cell types, such as keratocytes, epithelial cells, and endothelial cells. MSCs are considered a suitable candidate for corneal regeneration because of their promising therapeutic perspective and beneficial properties. MSCs compromise unique immunomodulation, anti-angiogenesis, and anti-inflammatory properties and secrete various growth factors, thus promoting corneal reconstruction. These effects in corneal engineering are mediated by MSCs differentiating into different lineages and paracrine action via exosomes. Early studies have proven the roles of MSC-derived exosomes in corneal regeneration by reducing inflammation, inhibiting neovascularization, and angiogenesis, and by promoting cell proliferation. This review highlights the contribution of MSCs and MSC-derived exosomes, their current usage status to overcome corneal disease, and their potential to restore different corneal layers as novel therapeutic agents. It also discusses feasible future possibilities, applications, challenges, and opportunities for future research in this field.

Advancements in Ocular Regenerative Therapies

The use of stem cells (SCs) has emerged as a promising avenue in ophthalmology, offering potential therapeutic solutions for various vision impairments and degenerative eye diseases. SCs possess the unique ability to self-renew and differentiate into specialised cell types, making them valuable tools for repairing damaged tissues and restoring visual function. Stem cell-based therapies hold significant potential for addressing conditions such as age-related macular degeneration (AMD), retinitis pigmentosa (RP), corneal disorders, and optic nerve damage. Therefore, researchers have explored different sources of stem cells, including embryonic stem cells (ESC), induced pluripotent stem cells (iPSCs), and adult stem cells, for ocular tissue regeneration. Preclinical studies and early-phase clinical trials have demonstrated promising outcomes, with some patients experiencing improved vision following stem cell-based interventions. However, several challenges remain, including optimising the differentiation protocols, ensuring transplanted cells' safety and long-term viability, and developing effective delivery methods. The field of stem cell research in ophthalmology witnesses a constant influx of new reports and discoveries. To effectively navigate these tons of information, it becomes crucial to summarise and systematise these findings periodically. In light of recent discoveries, this paper demonstrates the potential applications of stem cells in ophthalmology, focusing on their use in various eye tissues, including the cornea, retina, conjunctiva, iris, trabecular meshwork, lens, ciliary body, sclera, and orbital fat.

Corneal Reconstruction with EGFP-Labelled Limbal Mesenchymal Stem Cells in a Rabbit Model of Limbal Stem Cell Deficiency

Ocular surface reconstruction is essential for treating corneal epithelial defects and vision recovery. Stem cell-based therapy demonstrates promising results but requires further research to elucidate stem cell survival, growth, and differentiation after transplantation in vivo. This study examined the corneal reconstruction promoted by EGFP-labeled limbal mesenchymal stem cells (L-MSCs-EGFP) and their fate after transplantation. EGFP labeling allowed us to evaluate the migration and survival rates of the transferred cells. L-MSCs-EGFP seeded onto decellularized human amniotic membrane (dHAM) were transplanted into rabbits with a modeled limbal stem cell deficiency. The localization and viability of the transplanted cells in animal tissue were analyzed using histology, immunohistochemistry, and confocal microscopy up to 3 months after transplantation. EGFP-labeled cells remained viable for the first 14 days after transplantation. By the 90th day, epithelialization of the rabbit corneas reached 90%, but the presence of viable labeled cells was not observed within the newly formed epithelium. Although labeled cells demonstrated low survivability in host tissue, the squamous corneal-like epithelium was partially restored by the 30th day after transplantation of the tissue-engineered graft. Overall, this study paves the way for further optimization of transplantation conditions and studying the mechanisms of corneal tissue restoration.

Derivation of Limbal Stem Cells from Human Adult Mesenchymal Stem Cells for the Treatment of Limbal Stem Cell Deficiency

Approximately 10 million individuals have blindness due to limbal stem cell (LSCs) deficiency, one of the most challenging problems in ophthalmology. To replenish the LSC pool, an autologous extraocular cell source is appropriate, thereby avoiding the risk of immune rejection, the need for immunosuppression and the risk of damaging the contralateral eye. In recent years, adipose-derived mesenchymal stem cells (ADSCs) have been a key element in ocular regenerative medicine. In this study, we developed a protocol for deriving human LSCs from ADSCs compatible with the standard carrier human amniotic membrane, helping provide a stem cell pool capable of maintaining proper corneal epithelial homeostasis. The best protocol included an ectodermal induction step by culturing ADSCs with media containing fetal bovine serum, transforming growth factor-β inhibitor SB-505124, Wnt inhibitor IWP-2 and FGF2 for 7 days, followed by an LSC induction step of culture in modified supplemental hormonal epithelial medium supplemented with pigment epithelium-derived factor and keratinocyte growth factor for 10 additional days. The optimal differentiation efficiency was achieved when cells were cultured in this manner over vitronectin coating, resulting in up to 50% double-positive αp63/BMI-1 cells. The results of this project will benefit patients with LSC deficiency, aiding the restoration of vision.

Profile of biological characterizations and clinical application of corneal stem/progenitor cells

Corneal stem/progenitor cells are typical adult stem/progenitor cells. The human cornea covers the front of the eyeball, which protects the eye from the outside environment while allowing vision. The location and function demand the cornea to maintain its transparency and to continuously renew its epithelial surface by replacing injured or aged cells through a rapid turnover process in which corneal stem/progenitor cells play an important role. Corneal stem/progenitor cells include mainly corneal epithelial stem cells, corneal endothelial cell progenitors and corneal stromal stem cells. Since the discovery of corneal epithelial stem cells (also known as limbal stem cells) in 1971, an increasing number of markers for corneal stem/progenitor cells have been proposed, but there is no consensus regarding the definitive markers for them. Therefore, the identification, isolation and cultivation of these cells remain challenging without a unified approach. In this review, we systematically introduce the profile of biological characterizations, such as anatomy, characteristics, isolation, cultivation and molecular markers, and clinical applications of the three categories of corneal stem/progenitor cells.

Role of YAP as a Mechanosensing Molecule in Stem Cells and Stem Cell-Derived Hematopoietic Cells

Yes-associated protein (YAP) and WW domain-containing transcription regulator protein 1 (WWTR1, also known as TAZ) are transcriptional coactivators in the Hippo signaling pathway. Both are well-known regulators of cell proliferation and organ size control, and they have significant roles in promoting cell proliferation and differentiation. The roles of YAP and TAZ in stem cell pluripotency and differentiation have been extensively studied. However, the upstream mediators of YAP and TAZ are not well understood. Recently, a novel role of YAP in mechanosensing and mechanotransduction has been reported. The present review updates information on the regulation of YAP by mechanical cues such as extracellular matrix stiffness, fluid shear stress, and actin cytoskeleton tension in stem cell behaviors and differentiation. The review explores mesenchymal stem cell fate decisions, pluripotent stem cells (PSCs), self-renewal, pluripotency, and differentiation to blood products. Understanding how cells sense their microenvironment or niche and mimic those microenvironments in vitro could improve the efficiency of producing stem cell products and the efficacy of the products.

Mesenchymal Stromal Cell-Based Therapy for Dry Eye: Current Status and Future Perspectives

Dry eye is one of the most common chronic diseases in ophthalmology. It affects quality of life and has become a public health problem that cannot be ignored. The current treatment methods mainly include artificial tear replacement therapy, anti-inflammatory therapy, and local immunosuppressive therapy. These treatments are mainly limited to improvement of ocular surface discomfort and other symptoms. In recent years, regenerative medicine has developed rapidly, and ophthalmologists are working on new methods to treat dry eye. Mesenchymal stromal cells (MSCs) have anti-inflammatory, tissue repair, and immune regulatory effects, and have become a promising tool for the treatment of dry eye. These effects can also be produced by MSC-derived exosomes (MSC-Exos). As a cell-free therapy, MSC-Exos are hypoimmunogenic, serve more stable entities, and compared with MSCs, reduce the safety risks associated with the injection of live cells. This article reviews current knowledge about MSCs and MSC-Exos, and highlights the latest progress and future prospects of MSC-based therapy in dry eye treatment.

Corneal Epithelial Regeneration: Old and New Perspectives

Corneal blindness is the fifth leading cause of blindness worldwide, and therapeutic options are still often limited to corneal transplantation. The corneal epithelium has a strong barrier function, and regeneration is highly dependent on limbal stem cell proliferation and basement membrane remodeling. As a result of the lack of corneal donor tissues, regenerative medicine for corneal diseases affecting the epithelium is an area with quite advanced basic and clinical research. Surgery still plays a prominent role in the treatment of epithelial diseases; indeed, innovative surgical techniques have been developed to transplant corneal and non-corneal stem cells onto diseased corneas for epithelial regeneration applications. The main goal of applying regenerative medicine to clinical practice is to restore function by providing viable cells based on the use of a novel therapeutic approach to generate biological substitutes and improve tissue functions. Interest in corneal epithelium rehabilitation medicine is rapidly growing, given the exposure of the corneal outer layers to external insults. Here, we performed a review of basic, clinical and surgical research reports on regenerative medicine for corneal epithelial disorders, classifying therapeutic approaches according to their macro- or microscopic target, i.e., into cellular or subcellular therapies, respectively.

The role of BMP4 in adipose-derived stem cell differentiation: A minireview

Bone morphogenetic protein 4 (BMP4) is a member of the transforming growth factor beta (TGF-β) superfamily of cytokines responsible for stem cells’ commitment to differentiation, proliferation, and maturation. To date, various studies have utilized BMP4 as a chemical inducer for in vitro differentiation of human mesenchymal stem cells (MSCs) based on its potential. BMP4 drives in vitro differentiation of ADSC via TGF-β signaling pathway by interactions with BMP receptors leading to the activation of smad-dependent and smad-independent pathways. The BMP4 signaling pathways are regulated by intracellular and extracellular BMP4 antagonists. Extracellular BMP4 antagonist prevents interaction between BMP4 ligand to its receptors, while intracellular BMP4 antagonist shutdowns the smad-dependent pathways through multiple mechanisms. BMP4 proved as one of the popular differentiation factors to induce ADSC differentiation into cell from mesodermal origin. However, addition of all-trans retinoic acid is also needed in trans-differentiation of ADSC into ectodermal lineage cells. Suggesting that both BMP4 and RA signaling pathways may be necessary to be activated for in vitro trans-differentiation of ADSC.

[Regenerative medicine for the corneal epithelium : Cell therapy from bench to bedside]

In the case of thermal or caustic burns of the ocular surface, loss of limbal epithelial stem cells leads to compromised self-renewal of the corneal epithelium. This results in permanent loss of vision. In these situations, transplantation of cultured limbal epithelial cells on an amniotic membrane or fibrin gel as substrate (Holoclar®) can help to regenerate the corneal surface. The required cells are obtained from the healthy partner eye, if available. Adult stem cells from other parts of the body potentially serve as alternative cell sources: hair follicles, oral mucosa, mesenchymal stromal cells, or induced pluripotent stem cells (originally, e.g., skin fibroblasts). The reprogramming of such cells can be achieved with the help of transcription factors. In addition, work is being done on biosynthetic or synthetic matrices, which not only serve as substrate material for the transplantation but also support the functional properties of these cells (self-renewal, corneal epithelial-typical phenotype).

Corneal Regeneration Using Adipose-Derived Mesenchymal Stem Cells

Adipose-derived stem cells are a subtype of mesenchymal stem cell that offers the important advantage of being easily obtained (in an autologous manner) from low invasive procedures, rendering a high number of multipotent stem cells with the potential to differentiate into several cellular lineages, to show immunomodulatory properties, and to promote tissue regeneration by a paracrine action through the secretion of extracellular vesicles containing trophic factors. This secretome is currently being investigated as a potential source for a cell-free based regenerative therapy for human tissues, which would significantly reduce the involved costs, risks and law regulations, allowing for a broader application in real clinical practice. In the current article, we will review the existing preclinical and human clinical evidence regarding the use of such adipose-derived mesenchymal stem cells for the regeneration of the three main layers of the human cornea: the epithelium (derived from the surface ectoderm), the stroma (derived from the neural crest mesenchyme), and the endothelium (derived from the neural crest cells).

Factors influencing mesenchymal stromal cells in in vitro cellular models to study retinal pigment epithelial cell rescue

Mesenchymal stromal cells (MSC) stop or slow retinal pigment epithelium (RPE) and neuroretina (NR) degeneration by paracrine activity in oxidative stress-induced retinal degenerative diseases. However, it is mandatory to develop adequate in vitro models that allow testing new treatment strategies against oxidative stress before performing in vivo studies. The viable double- and triple-layered setups are composed of separate layers of NR, MSC, and RPE (NR-MSC-RPE, NR-RPE, MSC-RPE) partially mimic in vivo retinal conditions. In this study, the paracrine neuroprotective effect of each setup's microenvironment on hydrogen peroxide (H₂O₂)-stressed was compared with unstressed RPE cells. RPE cell proliferation viability was assessed on day 1, 3, and 6 using Alamar Blue® (10%), MTT (10%) and a cell viability/cytotoxicity assay kit followed by data analysis. The results showed that RPE cells, highly viable (> 90%) in mixed medium of DMEM and neurobasal A (1:1), lost 50% viability on exposure to 400 µM of H₂O₂ (P < 0.05). The unexposed groups differed significantly from exposed groups for RPE cell growth (RPE and [Formula: see text]RPE (P < 0.0001), NR-MSC-RPE, and NR-MSC-[Formula: see text]RPE (P < 0.05), NR-RPE and NR-[Formula: see text]RPE (P < 0.01), and MSC-RPE and MSC-[Formula: see text]RPE (P < 0.01). NR-[Formula: see text]RPE and NR-RPE supported RPE cell proliferation viability better than other setups (P < 0.01) and RPE cells proliferated 0.49-fold more in NR-MSC-[Formula: see text]RPE than NR-MSC-RPE. Thus, NR and MSC presence improved significantly each setup's microenvironment for cell rescue, nevertheless, each setup also showed limitations for its use as an in vitro study tool. Health of microenvironment of such setups depends on many factors including cell-secreted trophic factors.

Limbal Niche Cells and Three-Dimensional Matrigel-Induced Dedifferentiation of Mature Corneal Epithelial Cells

Purpose

To investigate the phenotypic changes of mature corneal epithelial cells (MCECs) that cocultured with limbal niche cells (LNCs) in three-dimensional Matrigel (3D Matrigel) in vitro.

Methods

MCECs were isolated from central corneas, and limbal epithelial progenitor cells (LEPCs) were isolated from limbal segments with Dispase II. LNCs were isolated and cultured from limbal niche using the collagenase A digestion method and identified with PCK/VIM/CD90/CD105/SCF/PDGFRβ. MCECs were cultured on 3D Matrigel (50%, v/v) with or without LNCs for 10 days. Expression of CK12 and p63α and clone formation test were used to compare the progenitor phenotypic changes for MCECs before and after induction using LEPCs as control.

Results

Homogeneous LNCs were isolated and identified as spindle shape and adherent to a plastic surface coated with 5% Matrigel. Double immunostaining of the fourth-passage LNCs was uniformly PCK⁻/VIM⁺/CD90⁺/CD105⁺/SCF⁺/PDGFRβ⁺. Reverse transcription and quantitative real-time polymerase chain reaction (RT-qPCR) revealed the decrease of PCK expression from the second passage and elevation of Vim, CD90, CD105, SCF, and PDGFRβ transcripts from the third passage, and the transcription level of Vim, CD90, CD105, SCF, and PDGFRβ was elevated statistically in the fourth passage compared to the first passage (P < 0.01). Both immunofluorescence (IF) staining for cross section and cytospin cells demonstrated that MCECs expressed higher CK12 while lower p63α than LEPCs (P < 0.01). Sphere growth formation was noticed as early as 24 hours in the MCEC + LNC group, 48 hours in the LEPC group, and 72 hours in the MCEC group. The diameters of the spheres were the biggest in the MCEC + LNC group (182.24 ± 57.91 µm), smaller in the LEPC group (125.71 ± 41.20 µm), and smallest in the MCEC group (109.39 ± 34.85 µm) by the end of the 10-day culture (P < 0.01). Double immunostaining with CK12/p63α showed that cells in the sphere formed from MCECs expressed CK12 but not p63α; in contrast, some cells in the MCEC + LNC group expressed CK12, but most of them expressed p63α. RT-qPCR revealed a significant reduction of CK12 transcript but elevation of p63α, Oct4, Nanog, Sox2, and SSEA4 (P < 0.05). Holoclone composed of cubic epithelial cells could be generated in the MCEC + LNC group but not in the other two groups.

Conclusions

The data shows that human MCEC cell phenotype could be induced to the dedifferentiation stage when cocultured with LNCs in 3D Matrigel that simulated the microenvironment of limbal stem cells in vitro.

Therapeutic Potential of Microvesicles in Cell Therapy and Regenerative Medicine of Ocular Diseases With an Especial Focus on Mesenchymal Stem Cells-Derived Microvesicles

These days, mesenchymal stem cells (MSCs), because of immunomodulatory and pro-angiogenic abilities, are known as inevitable factors in regenerative medicine and cell therapy in different diseases such as ocular disorder. Moreover, researchers have indicated that exosome possess an essential potential in the therapeutic application of ocular disease. MSC-derived exosome (MSC-DE) have been identified as efficient as MSCs for treatment of eye injuries due to their small size and rapid diffusion all over the eye. MSC-DEs easily transfer their ingredients such as miRNAs, proteins, and cytokines to the inner layer in the eye and increase the reconstruction of the injured area. Furthermore, MSC-DEs deliver their immunomodulatory cargos in inflamed sites and inhibit immune cell migration, resulting in improvement of autoimmune uveitis. Interestingly, therapeutic effects were shown only in animal models that received MSC-DE. In this review, we summarized the therapeutic potential of MSCs and MSC-DE in cell therapy and regenerative medicine of ocular diseases.

OUP accepted manuscript

The corneal epithelium serves to protect the underlying cornea from the external environment and is essential for corneal transparency and optimal visual function. Regeneration of this epithelium is dependent on a population of stem cells residing in the basal layer of the limbus, the junction between the cornea and the sclera. The limbus provides the limbal epithelial stem cells (LESCs) with an optimal microenvironment, the limbal niche, which strictly regulates their proliferation and differentiation. Disturbances to the LESCs and/or their niche can lead to the pathologic condition known as limbal stem cell deficiency (LSCD) whereby the corneal epithelium is not generated effectively. This has deleterious effects on the corneal and visual function, due to impaired healing and secondary corneal opacification. In this concise review, we summarize the characteristics of LESCs and their niche, and present the current and future perspectives in the management of LSCD with an emphasis on restoring the function of the limbal niche.

Generation of a Biomimetic Substitute of the Corneal Limbus Using Decellularized Scaffolds

Patients with severe limbal damage and limbal stem cell deficiency are a therapeutic challenge. We evaluated four decellularization protocols applied to the full-thickness and half-thickness porcine limbus, and we used two cell types to recellularize the decellularized limbi. The results demonstrated that all protocols achieved efficient decellularization. However, the method that best preserved the transparency and composition of the limbus extracellular matrix was the use of 0.1% SDS applied to the half-thickness limbus. Recellularization with the limbal epithelial cell line SIRC and human adipose-derived mesenchymal stem cells (hADSCs) was able to generate a stratified epithelium able to express the limbal markers p63, pancytokeratin, and crystallin Z from day 7 in the case of SIRC and after 14-21 days of induction when hADSCs were used. Laminin and collagen IV expression was detected at the basal lamina of both cell types at days 14 and 21 of follow-up. Compared with control native limbi, tissues recellularized with SIRC showed adequate picrosirius red and alcian blue staining intensity, whereas limbi containing hADSCs showed normal collagen staining intensity. These preliminary results suggested that the limbal substitutes generated in this work share important similarities with the native limbus and could be potentially useful in the future.

Corneal Epithelial Stem Cells-Physiology, Pathophysiology and Therapeutic Options

In the human cornea, regeneration of the epithelium is regulated by the stem cell reservoir of the limbus, which is the marginal region of the cornea representing the anatomical and functional border between the corneal and conjunctival epithelium. In support of this concept, extensive limbal damage, e.g., by chemical or thermal injury, inflammation, or surgery, may induce limbal stem cell deficiency (LSCD) leading to vascularization and opacification of the cornea and eventually vision loss. These acquired forms of limbal stem cell deficiency may occur uni- or bilaterally, which is important for the choice of treatment. Moreover, a variety of inherited diseases, such as congenital aniridia or dyskeratosis congenita, are characterized by LSCD typically occurring bilaterally. Several techniques of autologous and allogenic stem cell transplantation have been established. The limbus can be restored by transplantation of whole limbal grafts, small limbal biopsies or by ex vivo-expanded limbal cells. In this review, the physiology of the corneal epithelium, the pathophysiology of LSCD, and the therapeutic options will be presented.

Derivation and Characterization of EGFP-Labeled Rabbit Limbal Mesenchymal Stem Cells and Their Potential for Research in Regenerative Ophthalmology

The development of cell-based approaches to the treatment of various cornea pathologies, including limbal stem cell deficiency (LSCD), is an area of current interest in regenerative biomedicine. In this context, the shortage of donor material is urgent, and limbal mesenchymal stem cells (L-MSCs) may become a promising cell source for the development of these novel approaches, being established mainly within the rabbit model. In this study, we obtained and characterized rabbit L-MSCs and modified them with lentiviral transduction to express the green fluorescent protein EGFP (L-MSCs-EGFP). L-MSCs and L-MSCs-EGFP express not only stem cell markers specific for mesenchymal stem cells but also ABCG2, ABCB5, ALDH3A1, PAX6, and p63a specific for limbal epithelial stem cells (LESCs), as well as various cytokeratins (3/12, 15, 19). L-MSCs-EGFP have been proven to differentiate into adipogenic, osteogenic, and chondrogenic directions, as well as to transdifferentiate into epithelial cells. The possibility of using L-MSCs-EGFP to study the biocompatibility of various scaffolds developed to treat corneal pathologies was demonstrated. L-MSCs-EGFP may become a useful tool for studying regenerative processes occurring during the treatment of various corneal pathologies, including LSCD, with the use of cell-based technologies.

Application of mesenchymal stem cells in corneal regeneration

Due to delicate its structure, the cornea is susceptible to physical, chemical, and genetic damages. Corneal transplantation is the main treatment for serious corneal damage, but it faces significant challenges, including donor shortages and severe complications. In recent years, cell therapy is suggested as a novel alternative method for corneal regeneration. Regarding the unique characteristics of Mesenchymal stem cells including the potential to differentiate into discrete cell types, secretion of growth factors, mobilization potency, and availability from different sources; special attention has been paid to these cells in corneal engineering. Differentiation of MSCs into specialized corneal cells such as keratocytes, epithelial and endothelial cells is reported. Potential for Treatment of keratitis, reducing inflammation, and inhibition of neovascularization by MSCs, introducing them as novel agents for corneal repairing. In this review, various types of MSCs used to treat corneal injuries as well as their potential for restoring different corneal layers was investigated.

Priming human adipose-derived mesenchymal stem cells for corneal surface regeneration

Limbal stem cells (LSC) maintain the transparency of the corneal epithelium. Chemical burns lead the loss of LSC inducing an up-regulation of pro-inflammatory and pro-angiogenic factors, triggering corneal neovascularization and blindness. Adipose tissue-derived mesenchymal stem cells (AT-MSC) have shown promise in animal models to treat LSC deficiency (LSCD), but there are not studies showing their efficacy when primed with different media before transplantation. We cultured AT-MSC with standard medium and media used to culture LSC for clinical application. We demonstrated that different media changed the AT-MSC paracrine secretion showing different paracrine effector functions in an in vivo model of chemical burn and in response to a novel in vitro model of corneal inflammation by alkali induction. Treatment of LSCD with AT-MSC changed the angiogenic and inflammatory cytokine profile of mice corneas. AT-MSC cultured with the medium that improved their cytokine secretion, enhanced the anti-angiogenic and anti-inflammatory profile of the treated corneas. Those corneas also presented better outcome in terms of corneal transparency, neovascularization and histologic reconstruction. Priming human AT-MSC with LSC specific medium can potentiate their ability to improve corneal wound healing, decrease neovascularization and inflammation modulating paracrine effector functions in an in vivo optimized rat model of LSCD.

Extracellular Vesicles from Human Adipose-Derived Mesenchymal Stem Cells: A Review of Common Cargos

In recent years, the interest in adipose tissue mesenchymal cell–derived extracellular vesicles (AT-MSC-EVs) has increasingly grown. Numerous articles support the potential of human AT-MSC-EVs as a new therapeutic option for treatment of diverse diseases in the musculoskeletal and cardiovascular systems, kidney, skin, and immune system, among others. This approach makes use of the molecules transported inside of EVs, which play an important role in cell communication and in transmission of macromolecules. However, to our knowledge, there is no database where essential information about AT-MSC-EVs cargo molecules is gathered for easy reference. The aim of this study is to describe the different molecules reported so far in AT-MSC- EVs, their main molecular functions, and biological processes in which they are involved. Recently, the presence of 591 proteins and 604 microRNAs (miRNAs) has been described in human AT-MSC-EVs. The main molecular function enabled by both proteins and miRNAs present in human AT-MSC-EVs is the binding function. Signal transduction and gene silencing are the biological processes in which a greater number of proteins and miRNAs from human AT-MSC-EVs are involved, respectively. In this review we highlight the therapeutics effects of AT-MSC-EVs related with their participation in relevant biological processes including inflammation, angiogenesis, cell proliferation, apoptosis and migration, among others.

Evaluation of Clinical and Histological Outcomes of Adipose-Derived Mesenchymal Stem Cells in a Rabbit Corneal Alkali Burn Model

To assess effects of adipose-derived mesenchymal stem cells (AMSCs) in corneal alkali injuries in an experimental animal model. Twenty white New Zealand rabbits were included in the study. The animal models were randomly divided into 2 groups. Rabbits in the AMSC group (n = 10) received an intrastromal, a subconjunctival injection, and topical instillation of 0.5 ml totally of phosphate-buffered saline (PBS) containing 2 × 10⁶ AMSCs. In the control group (n = 10), rabbits received only 0.5 ml of PBS using the same methods. A masked investigator measured the corneal sensation, anterior chamber Inflammation (ACI), and conjunctival congestion. Additionally, a blind histological and immunohistochemical evaluation was made. In the AMSC group, the central corneal sensation was increased whereas ACI and conjunctival congestion were reduced compared to the control group in the 28 days of follow-up (p < 0.05). A statistically significant difference (p < 0.05) was noted between the two groups as recorded in the above parameters. Histological analysis showed that pathological vascularization was markedly reduced in the AMSC group which was consistent with the absence of factor VIII in the immunohistochemistry sections. There is a trend towards improved clinical outcomes including corneal sensation as well as acceleration in the restoration of normal corneal architecture in corneal alkali burns treated with AMSCs, results that support the need for further research in the field.

Preliminary studies of constructing a tissue-engineered lamellar corneal graft by culturing mesenchymal stem cells onto decellularized corneal matrix

AIM

To construct a competent corneal lamellar substitute in order to alleviate the shortage of human corneal donor.

METHODS

Rabbit mesenchymal stem cells (MSCs) were isolated from bone marrow and identified by flow cytometric, osteogenic and adipogenic induction. Xenogenic decellularized corneal matrix (XDCM) was generated from dog corneas. MSCs were seeded and cultured on XDCM to construct the tissue-engineered cornea. Post-transplantation biocompatibility of engineered corneal graft were tested by animal experiment. Rabbits were divided into two groups then underwent lamellar keratoplasty (LK) with different corneal grafts: 1) XDCM group (n=5): XDCM; 2) XDCM-MSCs groups (n=4): tissue-engineered cornea made up with XDCM and MSCs. The ocular surface recovery procedure was observed while corneal transparency, neovascularization and epithelium defection were measured and compared. In vivo on focal exam was performed 3mo postoperatively.

RESULTS

Rabbit MSCs were isolated and identified. Flow cytometry demonstrated isolated cells were CD90 positive and CD34, CD45 negative. Osteogenic and adipogenic induction verified their multipotent abilities. MSC-XDCM grafts were constructed and observed. In vivo transplantation showed the neovascularization in XDCM-MSC group was much less than that in XDCM group postoperatively. Post-transplant 3-month confocal test showed less nerve regeneration and bigger cell-absent area in XDCM-MSC group.

CONCLUSION

This study present a novel corneal tissue-engineered graft that could reduce post-operatively neovascularization and remain transparency, meanwhile shows that co-transplantation of MSCs may help increase corneal transplantation successful rate and enlarge the source range of corneal substitute to overcome cornea donor shortage.

Corneal Repair with Adhesive Cell Sheets of Fetal Cartilage-Derived Stem Cells

BACKGROUND

Corneal scarring or disease may lead to severe corneal opacification and consequently, severe loss of vision due to the complete loss of corneal epithelial cells. We studied the use of epithelial cell sheets differentiated from fetal cartilage-derived stem cells (FCSC) to resurface damaged cornea.

METHODS

The FCSC were isolated from the femoral head of immature cartilage tissue. The ability of the FCSCs to differentiate into corneal epithelial cells was evaluated using differentiation media at 2 days and 7 days post-seeding. A sheet fabricated of FCSCs was also used for the differentiation assay. The results of the in vitro studies were evaluated by immunocytochemistry and Western blots for corneal epithelial cell markers (CK3/12 and Pax6) and limbal epithelial stem cell markers (ABCG2 and p63). To test the material in vivo, an FCSC-sheet was applied as a treatment in a chemically burned rabbit model. The healing ability was observed histologically one week after treatment.

RESULTS

The in vitro experiments showed morphological changes in the FCSCs at two and seven days of culture. The differentiated cells from the FCSCs or the FCSC-sheet expressed corneal epithelial cells markers. FCSC were create cell sheet that successfully differentiated into corneal epithelial cells and had sufficient adhesion so that it could be fused to host tissue after suture to the ocular surface with silk suture. The implanted cell sheet maintained its transparency and the cells were alive a week after implantation.

CONCLUSION

These results suggest that carrier-free sheets fabricated of FCSCs have the potential to repair damaged corneal surfaces.

Stem cell-based therapeutic strategies for corneal epithelium regeneration

Any significant loss of vision or blindness caused by corneal damages is referred to as corneal blindness. Corneal blindness is the fourth most common cause of blindness worldwide, representing more than 5% of the total blind population. Currently, corneal transplantation is used to treat many corneal diseases. In some cases, implantation of artificial cornea (keratoprosthesis) is suggested after a patient has had a donor corneal transplant failure. The shortage of donors and the side effects of keratoprosthesis are limiting these approaches. Recently, researchers have been actively pursuing new approaches for corneal regeneration because of these limitations. Nowadays, tissue engineering of different corneal layers (epithelium, stroma, endothelium, or full thickness tissue) is a promising approach that has attracted a great deal of interest from researchers and focuses on regenerative strategies using different cell sources and biomaterials. Various sources of corneal and non-corneal stem cells have shown significant advantages for corneal epithelium regeneration applications. Pluripotent stem cells (embryonic stem cells and iPS cells), epithelial stem cells (derived from oral mucus, amniotic membrane, epidermis and hair follicle), mesenchymal stem cells (bone marrow, adipose-derived, amniotic membrane, placenta, umbilical cord), and neural crest origin stem cells (dental pulp stem cells) are the most promising sources in this regard. These cells could also be used in combination with natural or synthetic scaffolds to improve the efficacy of the therapeutic approach. As the ocular surface is exposed to external damage, the number of studies on regeneration of the corneal epithelium is rising. In this paper, we reviewed the stem cell-based strategies for corneal epithelium regeneration.

Corneal epithelium tissue engineering: recent advances in regeneration and replacement of corneal surface

Currently, many corneal diseases are treated by corneal transplantation, artificial corneal implantation or, in severe cases, keratoprosthesis. Owing to the shortage of cornea donors and the risks involved with artificial corneal implants, such as infection transmission, researchers continually seek new approaches for corneal regeneration. Corneal tissue engineering is a promising approach that has attracted much attention from researchers and is focused on regenerative strategies using various biomaterials in combination with different cell types. These constructs should have the ability to mimic the native tissue microenvironment and present suitable optical, mechanical and biological properties. In this article, we review studies that have focused on the current clinical techniques for corneal replacement. We also describe tissue-engineering and cell-based approaches for corneal regeneration.

Differentiation Induction of Human Stem Cells for Corneal Epithelial Regeneration

Deficiency of corneal epithelium causes vision impairment or blindness in severe cases. Transplantation of corneal epithelial cells is an effective treatment but the availability of the tissue source for those cells is inadequate. Stem cells can be induced to differentiate to corneal epithelial cells and used in the treatment. Multipotent stem cells (mesenchymal stem cells) and pluripotent stem cells (embryonic stem cells and induced pluripotent stem cells) are promising cells to address the problem. Various protocols have been developed to induce differentiation of the stem cells into corneal epithelial cells. The feasibility and efficacy of both human stem cells and animal stem cells have been investigated for corneal epithelium regeneration. However, some physiological aspects of animal stem cells are different from those of human stem cells, the protocols suited for animal stem cells might not be suitable for human stem cells. Therefore, in this review, only the investigations of corneal epithelial differentiation of human stem cells are taken into account. The available protocols for inducing the differentiation of human stem cells into corneal epithelial cells are gathered and compared. Also, the pathways involving in the differentiation are provided to elucidate the relevant mechanisms.

The Immunomodulatory Potential of Mesenchymal Stem Cells in a Retinal Inflammatory Environment

Retinal degenerative disorders are characterized by a local upregulation of inflammatory factors, infiltration with cells of the immune system, a vascular dysfunction and by the damage of retinal cells. There is still a lack of treatment protocols for these diseases. Mesenchymal stem cell (MSC)-based therapy using immunoregulatory, regenerative and differentiating properties of MSCs offers a promising treatment option. In this study, we analyzed the immunomodulatory properties of mouse bone marrow-derived MSCs after their intravitreal delivery to the inflammatory environment in the eye, caused by the application of pro-inflammatory cytokines IL-1β, TNF-α and IFN-γ. The intravitreal administration of these cytokines induces an increased expression of pro-inflammatory molecules such as IL-1α, IL-6, inducible nitric oxide synthase, TNF-α and vascular endothelial growth factor in the retina. However, a significant decrease in the expression of genes for all these pro-inflammatory molecules was observed after the intravitreal injection of MSCs. We further showed that an increased infiltration of the retina with immune cells, mainly with macrophages, which was observed after pro-inflammatory cytokine application, was significantly reduced after the intravitreal application of MSCs. The similar immunosuppressive effects of MSCs were also demonstrated in vitro in cultures of cytokine-stimulated retinal explants and MSCs. Overall, the results show that intravitreal application of MSCs inhibits the early retinal inflammation caused by pro-inflammatory cytokines, and propose MSCs as a promising candidate for stem cell-based therapy of retinal degenerative diseases.

In vitro potential of human mesenchymal stem cells for corneal epithelial regeneration

Aim: To determine the potential of mesenchymal stem cells (MSC) for corneal epithelial regeneration in vitro. Materials & methods: Bone marrow MSC (BM-MSC) and adipose tissue MSC were analyzed for corneal epithelial and mesenchymal markers, using limbal stem cells and corneal cells as controls. MSC with better potential were cultured with specific mediums for epithelial induction. Transepithelial electric resistance and wound healing assay with human corneal epithelial cells were performed. Results: BM-MSC showed better potential, increased corneal markers, and higher transepithelial electric resistance values when induced with limbal epithelial culture medium. Induced BM-MSC promoted better wound healing of human corneal epithelial cells by paracrine secretion. Conclusion: BM-MSC has potential for corneal epithelial induction in a protocol compatible with human application.

Cellular therapy of corneal epithelial defect by adipose mesenchymal stem cell-derived epithelial progenitors

Background

Persistent epithelial defects (PED), associated with limbal stem cell deficiency (LSCD), require ocular surface reconstruction with a stable corneal epithelium (CE). This study investigated CE reformation using human adipose mesenchymal stem cells (ADSC), which derived epithelial progenitors via mesenchymal-epithelial transition (MET).

Methods

STEMPRO human ADSC were cultured with specific inhibitors antagonizing glycogen synthase kinase-3 and transforming growth factor-β signaling, followed by culture under a defined progenitor cell targeted-epithelial differentiation condition to generate epithelial-like cells (MET-Epi), which were characterized for cell viability, mesenchymal, and epithelial phenotypes using immunofluorescence and flow cytometry. Tissue-engineered (TE) MET-Epi cells on fibrin gel were transplanted to corneal surface of the rat LSCD model caused by alkali injury. Epithelial healing, corneal edema, and haze grading, CE formation were assessed by fluorescein staining, slit lamp bio-microscopy, anterior segment optical coherence tomography, and immunohistochemistry.

Results

CD73^high/CD90^high/CD105^high/CD166^high/CD14^negative/CD31^negative human ADSC underwent MET, giving viable epithelial-like progenitors expressing δNp63, CDH1 (E-cadherin), epidermal growth factor receptor, integrin-β4, and cytokeratin (CK)-5, 9. Under defined epithelial differentiation culture, these progenitors generated MET-Epi cells expressing cell junction proteins ZO1 and occludin. When transplanted onto rat corneal surface with LSCD-induced PED, TE-MET-Epi achieved more efficient epithelial healing, suppressed corneal edema, and opacities, when compared to corneas without treatment or transplanted with TE-ADSC. CE markers (CK3, 12, and CDH1) were expressed on TE-MET-Epi-transplanted corneas but not in other control groups.

Conclusion

Human ADSC-derived epithelial-like cells, via MET, recovered the CE from PED associated with LSCD. ADSC can be a viable adult stem cell source for potential autologous epithelial cell-based therapy for corneal surface disorders.

Adipose-derived stem cells undergo differentiation after co-culture with porcine limbal epithelial stem cells

Mesenchymal stem cells (MSCs) are objects of interest in regenerative medicine. They are used for various therapies such as for the regeneration of bone, chondrocytes and other tissues. Adipose derived stem cells (ADSCs) inter alia are particularly easy to access, they are relatively abundant in fat tissue. ADSCs could be differentiated into many types of cells. To date, it has been proven that ADSCs only differentiate into mesodermal cell lineages. In this study, we present the differentiation of ADSCs into the corneal epithelium. Human ADSCs were placed in a co-culture with porcine limbal epithelial stem cells (LESCs). After 14 days of cultivation, total RNA was extracted for the analysis of the molecular markers (expression of genes of interest). The gene expression was assessed by real-time RT-qPCR. The expression of the surface molecular markers of ADSCs is modulated after co-culturing. We have observed the decrease in CD73, CD90 and CD105 mRNA expression, while the expression of mRNA coding for CK3 and CK12 mRNA was increased in ADSCs co-cultured with porcine limbal epithelial stem cells as compared to the control. We conclude that the co-culture of LESCs and ADSCs changed ADSCs' molecular markers gene expression indicating initiation of differentiation towards limbal cells.

Current Trends and Future Perspective of Mesenchymal Stem Cells and Exosomes in Corneal Diseases

The corneal functions (transparency, refractivity and mechanical strength) deteriorate in many corneal diseases but can be restored after corneal transplantation (penetrating and lamellar keratoplasties). However, the global shortage of transplantable donor corneas remains significant and patients are subject to life-long risk of immune response and graft rejection. Various studies have shown the differentiation of multipotent mesenchymal stem cells (MSCs) into various corneal cell types. With the unique properties of immunomodulation, anti-angiogenesis and anti-inflammation, they offer the advantages in corneal reconstruction. These effects are widely mediated by MSC differentiation and paracrine signaling via exosomes. Besides the cell-free nature of exosomes in circumventing the problems of cell-fate control and tumorigenesis, the vesicle content can be genetically modified for optimal therapeutic affinity. The pharmacology and toxicology, xeno-free processing with sustained delivery, scale-up production in compliant to Good Manufacturing Practice regulations, and cost-effectiveness are the current foci of research. Routes of administration via injection, topical and/or engineered bioscaffolds are also explored for its applicability in treating corneal diseases.

Differentiation of human embryonic stem cells derived mesenchymal stem cells into corneal epithelial cells after being seeded on decellularized SMILE-derived lenticules

AIM

To evaluate the feasibility of mesenchymal stem cells (MSCs) to differentiate into corneal epithelial cells after being seeded on the decellularized small incision lenticule extraction (SMILE)-derived lenticules.

METHODS

The fresh lenticules procured from patients undergoing SMILE for the correction of myopia were decellularized. The MSCs were subsequently cultivated on those denuded lenticules. The MSCs without lenticules were used as a control. The proliferation activity of the MSCs after seeding 24h was quantitatively determined with the Cell Counting Kit-8 (CCK-8) assay. Immunofluorescence staining and quantitative reverse transcription polymerase chain reaction (qRT-PCR) were used to assess the marker expression in differentiated MSCs.

RESULTS

The data showed that both fresh and decellularized lenticules could significantly promote the proliferation of MSCs, compared to that in control (P=0.02 for fresh lenticules, P=0.001 for decellularize ones, respectively). The MSCs seeded on both lenticules were positive for cytokeratin 3 (CK3) staining. The expression of CK3 increased 5-fold in MSCs seeded on fresh lenticules and 18-fold on decellularized ones, compared to that in control. There was a significant difference in the expression of CK3 in MSCs seeded on fresh and decellularized lenticules (P<0.001). The expression of CK8 and CK18 was similar in pure MSCs and MSCs seeded on fresh lenticules (P>0.05), while the expression of these markers was decreased in MSCs seeded on decellularized ones.

CONCLUSION

These results suggest that the decellularized lenticules might be more suitable for MSCs to differentiate into corneal epithelial cells, which offers the prospect of a novel therapeutic modality of SMILE-derived lenticules in regenerative corneal engineering.

A proof-of-concept clinical trial using mesenchymal stem cells for the treatment of corneal epithelial stem cell deficiency

Ocular stem cell transplantation derived from either autologous or allogeneic donor corneoscleral junction is a functional cell therapy to manage extensive and/or severe limbal stem cell deficiencies that lead to corneal epithelial failure. Mesenchymal stem cells have been properly tested in animal models of this ophthalmic pathology, but never in human eyes despite their potential advantages. We conducted a 6- to 12-month proof-of-concept, randomized, and double-masked pilot trial to test whether allogeneic bone marrow-derived mesenchymal stem cell transplantation (MSCT], n = 17) was as safe and as equally efficient as allogeneic cultivated limbal epithelial transplantation (CLET), (n = 11) to improve corneal epithelial damage due to limbal stem cell deficiency. Primary endpoints demanded combination of symptoms, signs, and the objective improvement of the epithelial phenotype in central cornea by in vivo confocal microscopy. This proof-of-concept trial showed that MSCT was as safe and efficacious as CLET. Global success at 6-12 months was 72.7%-77.8% for CLET cases and 76.5%-85.7% for MSCT cases (not significant differences). Central corneal epithelial phenotype improved in 71.4% and 66.7% of MSCT and CLET cases, respectively at 12 months (P = 1.000). There were no adverse events related to cell products. This trial suggests first evidence that MSCT facilitated improvement of a diseased corneal epithelium due to lack of its stem cells as efficiently as CLET. Consequently, not only CLET but also MSCT deserves more preclinical investigational resources before the favorable results of this proof-of-concept trial could be transformed into the larger numbers of the multicenter trials that would provide stronger evidence. (ClinicalTrials.gov number, NCT01562002.).

Hydrogen sulfide: a gaseous signaling molecule modulates tissue homeostasis: implications in ophthalmic diseases

Hydrogen sulfide (H2S) serves as a gasotransmitter in the regulation of organ development and maintenance of homeostasis in tissues. Its abnormal levels are associated with multiple human diseases, such as neurodegenerative disease, myocardial injury, and ophthalmic diseases. Excessive exposure to H2S could lead to cellular toxicity, orchestrate pathological process, and increase the risk of various diseases. Interestingly, under physiological status, H2S plays a critical role in maintaining cellular physiology and limiting damages to tissues. In mammalian species, the generation of H2S is catalyzed by cystathionine beta-synthase (CBS), cystathionine gamma-lyase (CSE), 3-mercapto-methylthio pyruvate aminotransferase (3MST) and cysteine aminotransferase (CAT). These enzymes are found inside the mammalian eyeballs at different locations. Their aberrant expression and the accumulation of substrates and intermediates can change the level of H2S by orders of magnitude, causing abnormal structures or functions in the eyes. Detailed investigations have demonstrated that H2S donors' administration could regulate intraocular pressure, protect retinal cells, inhibit oxidative stress and alleviate inflammation by modulating the function of intra or extracellular proteins in ocular tissues. Thus, several slow-releasing H2S donors have been shown to be promising drugs for treating multiple diseases. In this review, we discuss the biological function of H2S metabolism and its application in ophthalmic diseases.

The therapeutic application of mesenchymal stem cells at the ocular surface

The therapeutic potential of mesenchymal stem cells (MSCs) has been heralded by their multipotentiality and immunomodulatory capacity. MSCs migrate toward sites of tissue damage, where specific pro-inflammatory factors 'license' their immunosuppressive functions. Recent studies in animal models of ocular surface disease have demonstrated the potential of MSC-derived therapies to limit inflammation and promote tissue repair. Herein, we review the immunoregulatory mechanisms of MSCs, as well as strategies to harness their regenerative function at the cornea. We examine reports of the therapeutic application of MSCs in the setting of ocular surface inflammation; including corneal injury, transplantation, ocular surface autoimmunity and allergy.

From DMEK to Corneal Endothelial Cell Therapy: Technical and Biological Aspects

The main treatment available for restoration of the corneal endothelium is keratoplasty and DMEK provides faster visual recovery and better postoperative visual acuity when compared to DSAEK. However, the technical challenges related to this technique and the steep technical learning curve seem to prevent the overcoming of DSAEK in favor of DMEK. Furthermore, the outcome of lamellar keratoplasty techniques is influenced by problems related to corneal grafting tissue availability, management, and quality. On the other hand, improvements in the field of cell engineering have opened the way for the use of stem cells-derived corneal endothelial cells with regenerative intent. In this overview, latest findings in endothelial cell engineering are reported, and perspectives of clinical application of mesenchymal stem cells for corneal endothelial replacement and regeneration are evaluated.

Pluripotent Stem Cells and Other Innovative Strategies for the Treatment of Ocular Surface Diseases

The cornea provides two thirds of the refractive power of the eye and protection against insults such as infection and injury. The outermost tissue of the cornea is renewed by stem cells located in the limbus. Depletion or destruction of these stem cells may lead to blinding limbal stem cell deficiency (LSCD) that concerns millions of patients around the world. Innovative strategies based on adult stem cell therapies have been developed in the recent years but they are still facing numerous unresolved issues, and the long term results can be deceiving. Today there is a clear need to improve these therapies, and/or to develop new approaches for the treatment of LSCD. Here, we review the current cell-based therapies used for the treatment of ocular diseases, and discuss the potential of pluripotent stem cells (embryonic and induced pluripotent stem cells) in corneal repair. As the secretion of paracrine factors is known to have a crucial role in maintaining stem cell homeostasis and in wound repair, we also consider the therapeutic potential of a promising novel pathway, the exosomes. Exosomes are nano-sized vesicles that have the ability to transfer RNAs and proteins to recipient cells, and several studies demonstrated their role in cell protection and wound healing. Exosomes could circumvent the hurdles of stem-cell based approaches, and they could become a strong candidate as an alternative therapy for ocular surface diseases.

The Identification of Interferon-γ as a Key Supportive Factor for Retinal Differentiation of Murine Mesenchymal Stem Cells

Retinal disorders represent the main cause of decreased quality of vision and even blindness worldwide. The loss of retinal cells causes irreversible damage of the retina, and there are currently no effective treatment protocols for most retinal degenerative diseases. A promising approach for the treatment of retinal disorders is represented by stem cell-based therapy. The perspective candidates are mesenchymal stem cells (MSCs), which can differentiate into multiple cell types and produce a number of trophic and growth factors. In this study, we show the potential of murine bone marrow-derived MSCs to differentiate into cells expressing retinal markers and we identify the key supportive role of interferon-γ (IFN-γ) in the differentiation process. MSCs were cultured for 7 days with retinal extract and supernatant from T-cell mitogen concanavalin A-stimulated splenocytes, simulating the inflammatory site of retinal damage. MSCs cultured in such conditions differentiated to the cells expressing retinal cell markers such as rhodopsin, S antigen, retinaldehyde-binding protein, calbindin 2, recoverin, and retinal pigment epithelium 65. To identify a supportive molecule in the supernatants from activated spleen cells, MSCs were cultured with retinal extract in the presence of various T-cell cytokines. The expression of retinal markers was enhanced only in the presence of IFN-γ, and the supportive role of spleen cell supernatants was abrogated with the neutralization antibody anti-IFN-γ. In addition, differentiated MSCs were able to express a number of neurotrophic factors, which are important for retinal regeneration. Taken together, the results show that MSCs can differentiate into cells expressing retinal markers and that this differentiation process is supported by IFN-γ.

Inhibiting glycogen synthase kinase-3 and transforming growth factor-β signaling to promote epithelial transition of human adipose mesenchymal stem cells

BACKGROUND

This study was aimed to investigate the epithelial differentiation of human adipose-derived mesenchymal stem cells (ADSCs) by inhibiting glycogen synthase kinase-3 (GSK3) and transforming growth factor β (TGFβ) signaling.

METHODS AND RESULTS

STEMPRO human ADSCs at passage 2 were treated with CHIR99021 (GSK3 inhibitor), E-616452 (TGFβ1 receptor kinase inhibitor), A-83-01 (TGFβ type 1 receptor inhibitor), valproic acid (histone deacetylase inhibitor), tranylcypromine (monoamine oxidase inhibitor) and all-trans retinoic acid for 72 h. The mesenchymal-epithelial transition was shown by down-regulation of mesenchymal genes (Slug, Zinc Finger E-box Binding Homeobox 1 ZEB1, integrin α5 ITGA5 and vimentin VIM) and up-regulation of epithelial genes (E-cadherin, Epithelial Cell Adhesion Molecule EpCAM, Zonula Occludens-1 ZO-1, occludin, deltaN p63 δNp63, Transcription Factor 4 TCF4 and Twist Family bHLH Transcription Factor TWIST), compared to untreated ADSCs. Cell morphology and stress fiber pattern were examined and the treated cells became less migratory in scratch wound closure assay. The formation of cell junction complexes was observed under transmission electron microscopy. Global gene expression using GeneChip^® Human Genome U133 Array (Affymetrix) showed that the treatment up-regulated 540 genes (containing genes for cell cycle, cytoskeleton reorganization, chemotaxis, epithelium development and regulation of cell migration) and down-regulated 483 genes.

CONCLUSION

Human ADSCs were transited to epithelial lineage by inhibiting GSK3 and TGFβ signaling. It can be an adult stem cell source for epithelial cell-based therapy.

[New biomaterials and alternative stem cell sources for the reconstruction of the limbal stem cell niche]

Reconstruction of the limbal stem cell niche in patients with limbal stem cell insufficiency remains one of the most challenging tasks in the treatment of ocular surface diseases. Ex vivo expansion of limbal stem cells still has potential for optimization despite positive reports in centers worldwide. New biomaterials as well as alternative cell sources for the reconstruction of the limbal stem cell niche have been published in recent years. The aim of this review is to provide insight into new biomaterials and cell sources which may find their way into clinical routine in the upcoming decades.

Mesenchymal Stem and Progenitor Cells in Regeneration: Tissue Specificity and Regenerative Potential

It has always been an ambitious goal in medicine to repair or replace morbid tissues for regaining the organ functionality. This challenge has recently gained momentum through considerable progress in understanding the biological concept of the regenerative potential of stem cells. Routine therapeutic procedures are about to shift towards the use of biological and molecular armamentarium. The potential use of embryonic stem cells and invention of induced pluripotent stem cells raised hope for clinical regenerative purposes; however, the use of these interventions for regenerative therapy showed its dark side, as many health concerns and ethical issues arose in terms of using these cells in clinical applications. In this regard, adult stem cells climbed up to the top list of regenerative tools and mesenchymal stem cells (MSC) showed promise for regenerative cell therapy with a rather limited level of risk. MSC have been successfully isolated from various human tissues and they have been shown to offer the possibility to establish novel therapeutic interventions for a variety of hard-to-noncurable diseases. There have been many elegant studies investigating the impact of MSC in regenerative medicine. This review provides compact information on the role of stem cells, in particular, MSC in regeneration.

In vitro transdifferentiation of human skin keratinocytes to corneal epithelial cells

BACKGROUND AIMS

Skin keratinocytes (SKs) share the same surface ectodermal origin as that of corneal epithelium. In this study, the plasticity of epidermal keratinocytes was exploited to generate corneal epithelial-like cells, which might serve as an alternative source of autologous tissue for the treatment of bilateral limbal stem cell deficiency.

METHODS

Skin samples were subjected to collagenase digestion to isolate SKs and transdifferentiated to corneal epithelial-like cells using limbal fibroblast conditioned medium (LFCM). SKs and transdifferentiated corneal epithelial cells (TDCECs) were characterized using immunofluorescence and fluorescence-activated cell sorting. The propensity for expression of angiogenic genes in TDCECs was compared with cultured oral mucosal epithelial cells (COMEC) in vitro. RT(2) quantitative polymerase chain reaction profiler array was performed to study the signaling pathways involved in the transdifferentiation process.

RESULTS

The TDCECs obtained from SKs showed corneal epithelial-like morphology and expressed corneal epithelial markers, CK3 and CK12. Hematoxylin-eosin and immunohistochemistry showed stratified layers of TDCECs expressing CK 3/12, confirming the corneal epithelial phenotype. We found that the expression of several angiogenic and epithelial mesenchymal transition factors were down-regulated in TDCECs compared with COMEC, suggesting a lower capacity to induce angiogenesis in TDCECs. There was considerable difference in the signaling mechanisms between TDCECs and SKs on testing by RT(2) profiler array, signifying differences at the global gene profile. The comparison of TDCECs and limbal derived corneal epithelial cells showed similar gene expression.

DISCUSSION

Our study shows that SKs have the potential to transdifferentiate into corneal epithelial-like cells using LFCM.

Limbal Stem Cells from Aged Donors Are a Suitable Source for Clinical Application

Limbal stem cells (LSC) are the progenitor cells that maintain the transparency of the cornea. Limbal stem cell deficiency (LSCD) leads to corneal opacity, inflammation, scarring, and blindness. A clinical approach to treat this condition consists in LSC transplantation (LSCT) after ex vivo expansion of LSC. In unilateral LSCD, an autologous transplant is possible, but cases of bilateral LSCD require allogenic LSCT. Cadaveric donors represent the most important source of LSC allografts for treatment of bilateral LSCD when living relative donors are not available. To evaluate the suitability of aged cadaveric donors for LSCT, we compared three pools of LSC from donors of different ages (<60 years, 60–75 years, and >75 years). We evaluated graft quality in terms of percent of p63-positive (p63+) cells by immunofluorescence, colony forming efficiency, and mRNA and protein expression of p63, PAX6, Wnt7a, E-cadherin, and cytokeratin (CK) 12, CK3, and CK19. The results showed that LSC cultures from aged donors can express ≥3% of p63+ cells—considered as the minimum value for predicting favorable clinical outcomes after LSCT—suggesting that these cells could be a suitable source of LSC for transplantation. Our results also indicate the need to evaluate LSC graft quality criteria for each donor.