Corneal stem cells and tissue engineering: Current advances and future perspectives

The landscape of clinical trials in corneal regeneration: A systematic review of tissue engineering approaches in corneal disease

The limited availability of a healthy donor cornea and the incidence of allograft failure led researchers to seek other corneal substitutes via tissue engineering. Exploring the trend of clinical trials of the cornea with the vision of tissue engineering provides an opportunity to reveal future potential corneal substitutes. The results of this clinical trial are beneficial for future study designs to overcome the limitations of current therapeutic approaches. In this study, registered clinical trials of bio-based approaches were reviewed for corneal regeneration on March 22, 2024. Among the 3955 registered trials for the cornea, 392 trials were included in this study, which categorized in three main bio-based scaffolds, stem cells, and bioactive macromolecules. In addition to the acellular cornea and human amniotic membrane, several bio-based materials have been introduced as corneal substrates such as collagen, fibrin, and agarose. However, some synthetic materials have been introduced in recent studies to improve the desired properties of bio-based scaffolds for corneal substitutes. Nevertheless, new insights into corneal regenerative medicine have recently emerged from cell sheets with autologous and allogeneic cell sources. In addition, the future perspective of corneal regeneration is described through a literature review of recent experimental models.

Extracellular Vesicles Isolated from Equine Adipose-Derived Stromal Stem Cells (ASCs) Mitigate Tunicamycin-Induced ER Stress in Equine Corneal Stromal Stem Cells (CSSCs)

Corneal ulcers, characterized by severe inflammation of the cornea, can lead to serious, debilitating complications and may be vision-threatening for horses. In this study, we aimed to investigate the role of endoplasmic reticulum (ER) stress in corneal stem progenitor cell (CSSC) dysfunction and explore the potential of equine adipose-derived stromal stem cell (ASC)-derived extracellular vesicles (EVs) to improve corneal wound healing. We showed that CSSCs expressed high levels of CD44, CD45, and CD90 surface markers, indicating their stemness. Supplementation of the ER-stress-inducer tunicamycin to CSSCs resulted in reduced proliferative and migratory potential, accumulation of endoplasmic reticulum (ER)-stressed cells in the G0/G1 phase of the cell cycle, increased expression of proinflammatory genes, induced oxidative stress and sustained ER stress, and unfolded protein response (UPR). Importantly, treatment with EVs increased the proliferative activity and number of cells in the G2/Mitosis phase, enhanced migratory ability, suppressed the overexpression of proinflammatory cytokines, and upregulated the anti-inflammatory miRNA-146a-5p, compared to control and/or ER-stressed cells. Additionally, EVs lowered the expression of ER-stress master regulators and effectors (PERK, IRE1, ATF6, and XBP1), increased the number of mitochondria, and reduced the expression of Fis-1 and Parkin, thereby promoting metabolic homeostasis and protecting against apoptosis in equine CSSCs. Our findings demonstrate that MSCs-derived EVs represent an innovative and promising therapeutic strategy for the transfer of bioactive mediators which regulate various cellular and molecular signaling pathways.

Induced Pluripotent Stem Cell-Derived Corneal Cells: Current Status and Application

Deficiency and dysfunction of corneal cells leads to the blindness observed in corneal diseases such as limbal stem cell deficiency (LSCD) and bullous keratopathy. Regenerative cell therapies and engineered corneal tissue are promising treatments for these diseases [1]. However, these treatments are not yet clinically feasible due to inadequate cell sources. The discovery of induced pluripotent stem cells (iPSCs) by Shinya Yamanaka has provided a multitude of opportunities in research because iPSCs can be generated from somatic cells, thus providing an autologous and unlimited source for corneal cells. Compared to other stem cell sources such as mesenchymal and embryonic, iPSCs have advantages in differentiation potential and ethical concerns, respectively. Efforts have been made to use iPSCs to model corneal disorders and diseases, drug testing [2], and regenerative medicine [1]. Autologous treatments based on iPSCs can be exorbitantly expensive and time-consuming, but development of stem cell banks with human leukocyte antigen (HLA)- homozygous cell lines can provide cost- and time-efficient allogeneic alternatives. In this review, we discuss the early development of the cornea because protocols differentiating iPSCs toward corneal lineages rely heavily upon recapitulating this development. Differentiation of iPSCs toward corneal cell phenotypes have been analyzed with an emphasis on feeder-free, xeno-free, and well-defined protocols, which have clinical relevance. The application, challenges, and potential of iPSCs in corneal research are also discussed with a focus on hurdles that prevent clinical translation.

Polysaccharides, as biological macromolecule-based scaffolding biomaterials in cornea tissue engineering: A review

Corneal-related diseases and injuries are the leading causes of vision loss, estimated to affect over 10 million people worldwide. Currently, cadaveric corneal grafts are considered the gold standard of treatment to restore cornea-related vision. However, this treatment modality faces different challenges such as donor shortage and graft failure. Therefore, the need for alternative solutions continues to grow. Tissue engineering has dramatically progressed to produce artificial cornea implants in order to repair, regenerate, or replace the damaged cornea. In this regard, a variety of polysaccharides such as cellulose, chitosan, alginate, agarose, and hyaluronic acid have been widely explored as scaffolding biomaterials for the production of tissue-engineered cornea. These polymers are known for their excellent biocompatibility, versatile properties, and processability. Recent progress and future perspectives of polysaccharide-based biomaterials in cornea tissue engineering is reviewed here.

Regenerative therapy for the Cornea

The cornea is the outmost layer of the eye, unique in its transparency and strength. The cornea not only transmits the light essential for vision, also refracts light, giving focus to images. Each of the three layers of the cornea has properties essential for the function of vision. Although the epithelium can often recover from injury quickly by cell division, loss of limbal stem cells can cause severe corneal surface abnormalities leading to corneal blindness. Disruption of the stromal extracellular matrix and loss of cells determining this structure, the keratocytes, leads to corneal opacity. Corneal endothelium is the inner part of the cornea without self-renewal capacity. It is very important to maintain corneal dehydration and transparency. Permanent damage to the corneal stroma or endothelium can be effectively treated by corneal transplantation; however, there are drawbacks to this procedure, including a shortage of donors, the need for continuing treatment to prevent rejection, and limits to the survival of the graft, averaging 10-20 years. There exists a need for new strategies to promote regeneration of the stromal structure and restore vision. This review highlights critical contributions in regenerative medicine with the aim of corneal reconstruction after injury or disease. These approaches include corneal stromal stem cells, corneal limbal stem cells, embryonic stem cells, and other adult stem cells, as well as induced pluripotent stem cells. Stem cell-derived trophic factors in the forms of secretomes or exosomes for corneal regeneration are also discussed. Corneal sensory nerve regeneration promoting corneal transparency is discussed. This article provides description of the up-to-date options for corneal regeneration and presents exciting possible avenues for future studies toward clinical applications for corneal regeneration.

Stem Cells in the Path of Light, from Corneal to Retinal Reconstruction

The future of eye reconstruction invariably includes stem cells transplantation. Corneal limbus, corneal stroma, trabeculum, retinal cells, optic nerve, and all structures that are irreversibly damaged and have no means to be repaired or replaced, through conventional treatment or surgery, represent targets for stem cell reconstruction. This review tries to answer the question if there is any clinical validation for stem therapies, so far, starting from the cornea and, on the path of light, arriving to the retina. The investigation covers the last 10 years of publications. From 2385 published sources, we found 56 clinical studies matching inclusion criteria, 39 involving cornea, and 17 involving retina. So far, corneal epithelial reconstruction seems well validated clinically. Enough clinical data are collected to allow some form of standardization for the stem cell transplant procedures. Cultivated limbal epithelial stem cells (CLET), simple limbal epithelial transplant (SLET), and oral mucosa transplantation are implemented worldwide. In comparison, far less patients are investigated in retinal stem reconstructions, with lower anatomical and clinical success, so far. Intravitreal, subretinal, and suprachoroidal approach for retinal stem therapies face specific challenges.

Screening for Novel LOX and SOD1 Variants in Keratoconus Patients from Brazil

Purpose

To investigate the presence of the variants of lysyl oxygenase (LOX) and superoxide dismutase 1 (SOD1) genes in Brazilian patients with advanced keratoconus.

Methods

Donor genomic DNA extracted from blood samples was screened for 5'UTR, exonic LOX, and SOD1 variants in a subset of 26 patients presenting with advanced keratoconus (KISA > 1000% and I–S > 2.0) by Sanger sequencing. The impact of non-synonymous amino acid changes was evaluated by SIFT, PMUT, and PolyPhen algorithms. The Mutation Taster tool was used to evaluate the potential impact of formation of new donor and acceptor splice sites in the promoter region of affected volunteers carrying sequence variants. A 7-base SOD1 deletion (IVS2 + 50del7bp) previously associated with keratoconus was screened in 140 patients presenting classical keratoconus by gel fragment analysis, and positive samples were sequenced for confirmation.

Results

We found an unreported missense variant in LOX exon 6 in one heterozygous patient, leading to substitution of proline with threonine at residue 392 (p. Thr392Pro) of LOX protein sequence. This mutation was predicted to be potentially damaging to LOX protein. Another LOX variant, Arg158Gln, was also detected in another patient but predicted to be non-pathogenic. Two additional new polymorphisms in LOX 5'UTR region (–116C > T and –58C > T) were found in two patients presenting with advanced keratoconus and were predicted to modulate or create donor/acceptor splice sites in LOX transcripts. Additionally, SOD1 deletion was detected in one patient presenting with severe keratoconus, not in control samples.

Conclusion

We described three novel LOX polymorphisms identified for the first time in Brazilian patients with advanced keratoconus, as well as a previously described SOD1 deletion strongly associated with keratoconus. A possible role of these variants in modulating transcript levels in the cornea of affected individual requires further investigation.

Clinical Outcomes From Cultivated Allogenic Stem Cells vs. Oral Mucosa Epithelial Transplants in Total Bilateral Stem Cells Deficiency

Total bilateral limbal stem cell deficiency results from various pathologies, from burns (either chemical or physical) to Sjogren Syndrome, aniridia or ocular cicatricial pemphigoid. After the loss of stem cells, normal corneal epithelium is replaced by a more opaque and vascularized conjunctival epithelium, causing loss of vision. After 1997, cultivation techniques for limbal stem cells became possible. In parallel, cultivation techniques for oral mucosa epithelial cells were also available. The aim of our review was to summarize the clinical outcomes following allogenic cultured limbal stem cell transplant (allogenic CLET), and on the other hand, oral mucosa derived epithelium transplant (cultivated oral mucosa epithelial transplant-COMET or cultivated autologous oral mucosal epithelial cell sheet-CAOMECS), in the case of total bilateral limbal stem cell loss. Thirty studies matching the inclusion criteria were found. The clinical improvement in both methods was reported similar, with percentages higher than 50% of the treated cases. However, the comparison between studies was difficult to achieve due to the lack of a universal and objective grading tool for assessing post-operative results. The definition of clinical improvement was problematic, because success was defined differently, depending on the study. Moreover, some of the studies followed both autologous and allogenic CLET, but described the results together, for both procedures, and therefore it was impossible to analyze them separately. COMET presented some advantages compared to CLET. By using autologous cells, there was no risk of immune activation and no immunosuppression was needed. COMET, however, might be associated with increased risk of persistent epithelial defects and graft failure, compared with allogenic CLET.

Exploring the Key Genes and Pathways in the Formation of Corneal Scar Using Bioinformatics Analysis

The Corneal wound healing results in the formation of opaque corneal scar. In fact, millions of people around the world suffer from corneal scars, leading to loss of vision. This study aimed to identify the key changes of gene expression in the formation of opaque corneal scar and provided potential biomarker candidates for clinical treatment and drug target discovery. We downloaded Gene expression dataset GSE6676 from NCBI-GEO, and analyzed the Differentially Expressed Genes (DEGs), Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment pathway analyses, and protein-protein interaction (PPI) network. A total of 1377 differentially expressed genes were identified and the result of Functional enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) identification and protein-protein interaction (PPI) networks were performed. In total, 7 hub genes IL6 (interleukin-6), MMP9 (matrix metallopeptidase 9), CXCL10 (C-X-C motif chemokine ligand 10), MAPK8 (mitogen-activated protein kinase 8), TLR4 (toll-like receptor 4), HGF (hepatocyte growth factor), EDN1 (endothelin 1) were selected. In conclusion, the DEGS, Hub genes and signal pathways identified in this study can help us understand the molecular mechanism of corneal scar formation and provide candidate targets for the diagnosis and treatment of corneal scar.

Corneal injury: Clinical and molecular aspects

Currently, over 10 million people worldwide are affected by corneal blindness. Corneal trauma and disease can cause irreversible distortions to the normal structure and physiology of the cornea often leading to corneal transplantation. However, donors are in short supply and risk of rejection is an ever-present concern. Although significant progress has been made in recent years, the wound healing cascade remains complex and not fully understood. Tissue engineering and regenerative medicine are currently at the apex of investigation in the pursuit of novel corneal therapeutics. This review uniquely integrates the clinical and cellular aspects of both corneal trauma and disease and provides a comprehensive view of the most recent findings and potential therapeutics aimed at restoring corneal homeostasis.

Dental Mesenchymal Stem Cell-Based Translational Regenerative Dentistry: From Artificial to Biological Replacement

Dentistry is a continuously changing field that has witnessed much advancement in the past century. Prosthodontics is that branch of dentistry that deals with replacing missing teeth using either fixed or removable appliances in an attempt to simulate natural tooth function. Although such "replacement therapies" appear to be easy and economic they fall short of ever coming close to their natural counterparts. Complications that arise often lead to failures and frequent repairs of such devices which seldom allow true physiological function of dental and oral-maxillofacial tissues. Such factors can critically affect the quality of life of an individual. The market for dental implants is continuously growing with huge economic revenues. Unfortunately, such treatments are again associated with frequent problems such as peri-implantitis resulting in an eventual loss or replacement of implants. This is particularly influential for patients having co-morbid diseases such as diabetes or osteoporosis and in association with smoking and other conditions that undoubtedly affect the final treatment outcome. The advent of tissue engineering and regenerative medicine therapies along with the enormous strides taken in their associated interdisciplinary fields such as stem cell therapy, biomaterial development, and others may open arenas to enhancing tissue regeneration via designing and construction of patient-specific biological and/or biomimetic substitutes. This review will overview current strategies in regenerative dentistry while overviewing key roles of dental mesenchymal stem cells particularly those of the dental pulp, until paving the way to precision/translational regenerative medicine therapies for future clinical use.

Generation of Femtosecond Laser-Cut Decellularized Corneal Lenticule Using Hypotonic Trypsin-EDTA Solution for Corneal Tissue Engineering

Purpose

To establish an optimized and standardized protocol for the development of optimal scaffold for bioengineering corneal substitutes, we used femtosecond laser to process human corneal tissue into stromal lenticules and studied to find the most efficient decellularization method among various reagents with different tonicities.

Methods

The decellularization efficacy of several agents (0.1%, 0.25%, and 0.5% of Triton X-100, SDS, and trypsin-EDTA (TE), resp.) with different tonicities was evaluated. Of all protocols, the decellularization methods, which efficiently removed nuclear materials examined as detected by immunofluorescent staining, were quantitatively tested for sample DNA and glycosaminoglycan (GAG) contents, recellularization efficacy, and biocompatibilities.

Results

0.5% SDS in hypertonic and isotonic buffer, 0.25% TE in hypotonic buffer, and 0.5% TE in all tonicities completely decellularized the corneal lenticules. Of the protocols, decellularization with hypotonic 0.25 and 0.5% TE showed the lowest DNA contents, while the GAG content was the highest. Furthermore, the recellularization efficacy of the hypotonic TE method was better than that of the SDS-based method. Hypotonic TE-treated decellularized corneal lenticules (DCLs) were sufficiently transparent and biocompatible.

Conclusion

We generated an ideal protocol for DCLs using a novel method. Furthermore, it is possible to create a scaffold using a bioengineered corneal substitute.

Unrestricted somatic stem cells, as a novel feeder layer: Ex vivo culture of human limbal stem cells

Ex vivo culture of limbal stem cells (LSCs) is a current promising approach for reconstruction of the ocular surface. In this context, 3T3 feeder layer cells (mouse embryo fibroblast) are generally utilized to maintain and expand LSCs. The aim of this study is to develop a novel culture method (animal-derived products free) to expand LSCs, using umbilical cord derived human unrestricted somatic stem cells (hUSSCs) instead of 3T3 cell with an emphasis on maintaining of the Stemness in LSCs. Using flow-cytometer, isolated hUSSCs were characterized for CD105, CD90, CD166, CD34, CD45, CD31 cell surface markers and their differentiation capability into adipogenic as well as osteogenic lineages were evaluated. In addition to colony-forming efficiency (CFE), epithelial lineage differentiation and karyotyping, LSC properties were evaluated for ABCG2, ΔNP63-α, CK19, CK3, and CK12 mRNA and protein expressions using quantitative RT-PCR (qRT-PCR) and immunocytochemistry, when these cells were co-cultured with hUSSCs (in comparison with 3T3 feeder layer). LSCs, co-cultured with hUSSCs, showed normal karyotype (46, XX), while they could efficiently form colony (86 ± 3) and display up-regulation of the genes associated with stemness and down-regulation of corneal epithelial differentiation genes. Consistent with 3T3 feeder cells, hUSSCs with spindle-shaped morphology and quick splitting up properties had ability to preserve the stem like-cell phenotype of LSCs. These findings were confirmed by qRT-PCR and flow-cytometer. Findings of present study suggest hUSSCs as a promising alternative method for 3T3 feeder layer cells, to preserve growth and stemness of LSCs ex vivo culture.

Outcome of repeat penetrating keratoplasty in eyes with failed penetrating keratoplasty

Objectives:

To report the outcomes of penetrating keratoplasty (PKP) surgeries in eyes with failed PKP.

Methods:

This was a retrospective, non-comparative, descriptive case series. Thirty eyes of 30 patients with failed PKP comprised our study group, they were reviewed from January 2007 to December 2012 at the King Khaled Eye Specialist Hospital, Riyadh, Kingdom of Saudi Arabia. Data were collected on best corrected visual acuity before and after one week, one month, 3 months, 6 months, one year, and 2 years following PKP. Intraoperative and postoperative complications, changes in intraocular pressure (IOP), additional surgical procedure and other ocular comorbidities were also documented. The visual outcomes at 6 months and one year were associated with risk factors.

Results:

Before intervention, 18 (60%) eyes had vision <20/400. Vision was 20/20 to 20/60 in 10 (30%) eyes at 6 months, 17 (57%) eyes at 12 months, and 22 (73%) eyes at 24 months. The variation in IOP at different follow up periods was not significant (p=0.2). The presence of other ocular comorbidity was not significantly associated with functional visual outcome (p=0.4). Additional surgical procedure after repeat PKP enabled a regain of excellent vision in 9 (47%) eyes at one year. The numbers for past corneal surgeries were significantly associated with the visual outcome at 6 months.

Conclusion:

Penetrating keratoplasty to manage failed PKP resulted in reducing visual disabilities.

Concise Review: Stem Cells for Corneal Wound Healing

Corneal wound healing is a complex process that occurs in response to various injuries and commonly used refractive surgery. It is a significant clinical problem, which may lead to serious complications due to either incomplete (epithelial) or excessive (stromal) healing. Epithelial stem cells clearly play a role in this process, whereas the contribution of stromal and endothelial progenitors is less well studied. The available evidence on stem cell participation in corneal wound healing is reviewed, together with the data on the use of corneal and non-corneal stem cells to facilitate this process in diseased or postsurgical conditions. Important aspects of corneal stem cell generation from alternative cell sources, including pluripotent stem cells, for possible transplantation upon corneal injuries or in disease conditions are also presented. Stem Cells 2017;35:2105-2114.

Three-Dimensional Construction of a Rabbit Anterior Corneal Replacement for Lamellar Keratoplasty

The aim of this study was to construct a rabbit anterior corneal replacement for transplantation using acellular porcine corneal matrix (APCM) and rabbit epithelial or stromal cells. APCM was prepared from fresh porcine cornea treated with 0.5% (wt./vol.) sodium dodecyl sulfate (SDS) solution. The expanded stromal cells were first injected into APCM parallel to its surface and were cultured in a shaking culture system for 7 days to obtain the stromal construct. Next, corneal epithelial cells were cultured on the stromal construct surface for another 7 days to obtain rabbit anterior corneal lamella. The construct had a phenotype similar to that of normal cornea, with high expression of cytokeratin 3 in the epithelial cell layer and vimentin in the stromal cells. More importantly, the construct integrated well with the implanted host corneal tissue, and the implant cornea maintained transparency in the 6-month follow-up, although there was a slight haze in the central corneal area. The endothelium in the surgery cornea had a similar cell density and mosaic pattern with normal cornea as shown by confocal laser corneal microscopy, and the regenerated corneal epithelial cells on the implant surface showed a similar morphology to that of natural epithelial cells. These results demonstrate that the constructed anterior corneal replacement exhibits an excellent biological property for lamellar keratoplasty and might be a possible alternative to human corneal tissue in the future.