Anatomical Features and Cell-Cell Interactions in the Human Limbal Epithelial Stem Cell Niche

Wnt/β-catenin signaling in corneal epithelium development, homeostasis, and pathobiology

The corneal epithelium is located on the most anterior surface of the eyeball and protects against external stimuli. The development of the corneal epithelium and the maintenance of corneal homeostasis are essential for the maintenance of visual acuity. It has been discovered recently via the in-depth investigation of ocular surface illnesses that the Wnt/β-catenin signaling pathway is necessary for the growth and stratification of corneal epithelial cells as well as the control of endothelial cell stability. In addition, the Wnt/β-catenin signaling pathway is directly linked to the development of common corneal illnesses such as keratoconus, fungal keratitis, and corneal neovascularization. This review mainly summarizes the role of the Wnt/β-catenin signaling pathway in the development, homeostasis, and pathobiology of cornea, hoping to provide new insights into the study of corneal epithelium and the treatment of related diseases.

Limbal stem cell therapy

PURPOSE OF REVIEW

To highlight the progress and future direction of limbal stem cell (LSC) therapies for the treatment of limbal stem cell deficiency (LSCD).

RECENT FINDINGS

Direct LSC transplantation have demonstrated good long-term outcomes. Cultivated limbal epithelial transplantation (CLET) has been an alternative to treat severe to total LSCD aiming to improve the safety and efficacy of the LSC transplant. A prospective early-stage uncontrolled clinical trial shows the feasibility and safety of CLET manufactured under xenobiotic free conditions. Other cell sources for repopulating of the corneal epithelium such as mesenchymal stem cells (MSCs) and induced pluripotent stem cells are being investigated. The first clinical trials of using MSCs showed short-term results, but long-term efficacy seems to be disappointing. A better understanding of the niche function and regulation of LSC survival and proliferation will lead to the development of medical therapies to rejuvenate the residual LSCs found in a majority of eyes with LSCD in vivo. Prior efforts have been largely focused on improving LSC transplantation. Additional effort should be placed on improving the accuracy of diagnosis and staging of LSCD, and implementing standardized outcome measures which enable comparison of efficacy of different LSCD treatments for different severity of LSCD. The choice of LSCD treatment will be customized based on the severity of LSCD in the future.

SUMMARY

New approaches for managing different stages of LSCD are being developed. This concise review summarizes the progresses in LSC therapies for LSCD, underlying mechanisms, limitations, and future areas of development.

Clinical and pathologic characterization of a mouse model of graded limbal stem cell deficiency

Limbal stem cell deficiency (LSCD) is a clinically challenging eye disease caused by damage to limbal stem cells (LSCs). Currently, the international consensus classifies LSCD into three clinical stages based on the disease severity. However, no existing animal models attempt to replicate the varying degrees of LSCD observed in clinical cases. The present study demonstrates an easy-to-create, reproducible, and reliable mouse model of graded LSCD. To achieve mild, moderate, or severe LSCD, filter paper rings with a variety of central angles (90°, 180°, or 270°) are utilized to deliver alkali burns to different sizes of the limbal area (1, 2, or 3 quarters). The animal model has successfully resulted in the development of clinical signs and pathological manifestations in escalating severity that are similarly observed in the three clinical stages of LSCD. Our study thus provides new insights into distinct pathological features underlying different grades of LSCD and serves as a new tool for further exploring the disease mechanisms and developing new effective therapeutics for repairing damaged LSCs.

Influence of Organ Culture on the Characteristics of the Human Limbal Stem Cell Niche

Organ culture storage techniques for corneoscleral limbal (CSL) tissue have improved the quality of corneas for transplantation and allow for longer storage times. Cultured limbal tissue has been used for stem cell transplantation to treat limbal stem cell deficiency (LSCD) as well as for research purposes to assess homeostasis mechanisms in the limbal stem cell niche. However, the effects of organ culture storage conditions on the quality of limbal niche components are less well described. Therefore, in this study, the morphological and immunohistochemical characteristics of organ-cultured limbal tissue are investigated and compared to fresh limbal tissues by means of light and electron microscopy. Organ-cultured limbal tissues showed signs of deterioration, such as edema, less pronounced basement membranes, and loss of the most superficial layers of the epithelium. In comparison to the fresh limbal epithelium, organ-cultured limbal epithelium showed signs of ongoing proliferative activity (more Ki-67+ cells) and exhibited an altered limbal epithelial phenotype with a loss of N-cadherin and desmoglein expression as well as a lack of precise staining patterns for cytokeratin ((CK)14, CK17/19, CK15). The analyzed extracellular matrix composition was mainly intact (collagen IV, fibronectin, laminin chains) except for Tenascin-C, whose expression was increased in organ-cultured limbal tissue. Nonetheless, the expression patterns of cell-matrix adhesion proteins varied in organ-cultured limbal tissue compared to fresh limbal tissue. A decrease in the number of melanocytes (Melan-A+ cells) and Langerhans cells (HLA-DR+, CD1a+, CD18+) was observed in the organ-cultured limbal tissue. The organ culture-induced alterations of the limbal epithelial stem cell niche might hamper its use in the treatment of LSCD as well as in research studies. In contrast, reduced numbers of donor-derived Langerhans cells seem associated with better clinical outcomes. However, there is a need to consider the preferential use of fresh CSL for limbal transplants and to look at ways of improving the limbal stem cell properties of stored CSL tissue.

Promotion of uterine reconstruction by a tissue-engineered uterus with biomimetic structure and extracellular matrix microenvironment

The recurrence rate for severe intrauterine adhesions is as high as 60%, and there is still lack of effective prevention and treatment. Inspired by the nature of uterus, we have developed a bilayer scaffold (ECM-SPS) with biomimetic heterogeneous features and extracellular matrix (ECM) microenvironment of the uterus. As proved by subtotal uterine reconstruction experiments, the mechanical and antiadhesion properties of the bilayer scaffold could meet the requirement for uterine repair. With the modification with tissue-specific cell-derived ECM, the ECM-SPS had the ECM microenvironment signatures of both the endometrium and myometrium and exhibited the property of inducing stem cell-directed differentiation. Furthermore, the ECM-SPS has recruited more endogenous stem cells to promote endometrial regeneration at the initial stage of repair, which was accompanied by more smooth muscle regeneration and a higher pregnancy rate. The reconstructed uterus could also sustain normal pregnancy and live birth. The ECM-SPS may thereby provide a potential treatment for women with severe intrauterine adhesions.

Toward Corneal Limbus In Vitro Model: Regulation of hPSC-LSC Phenotype by Matrix Stiffness and Topography During Cell Differentiation Process

A functional limbal epithelial stem cells (LSC) niche is a vital element in the regular renewal of the corneal epithelium by LSCs and maintenance of good vision. However, little is known about its unique structure and mechanical properties on LSC regulation, creating a significant gap in development of LSC-based therapies. Herein, the effect of mechanical and architectural elements of the niche on human pluripotent derived LSCs (hPSC-LSC) phenotype and growth is investigated in vitro. Specifically, three formulations of polyacrylamide gels with different controlled stiffnesses are used for culture and characterization of hPSC-LSCs from different stages of differentiation. In addition, limbal mimicking topography in polydimethylsiloxane is utilized for culturing hPSC-LSCs at early time point of differentiation. For comparison, the expression of selected key proteins of the corneal cells is analyzed in their native environment through whole mount staining of human donor corneas. The results suggest that mechanical response and substrate preference of the cells is highly dependent on their developmental stage. In addition, data indicate that cells may carry possible mechanical memory from previous culture matrix, both highlighting the importance of mechanical design of a functional in vitro limbus model.

MicroRNA and Protein Cargos of Human Limbal Epithelial Cell-Derived Exosomes and Their Regulatory Roles in Limbal Stromal Cells of Diabetic and Non-Diabetic Corneas

Epithelial and stromal/mesenchymal limbal stem cells contribute to corneal homeostasis and cell renewal. Extracellular vesicles (EVs), including exosomes (Exos), can be paracrine mediators of intercellular communication. Previously, we described cargos and regulatory roles of limbal stromal cell (LSC)-derived Exos in non-diabetic (N) and diabetic (DM) limbal epithelial cells (LECs). Presently, we quantify the miRNA and proteome profiles of human LEC-derived Exos and their regulatory roles in N- and DM-LSC. We revealed some miRNA and protein differences in DM vs. N-LEC-derived Exos' cargos, including proteins involved in Exo biogenesis and packaging that may affect Exo production and ultimately cellular crosstalk and corneal function. Treatment by N-Exos, but not by DM-Exos, enhanced wound healing in cultured N-LSCs and increased proliferation rates in N and DM LSCs vs. corresponding untreated (control) cells. N-Exos-treated LSCs reduced the keratocyte markers ALDH3A1 and lumican and increased the MSC markers CD73, CD90, and CD105 vs. control LSCs. These being opposite to the changes quantified in wounded LSCs. Overall, N-LEC Exos have a more pronounced effect on LSC wound healing, proliferation, and stem cell marker expression than DM-LEC Exos. This suggests that regulatory miRNA and protein cargo differences in DM- vs. N-LEC-derived Exos could contribute to the disease state.

Long-term PM2.5 exposure disrupts corneal epithelial homeostasis by impairing limbal stem/progenitor cells in humans and rat models

BACKGROUND

Limbal stem/progenitor cells (LSPCs) play a crucial role in maintaining corneal health by regulating epithelial homeostasis. Although PM2.5 is associated with the occurrence of several corneal diseases, its effects on LSPCs are not clearly understood.

METHODS

In this study, we explored the correlation between PM2.5 exposure and human limbal epithelial thickness measured by Fourier-domain Optical Coherence Tomography in the ophthalmologic clinic. Long- and short-term PM2.5 exposed-rat models were established to investigate the changes in LSPCs and the associated mechanisms.

RESULTS

We found that people living in regions with higher PM2.5 concentrations had thinner limbal epithelium, indicating the loss of LSPCs. In rat models, long-term PM2.5 exposure impairs LSPCs renewal and differentiation, manifesting as corneal epithelial defects and thinner epithelium in the cornea and limbus. However, LSPCs were activated in short-term PM2.5-exposed rat models. RNA sequencing implied that the circadian rhythm in LSPCs was perturbed during PM2.5 exposure. The mRNA level of circadian genes including Per1, Per2, Per3, and Rev-erbα was upregulated in both short- and long-term models, suggesting circadian rhythm was involved in the activation and dysregulation of LSPCs at different stages. PM2.5 also disturbed the limbal microenvironment as evidenced by changes in corneal subbasal nerve fiber density, vascular density and permeability, and immune cell infiltration, which further resulted in the circadian mismatches and dysfunction of LSPCs.

CONCLUSION

This study systematically demonstrates that PM2.5 impairs LSPCs and their microenvironment. Moreover, we show that circadian misalignment of LSPCs may be a new mechanism by which PM2.5 induces corneal diseases. Therapeutic options that target circadian rhythm may be viable options for improving LSPC functions and alleviating various PM2.5-associated corneal diseases.

Cell-Cell and Cell-Matrix Interactions at the Presumptive Stem Cell Niche of the Chick Corneal Limbus

(1) Background: Owing to its ready availability and ease of acquisition, developing chick corneal tissue has long been used for research purposes. Here, we seek to ascertain the three-dimensional microanatomy and spatiotemporal interrelationships of the cells (epithelial and stromal), extracellular matrix, and vasculature at the corneo-scleral limbus as the site of the corneal stem cell niche of the chicken eye. (2) Methods: The limbus of developing (i.e., embryonic days (E) 16 and 18, just prior to hatch) and mature chicken eyes was imaged using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the volume electron microscopy technique, serial-block face SEM (SBF-SEM), the latter technique allowing us to generate three-dimensional reconstructions from data sets of up to 1000 serial images; (3) Results: Data revealed that miniature limbal undulations of the embryonic basement membrane, akin to Palisades of Vogt (PoV), matured into distinct invaginations of epithelial cells that extended proximally into a vascularized limbal stroma. Basal limbal epithelial cells, moreover, occasionally exhibited a high nuclear:cytoplasmic ratio, which is a characteristic feature of stem cells. SBF-SEM identified direct cell-cell associations between corneal epithelial and stromal cells at the base of structures akin to limbal crypts (LCs), with cord-like projections of extracellular matrix extending from the basal epithelial lamina into the subjacent stroma, where they made direct contact with stomal cells in the immature limbus. (4) Conclusion: Similarities with human tissue suggest that the corneal limbus of the mature chicken eye is likely the site of a corneal stem cell niche. The ability to study embryonic corneas pre-hatch, where we see characteristic niche-like features emerge, thus provides an opportunity to chart the development of the limbal stem cell niche of the cornea.

Wnt activation as a potential therapeutic approach to treat partial limbal stem cell deficiency

Limbal epithelial stem/progenitor cells (LSCs) are adult stem cells located at the limbus, tightly regulated by their niche involving numerous signaling pathways, such as Wnt. Wnt proteins are secreted morphogens that play critical roles in embryonic development, stem cell proliferation, self-renewal, tissue regeneration, and remodeling in adults. It has been shown that a small molecule Wnt mimic could improve LSCs expansion ex vivo. Damage to the LSCs and/or their niche can lead to limbal stem cell deficiency (LSCD), a condition that can cause corneal blindness and is difficult to treat. This study explored if repopulating residual LSCs in partial LSCD through Wnt activation could be a novel therapeutic approach. To mimic LSCD due to a chemical injury, single cultured LSCs were exposed to various concentrations of sodium hydroxide. A progressive loss of the LSCs phenotype was observed: the percentage of p63bright cells and cytokeratin (K)14+ cells decreased while the percentage of K12+ increased. Wnt activation was attained by treating the LSCs with lithium chloride (LiCl) and a small-molecule Wnt mimic, respectively. After 18 h of treatment, LSCs proliferation was increased, and the LSCs phenotype was recovered, while the untreated cells did not proliferate and lost their phenotype. The percentage of p63bright cells was significantly higher in the Wnt mimic-treated cells compared with untreated cells, while the percentage of K12+ cells was significantly lower. These findings suggest that local Wnt activation may rescue LSCs upon alkaline injury.

Opposing Roles of Blood-Borne Monocytes and Tissue-Resident Macrophages in Limbal Stem Cell Damage after Ocular Injury

Limbal stem cell (LSC) deficiency is a frequent and severe complication after chemical injury to the eye. Previous studies have assumed this is mediated directly by the caustic agent. Here we show that LSC damage occurs through immune cell mediators, even without direct injury to LSCs. In particular, pH elevation in the anterior chamber (AC) causes acute uveal stress, the release of inflammatory cytokines at the basal limbal tissue, and subsequent LSC damage and death. Peripheral C-C chemokine receptor type 2 positive/CX3C motif chemokine receptor 1 negative (CCR2+ CX3CR1-) monocytes are the key mediators of LSC damage through the upregulation of tumor necrosis factor-alpha (TNF-α) at the limbus. In contrast to peripherally derived monocytes, CX3CR1+ CCR2- tissue-resident macrophages have a protective role, and their depletion prior to injury exacerbates LSC loss and increases LSC vulnerability to TNF-α-mediated apoptosis independently of CCR2+ cell infiltration into the tissue. Consistently, repopulation of the tissue by new resident macrophages not only restores the protective M2-like phenotype of macrophages but also suppresses LSC loss after exposure to inflammatory signals. These findings may have clinical implications in patients with LSC loss after chemical burns or due to other inflammatory conditions.

Enhanced adipose-derived stem cells with IGF-1-modified mRNA promote wound healing following corneal injury

The cornea serves as an important barrier structure to the eyeball and is vulnerable to injuries, which may lead to scarring and blindness if not treated promptly. To explore an effective treatment that could achieve multi-dimensional repair of the injured cornea, the study herein innovatively combined modified mRNA (modRNA) technologies with adipose-derived mesenchymal stem cells (ADSCs) therapy, and applied IGF-1 modRNA (modIGF1)-engineered ADSCs (ADSCmodIGF1) to alkali-burned corneas in mice. The therapeutic results showed that ADSCmodIGF1 treatment could achieve the most extensive recovery of corneal morphology and function when compared not only with simple ADSCs but also IGF-1 protein eyedrops, which was reflected by the healing of corneal epithelium and limbus, the inhibition of corneal stromal fibrosis, angiogenesis and lymphangiogenesis, and also the repair of corneal nerves. In vitro experiments further proved that ADSCmodIGF1 could more significantly promote the activity of trigeminal ganglion cells and maintain the stemness of limbal stem cells than simple ADSCs, which were also essential for reconstructing corneal homeostasis. Through a combinatorial treatment regimen of cell-based therapy with mRNA technology, this study highlighted comprehensive repair in the damaged cornea and showed the outstanding application prospect in the treatment of corneal injury.

Evaluation of the factors that influence surgical outcome in conjunctival-limbal allograft transplantation

OBJECTIVE

To evaluate the surgical results and the variables affecting the outcomes in conjunctival-limbal allograft transplantation (CLAL).

METHODS

Patients who underwent CLAL for limbal stem cell deficiency (LSCD) between 2007 and 2019 were included in the study. LSCD staging was performed according to the staging system developed by the 'Limbal Stem Cell Working Group'. Stage 1C and higher stage LSCD patients were included in the study. 'Successful surgical outcome' was defined as improvement in LSCD stage at 1 year postoperatively.

RESULTS

A total of 19 eyes of 19 LSCD patients were included. The mean age of the patients was 40.21 ± 14.65 (6-65) years, and the male/female ratio was 12/7. CLAL was performed in 9 (47.3%) patients with Stage 2B, 9 (47.3%) patients with Stage 3 and 1 (5.4%) with Stage 1C. LSCD aetiology; chemical injury (12), vernal keratoconjunctivitis (2), aniridia (1), corneal degeneration (1), and unknown (3). Surgery was successful in 52.6% of cases. Surgical success was associated with lower LSCD stage (p = 0.04). Lower grades of chemical injury at presentation and a longer time interval between injury and CLAL were associated with higher surgical success (p = 0.001; p = 0.001). The mean postoperative follow-up time was 50.77 ± 29.46 (6-98) months.

CONCLUSIONS

Despite graft rejection and long-term use of immunosuppressants, CLAL is still one of the most preferred techniques in the treatment of bilateral LSCD. Preoperative LSCD stage and degree of chemical burn are important factors affecting the surgical outcome. Also, CLAL surgery should not be rushed and should be performed when inflammation has subsided.

Single mRNA detection of Wnt signaling pathway in the human limbus

Limbal epithelial stem/progenitor cells (LSCs) are adult stem cells located at the limbus, tightly regulated by their close microenvironment. It has been shown that Wnt signaling pathway is crucial for LSCs regulation. Previous differential gene profiling studies confirmed the preferential expression of specific Wnt ligands (WNT2, WNT6, WNT11, WNT16) and Wnt inhibitors (DKK1, SFRP5, WIF1, FRZB) in the limbal region compared to the cornea. Among all frizzled receptors, frizzled7 (Fzd7) was found to be preferentially expressed in the basal limbal epithelium. However, the exact localization of Wnt signaling molecules-producing cells in the limbus remains unknown. The current study aims to evaluate the in situ spatial expression of these 4 Wnt ligands, 4 Wnt inhibitors, and Fzd7. Wnt ligands, DKK1, and Fzd7 expression were scattered within the limbal epithelium, at a higher abundance in the basal layer than the superficial layer. SFRP5 expression was diffuse among the limbal epithelium, whereas WIF1 and FRZB expression was clustered at the basal limbal epithelial layer corresponding to the areas of high levels of Fzd7 expression. Quantitation of the fluorescence intensity showed that all 4 Wnt ligands, 3 Wnt inhibitors (WIF1, DKK1, FRZB), and Fzd7 were highly expressed at the basal layer of the limbus, then in a decreasing gradient toward the superficial layer (P < 0.05). The expression levels of all 4 Wnt ligands, FRZB, and Fzd7 in the basal epithelial layer were higher in the limbus than the central cornea (P < 0.05). All 4 Wnt ligands, 4 Wnt inhibitors, and Fzd7 were also highly expressed in the limbal stroma immediately below the epithelium but not in the corneal stroma (P < 0.05). In addition, Fzd7 had a preferential expression in the superior limbus compared to other limbal quadrants (P < 0.05). Taken together, the unique expression patterns of the Wnt molecules in the limbus suggests the involvement of both paracrine and autocrine effects in LSCs regulation, and a fine balance between Wnt activators and inhibitors to govern LSC fate.

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.

Corneal stem cells niche and homeostasis impacts in regenerative medicine; concise review

The limbal stem cells niche (LSCN) is an optimal microenvironment that provides the limbal epithelial stem cells (LESCs) and strictly regulates their proliferation and differentiation. Disturbing the LSCN homeostasis can lead to limbal stem cell dysfunction (LSCD) and subsequent ocular surface aberrations, such as corneal stromal inflammation, persistent epithelial defects, corneal neovascularisation, lymphangiogenesis, corneal opacification, and conjunctivalization. As ocular surface disorders are considered the second main cause of blindness, it becomes crucial to explore different therapeutic strategies for restoring the functions of the LSCN. A major limitation of corneal transplantation is the current shortage of donor tissue to meet the requirements worldwide. In this context, it becomes mandatory to find an alternative regenerative medicine, such as using cultured limbal epithelial/stromal stem cells, inducing the production of corneal like cells by using other sources of stem cells, and using tissue engineering methods aiming to produce the three-dimensional (3D) printed cornea. Limbal epithelial stem cells have been considered the magic potion for eye treatment. Epithelial and stromal stem cells in the limbal niche hold the responsibility of replenishing the corneal epithelium. These stem cells are being used for transplantation to maintain corneal epithelial integrity and ultimately sustain optimal vision. In this review, we summarised the characteristics of the LSCN and their current and future roles in restoring corneal homeostasis in eyes with LSCD.

Targeting Type I IFN/STAT1 signaling inhibited and reversed corneal squamous metaplasia in Aire-deficient mouse

Corneal transparency and integrity are essential for obtaining good vision; nevertheless, squamous metaplasia (SQM) of ocular epithelium is a kind of serious blinding corneal diseases, without therapeutic medication in clinic. Here, we found that deficiency of the autoimmune regulator (AIRE) in corneas spontaneously developed corneal plaques. Using corneal abrasion model, we revealed that deletion of Aire not only resulted in delayed corneal re-epithelialization, but also promoted a cell-fate transition from transparent corneal epithelium to keratinized epithelium, histopathologically characterized with SQM based on the transcriptomic analysis. Mechanistically, Aire-deficient corneas led to the heightened Type I interferon (IFN-I)/STAT1 signaling after abrasion. Pharmacological blockade of IFN-I/JAK/STAT1 signaling in Aire-knockout (KO) corneas not only accelerated epithelial wound healing, but also alleviated corneal plaques and SQM. Collectively, our findings revealed critical roles of AIRE in governing corneal epithelial homeostasis and pathologic keratinization, and further identified IFN-I/STAT1 signaling as a potential target for treating ocular surface diseases with SQM, and even for treating pathological scenarios related to SQM in other tissues.

Notch1 signaling in keratocytes maintains corneal transparency by suppressing VEGF expression

The cornea fends off chemicals, dirt, and infectious particles and provides most of the eye's focusing power. Corneal transparency is of paramount importance to normal vision, yet how it is established and maintained remains unclear. Here, we ablated Notch1 in keratocytes using Twist2-Cre mice and found that Twist2-Cre; Notch1^f/f mice developed stroma expansion and neovascularization, followed by hyperproliferation and metaplasia of corneal epithelial progenitor cells and plaque formation at central cornea, leading to loss of transparency. Development of these phenotypes does not involve bacteria-caused inflammation; instead, Notch1 deletion upregulates Vegfa and Vegfc via Hif1α in keratocytes. Vascular endothelial growth factor (VEGF) receptor inhibitor axitinib prevented development of these anomalies in Twist2-Cre; Notch1^f/f mice, suggesting that VEGFs secreted by keratocytes promote not only neovascularization but also proliferation and metaplasia of epithelial progenitor cells at central cornea. This study uncovers a Notch1-Hif1α-VEGF pathway in keratocytes that maintains corneal transparency and represents a potential target for treatment of related corneal disorders.

Identification, Isolation, and Characterization of Melanocyte Precursor Cells in the Human Limbal Stroma

Given their vital role in the homeostasis of the limbal stem cell niche, limbal melanocytes have emerged as promising candidates for tissue engineering applications. This study aimed to isolate and characterize a population of melanocyte precursors in the limbal stroma, compared with melanocytes originating from the limbal epithelium, using magnetic-activated cell sorting (MACS) with positive (CD117/c-Kit microbeads) or negative (CD326/EpCAM or anti-fibroblast microbeads) selection approaches. Both approaches enabled fast and easy isolation and cultivation of pure limbal epithelial and stromal melanocyte populations, which differed in phenotype and gene expression, but exhibited similar functional properties regarding proliferative potential, pigmentation, and support of clonal growth of limbal epithelial stem/progenitor cells (LEPCs). In both melanocyte populations, limbus-specific matrix (laminin 511-E8) and soluble factors (LEPC-derived conditioned medium) stimulated melanocyte adhesion, dendrite formation, melanogenesis, and expression of genes involved in UV protection and immune regulation. The findings provided not only a novel protocol for the enrichment of pure melanocyte populations from limbal tissue applying easy-to-use MACS technology, but also identified a population of stromal melanocyte precursors, which may serve as a reservoir for the replacement of damaged epithelial melanocytes and an alternative resource for tissue engineering applications.

A Broad Overview on Pituitary Adenylate Cyclase-Activating Polypeptide Role in the Eye: Focus on Its Repairing Effect in Cornea

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) is a neuropeptide with widespread distribution throughout the central and peripheral nervous system as well as in many other peripheral organs. It plays cytoprotective effects mediated mainly through the activation of specific receptors. PACAP is known to play pleiotropic effects on the eye, including the cornea, protecting it against different types of insult. This review firstly provides an overview of the anatomy of the cornea and summarizes data present in literature about PACAP’s role in the eye and, in particular, in the cornea, either in physiological or pathological conditions.

Limbal Stem Cell Deficiency: Demographics and Clinical Characteristics of a Large Retrospective Series at a Single Tertiary Referral Center

PURPOSE

The aim of this study was to characterize a large cohort of patients presenting to a single referral center for limbal stem cell deficiency (LSCD).

METHODS

A retrospective chart review of all patients with a clinical diagnosis of LSCD from 2002 to 2015 was performed. Demographics, etiology, previous ocular surgeries, visual acuity, and treatment were assessed.

RESULTS

Seven hundred thirty-eight eyes of 434 patients (51.4% male subjects) were diagnosed with LSCD. The mean presenting age was 42.9 years, 70% presented with bilateral disease, and overall vision was poor. The most common etiologies were congenital aniridia (30.9%), chemical or thermal injuries (20.6%), contact lens (16.8%), Stevens-Johnson syndrome (SJS, 10.4%), and iatrogenic (7.3%). Congenital aniridia had a significantly increased association with glaucoma or ocular hypertension (P < 0.0001). Chemical or thermal injuries (P = 0.0007), SJS (P < 0.0001), and mucous membrane pemphigoid (P < 0.0001) had a significantly increased association with eyelid pathology. The mean logMAR best corrected visual acuity (analysis excluded eyes with light perception and no light perception) at presentation was 1.145 (∼20/280). Keratoplasty performed (before presentation at our center) without first addressing the LSCD was seen in 80 eyes; all keratoplasties failed due to recurrence of the LSCD.

CONCLUSIONS

Patients presenting with LSCD were on average middle aged without sex predominance. LSCD was most commonly bilateral, comprised a wide range of etiologies, and decreased vision substantially. Ocular comorbidities may need to be treated before treating the LSCD surgically. Finally, not addressing the LSCD (primary issue) first can result in keratoplasty failure.

Regulation of Limbal Epithelial Stem Cells: Importance of the Niche

Limbal epithelial stem/progenitor cells (LSCs) reside in a niche that contains finely tuned balances of various signaling pathways including Wnt, Notch, BMP, Shh, YAP, and TGFβ. The activation or inhibition of these pathways is frequently dependent on the interactions of LSCs with various niche cell types and extracellular substrates. In addition to receiving molecular signals from growth factors, cytokines, and other soluble molecules, LSCs also respond to their surrounding physical structure via mechanotransduction, interaction with the ECM, and interactions with other cell types. Damage to LSCs or their niche leads to limbal stem cell deficiency (LSCD). The field of LSCD treatment would greatly benefit from an understanding of the molecular regulation of LSCs in vitro and in vivo. This review synthesizes current literature around the niche factors and signaling pathways that influence LSC function. Future development of LSCD therapies should consider all these niche factors to achieve improved long-term restoration of the LSC population.

Biofabrication of Artificial Stem Cell Niches in the Anterior Ocular Segment

The anterior segment of the eye is a complex set of structures that collectively act to maintain the integrity of the globe and direct light towards the posteriorly located retina. The eye is exposed to numerous physical and environmental insults such as infection, UV radiation, physical or chemical injuries. Loss of transparency to the cornea or lens (cataract) and dysfunctional regulation of intra ocular pressure (glaucoma) are leading causes of worldwide blindness. Whilst traditional therapeutic approaches can improve vision, their effect often fails to control the multiple pathological events that lead to long-term vision loss. Regenerative medicine approaches in the eye have already had success with ocular stem cell therapy and ex vivo production of cornea and conjunctival tissue for transplant recovering patients' vision. However, advancements are required to increase the efficacy of these as well as develop other ocular cell therapies. One of the most important challenges that determines the success of regenerative approaches is the preservation of the stem cell properties during expansion culture in vitro. To achieve this, the environment must provide the physical, chemical and biological factors that ensure the maintenance of their undifferentiated state, as well as their proliferative capacity. This is likely to be accomplished by replicating the natural stem cell niche in vitro. Due to the complex nature of the cell microenvironment, the creation of such artificial niches requires the use of bioengineering techniques which can replicate the physico-chemical properties and the dynamic cell-extracellular matrix interactions that maintain the stem cell phenotype. This review discusses the progress made in the replication of stem cell niches from the anterior ocular segment by using bioengineering approaches and their therapeutic implications.

A Decellularized Human Limbal Scaffold for Limbal Stem Cell Niche Reconstruction

The transplantation of ex vivo expanded limbal epithelial progenitor cells (LEPCs) on amniotic membrane or fibrin gel is an established therapeutic strategy to regenerate the damaged corneal surface in patients with limbal stem cell deficiency (LSCD), but the long-term success rate is restricted. A scaffold with niche-specific structure and extracellular matrix (ECM) composition might have the advantage to improve long-term clinical outcomes, in particular for patients with severe damage or complete loss of the limbal niche tissue structure. Therefore, we evaluated the decellularized human limbus (DHL) as a biomimetic scaffold for the transplantation of LEPCs. Corneoscleral tissue was decellularized by sodium deoxycholate and deoxyribonuclease I in the presence or absence of dextran. We evaluated the efficiency of decellularization and its effects on the ultrastructure and ECM composition of the human corneal limbus. The recellularization of these scaffolds was studied by plating cultured LEPCs and limbal melanocytes (LMs) or by allowing cells to migrate from the host tissue following a lamellar transplantation ex vivo. Our decellularization protocol rapidly and effectively removed cellular and nuclear material while preserving the native ECM composition. In vitro recellularization by LEPCs and LMs demonstrated the good biocompatibility of the DHL and intrastromal invasion of LEPCs. Ex vivo transplantation of DHL revealed complete epithelialization as well as melanocytic and stromal repopulation from the host tissue. Thus, the generated DHL scaffold could be a promising biological material as a carrier for the transplantation of LEPCs to treat LSCD.

Pathophysiology of aniridia-associated keratopathy: Developmental aspects and unanswered questions

Aniridia, a rare congenital disease, is often characterized by a progressive, pronounced limbal insufficiency and ocular surface pathology termed aniridia-associated keratopathy (AAK). Due to the characteristics of AAK and its bilateral nature, clinical management is challenging and complicated by the multiple coexisting ocular and systemic morbidities in aniridia. Although it is primarily assumed that AAK originates from a congenital limbal stem cell deficiency, in recent years AAK and its pathogenesis has been questioned in the light of new evidence and a refined understanding of ocular development and the biology of limbal stem cells (LSCs) and their niche. Here, by consolidating and comparing the latest clinical and preclinical evidence, we discuss key unanswered questions regarding ocular developmental aspects crucial to AAK. We also highlight hypotheses on the potential role of LSCs and the ocular surface microenvironment in AAK. The insights thus gained lead to a greater appreciation for the role of developmental and cellular processes in the emergence of AAK. They also highlight areas for future research to enable a deeper understanding of aniridia, and thereby the potential to develop new treatments for this rare but blinding ocular surface disease.

Melanocytes as emerging key players in niche regulation of limbal epithelial stem cells

PURPOSE

Limbal melanocytes (LMel) represent essential components of the corneal epithelial stem cell niche and are known to protect limbal epithelial stem/progenitor cells (LEPCs) from UV damage by transfer of melanosomes. Here, we explored additional functional roles for LMel in niche homeostasis, immune regulation and angiostasis.

METHODS

Human corneoscleral tissues were morphologically analyzed in normal, inflammatory and wound healing conditions. The effects of LMel on LEPCs were analyzed in direct and indirect co-culture models using electron microscopy, immunocytochemistry, qRT-PCR, Western blotting and functional assays; limbal mesenchymal stromal cells and murine embryonic 3T3 fibroblasts served as controls. The immunophenotype of LMel was assessed by flow cytometry before and after interferon-γ stimulation, and their immunomodulatory properties were analyzed by mixed lymphocytes reaction, monocyte adhesion assays and cytometric bead arrays. Their angiostatic effects on human umbilical cord endothelial cells (HUVECs) were evaluated by proliferation, migration, and tube formation assays.

RESULTS

LMel and LEPCs formed structural units in the human limbal stem cell niche in situ, which could be functionally replicated, including melanosome transfer, by co-cultivation in vitro. LMel supported LEPCs during clonal expansion and during epithelial wound healing by stimulating proliferation and migration, and suppressed their differentiation through direct contact and paracrine effects. Under inflammatory conditions, LMel were increased in numbers and upregulated expression of ICAM-1 and MHC II molecules (HLA-DR), but lacked expression of HLA-G, -DP, -DQ and costimulatory molecules CD80 and CD86. They were also found to be potent suppressors of alloreactive T- cell proliferation and cytokine secretion, which largely depended on direct cell-cell interaction. Moreover, the LMel secretome exerted angiostatic activity by inhibiting vascular endothelial cell proliferation and capillary network formation.

CONCLUSION

These findings suggest that LMel are not only professional melanin-producing cells, but exert various non-canonical functions in limbal niche homeostasis by regulating LEPC maintenance, immune responses, and angiostasis. Their potent regulatory, immunomodulatory and anti-angiogenic properties may have important implications for future regenerative cell therapies.

Bioprinting of dual ECM scaffolds encapsulating limbal stem/progenitor cells in active and quiescent statuses

Limbal stem cell deficiency and corneal disorders are among the top global threats for human vision. Emerging therapies that integrate stem cell transplantation with engineered hydrogel scaffolds for biological and mechanical support are becoming a rising trend in the field. However, methods for high-throughput fabrication of hydrogel scaffolds, as well as knowledge of the interaction between limbal stem/progenitor cells (LSCs) and the surrounding extracellular matrix (ECM) are still much needed. Here, we employed digital light processing (DLP)-based bioprinting to fabricate hydrogel scaffolds encapsulating primary LSCs and studied the ECM-dependent LSC phenotypes. The DLP-based bioprinting with gelatin methacrylate (GelMA) or hyaluronic acid glycidyl methacrylate (HAGM) generated microscale hydrogel scaffolds that could support the viability of the encapsulated primary rabbit LSCs (rbLSCs) in culture. Immunocytochemistry and transcriptional analysis showed that the encapsulated rbLSCs remained active in GelMA-based scaffolds while exhibited quiescence in the HAGM-based scaffolds. The primary human LSCs encapsulated within bioprinted scaffolds showed consistent ECM-dependent active/quiescent statuses. Based on these results, we have developed a novel bioprinted dual ECM 'Yin-Yang' model encapsulating LSCs to support both active and quiescent statues. Our findings provide valuable insights towards stem cell therapies and regenerative medicine for corneal reconstruction.

Characterisation of corneas following different time and storage methods for their use as a source of stem-like limbal epithelial cells

The transplantation of expansions of limbal epithelial stem cells (LESC) remains one of the most efficient therapies for the treatment of limbal stem cell deficiency (LSCD) to date. However, the available donor corneas are scarce, and the corneas conserved for long time, under hypothermic conditions (after 7 days) or in culture (more than 28 days), are usually discarded due to poor viability of the endothelial cells. To establish an objective criterion for the utilisation or discarding of corneas as a source of LESC, we characterized, by immunohistochemistry analysis, donor corneas conserved in different conditions and for different periods of time. We also studied the potency of LESCs isolated from these corneas and maintained in culture up to 3 cell passages. We hoped that the study of markers of LESCs present in both the corneoscleral histological sections and the cell cultures would show the adequacy of the methods used for cell isolation and how fit the LESC enrichment of the obtained cell populations to be expanded was. Thus, the expressions of markers of the cells residing in the human limbal and corneal epithelium (cytokeratin CK15 and CK12, vimentin, Collagen VII, p63α, ABCG2, Ki67, Integrin β4, ZO1, and melan A) were analysed in sections of corneoscleral tissues conserved in hypothermic conditions for 2-9 days with post-mortem time (pmt) < 8 h or for 1 day with pmt > 16 h, and in sclerocorneal rims maintained in an organ culture medium for 29 days. Cell populations isolated from donor corneoscleral tissues were also assessed based on these markers to verify the adequacy of isolation methods and the potential of expanding LESCs from these tissues. Positivity for several putative stem cell markers such as CK15 and p63α was detected in all corneoscleral tissues, although a decrease was recorded in the ones conserved for longer times. The barrier function and the ability to adhere to the extracellular matrix were maintained in all the analysed tissues. In limbal epithelial cell cultures, a simultaneous decrease in the melan A melanocyte marker and the putative stem cell markers was detected, suggesting a close relationship between the melanocytes and the limbal stem cells of the niche. Holoclones stained with putative stem cell markers were obtained from long-term, hypothermic, stored sclerocorneal rims. The results showed that the remaining sclerocorneal rims after corneal transplantation, which were conserved under hypothermic conditions for up to 7 days and would have been discarded at a first glance, still maintained their potential as a source of LESC cultures.

Stem Cell Niche Microenvironment: Review

The cornea comprises a pool of self-regenerating epithelial cells that are crucial to preserving clarity and visibility. Limbal epithelial stem cells (LESCs), which live in a specialized stem cell niche (SCN), are crucial for the survival of the human corneal epithelium. They live at the bottom of the limbal crypts, in a physically enclosed microenvironment with a number of neighboring niche cells. Scientists also simplified features of these diverse microenvironments for more analysis in situ by designing and recreating features of different SCNs. Recent methods for regenerating the corneal epithelium after serious trauma, including burns and allergic assaults, focus mainly on regenerating the LESCs. Mesenchymal stem cells, which can transform into self-renewing and skeletal tissues, hold immense interest for tissue engineering and innovative medicinal exploration. This review summarizes all types of LESCs, identity and location of the human epithelial stem cells (HESCs), reconstruction of LSCN and artificial stem cells for self-renewal.

Canonical NF-κB signaling maintains corneal epithelial integrity and prevents corneal aging via retinoic acid

Disorders of the transparent cornea affect millions of people worldwide. However, how to maintain and/or regenerate this organ remains unclear. Here, we show that Rela (encoding a canonical NF-κB subunit) ablation in K14+ corneal epithelial stem cells not only disrupts corneal regeneration but also results in age-dependent epithelial deterioration, which triggers aberrant wound-healing processes including stromal remodeling, neovascularization, epithelial metaplasia, and plaque formation at the central cornea. These anomalies are largely recapitulated in normal mice that age naturally. Mechanistically, Rela deletion suppresses expression of Aldh1a1, an enzyme required for retinoic acid synthesis from vitamin A. Retinoic acid administration blocks development of ocular anomalies in Krt14-Cre; Relaf/f mice and naturally aged mice. Moreover, epithelial metaplasia and plaque formation are preventable by inhibition of angiogenesis. This study thus uncovers the major mechanisms governing corneal maintenance, regeneration, and aging and identifies the NF-κB-retinoic acid pathway as a therapeutic target for corneal disorders.

Can Human Oral Mucosa Stem Cells Differentiate to Corneal Epithelia?

Human oral mucosa stem cells (hOMSCs) arise from the neural crest, they can self-renew, proliferate, and differentiate to several cell lines and could represent a good source for application in tissue engineering. Because of their anatomical location, hOMSCs are easy to isolate, have multilineage differentiation capacity and express embryonic stem cells markers such as—Sox2, Oct3/4 and Nanog. We have used SHEM (supplemented hormonal epithelial medium) media and cultured hOMSCs over human amniotic membrane and determined the cell’s capacity to differentiate to an epithelial-like phenotype and to express corneal specific epithelial markers—CK3, CK12, CK19, Pan-cadherin and E-cadherin. Our results showed that hOMSCs possess the capacity to attach to the amniotic membrane and express CK3, CK19, Pan-Cadherin and E-Cadherin without induction with SHEM media and expressed CK12 or changed the expression pattern of E-Cadherin to a punctual-like feature when treated with SHEM media. The results observed in this study show that hOMSCs possess the potential to differentiate toward epithelial cells. In conclusion, our results revealed that hOMSCs readily express markers for corneal determination and could provide the ophthalmology field with a therapeutic alternative for tissue engineering to achieve corneal replacement when compared with other techniques. Nevertheless, further studies are needed to develop a predictable therapeutic alternative for cornea replacement.

A Comparison of Methods for Isolation of Limbal Niche Cells: Maintenance of Limbal Epithelial Stem/Progenitor Cells

Purpose

Limbal niche cells (LNCs) play a vital role in the maintenance of limbal epithelial stem/progenitor cells (LESCs). Four methods have been reported to isolate and expand LNCs: digestion by collagenase alone (C-LNC), collagenase following dispase removal of the limbal epithelium (DC-LNC), dissection of dispase-isolated limbal epithelial sheets (D-LNC), and explant cultures of limbal stromal tissues (Ex-LNC). This study aimed to isolate LNCs using those four methods and to compare their capacity to maintain LESCs.

Methods

LNCs were isolated from the rat corneal limbus by the following methods: C-LNC, DC-LNC, D-LNC, and Ex-LNC. Quantitative real-time PCR and immunofluorescence staining were used to analyze the expression of embryonic stem cell (ESC) markers. The ability to maintain LESCs was assessed on the basis of colony-forming capacity and the expression of progenitor, proliferation, and differentiation markers in three-dimensional (3D) Matrigel and Transwell systems. Notch signaling of LESCs supported by different LNCs in Transwell inserts was analyzed by quantitative real-time PCR.

Results

DC-LNCs exhibited lower expression of CK12 during isolation and expansion. Among P4-expanded LNCs, DC-LNCs expressed significantly higher levels of Sox2, Oct4, Nanog, and N-cadherin than C-LNCs, D-LNCs, and Ex-LNCs. Compared with other LNCs, DC-LNCs were more effective in maintaining LESCs with higher holoclone-forming efficiency, greater expression of ΔNp63α and Ki67, and lower expression of CK12. DC-LNCs were also more capable of downregulating Notch signaling of LESCs.

Conclusions

DC-LNCs were more effective in expressing ESC markers and maintaining LESCs compared to other LNCs. This study identifies an optimal method for the isolation of LNCs in tissue engineering and ocular surface reconstruction.

Molecular identity of human limbal heterogeneity involved in corneal homeostasis and privilege

PURPOSE

The corneal limbus maintains the homeostasis, immune and angiogenic privilege of cornea. This study aimed to depict the landscape of human limbal tissues by single-cell RNA sequencing (scRNA-seq).

METHODS

Single cells of human limbus collected from donor corneas were subjected to 10x scRNA-seq, followed by clustering cell types through the t-distributed stochastic neighbor embedding (t-SNE) and unbiased computational informatic analysis. Immunofluorescent staining was performed using human corneas to validate the analysis results.

RESULTS

47,627 cells acquired from six human limbal tissues were collected and subjected to scRNA-seq. 14 distinct clusters were identified and 8 cell types were annotated with representative markers. In-depth dissection revealed three limbal epithelial cell subtypes and refined the X-Y-Z hypothesis of corneal epithelial maintenance. We further unveiled two cell states with higher stemness (TP63⁺ and CCL20⁺ cells), and two other differentiated cell states (GPHA2⁺ and KRT6B ⁺ cells) in homeostatic limbal stem/progenitor cells (LSPCs) that differ in transcriptional profiles. Cell-cell communication analysis revealed the central role of LSPCs and their bidirectional regulation with various niche cells. Moreover, comparative analysis between limbus and skin deciphered the pivotal contribution of limbal immune cells, vascular and lymphatic endothelial cells to corneal immune and angiogenic privilege.

CONCLUSIONS

The human limbus atlas provided valuable resources and foundations for understanding corneal biology, disease and potential interventions.

Limbal stem cell diseases

The function of limbal stem/progenitor cells (LSCs) is critical to maintain corneal epithelial homeostasis. Many external insults and intrinsic defects can be deleterious to LSCs and their niche microenvironment, resulting in limbal stem cell dysfunction or deficiency (LSCD). Ocular comorbidities, frequent in eyes with LSCD, can exacerbate the dysfunction of residual LSCs, and limit the survival of transplanted LSCs. Clinical presentation and disease evolution vary among different etiologies of LSCD. New ocular imaging modalities and molecular markers are now available to standardize the diagnosis criteria and stage the severity of the disease. Medical therapies may be sufficient to reverse the disease if residual LSCs are present. A stepwise approach should be followed to optimize the ocular surface, eliminate the causative factors and treat comorbid conditions, before considering surgical interventions. Furthermore, surgical options are selected depending on the severity and laterality of the disease. The standardized diagnostic criteria to stage the disease is necessary to objectively evaluate and compare the efficacy of the emerging customized therapies.

[Simple limbal epithelial transplantation (SLET) : A simple technique for the treatment of unilateral complete limbal stem cell deficiency. Video article]

OBJECTIVE

The aim of simple limbal epithelial transplantation (SLET) is the regeneration of the corneal surface in unilateral complete limbal stem cell deficiency (LSCD).

INDICATIONS

SLET is indicated for unilateral complete LSCD.

CONTRAINDICATIONS

Contraindications include bilateral LSCD, severe corneal thinning, pronounced keratoconjunctivitis sicca, chronic inflammatory condition of the ocular surface, malposition of the eyelids and pronounced adhesions of the conjunctiva with trichiasis.

SURGICAL TECHNIQUE

A 1‑h biopsy is obtained from the superior limbus of the healthy donor eye. A 360° peritomy is performed on the LSCD eye and pannus tissue covering the cornea is removed. An amniotic membrane (AM) is glued to the corneal surface with fibrin. The donor tissue is then divided into 8-10 small pieces, which are placed on the AM sparing the visual axis and fixed by fibrin glue. A contact lens is placed on the eye. A surgical video, which is available online, shows the surgical technique in detail.

FOLLOW-UP

Examinations are necessary within the first postoperative week and 1 month after SLET, then every 3 months within the first postoperative year. Antibiotic eye drops should be applied 5 times daily until complete epithelialization. Topical steroids should be applied 6 times daily in the early postoperative period and can be tapered thereafter. Artificial tears can improve epithelial healing. Ideally, all eye drops should be preservative-free. The contact lens can be removed after 7-10 days. The AM dissolves within a few weeks. An epithelialization of the corneal surface can be observed by the second postoperative week.

EVIDENCE

A recent systematic review reported a stable epithelialized corneal surface in 78% of SLET cases after 1.5 years. An improvement of visual acuity of at least two lines was found in 69% of SLET cases.

Transcriptome analysis identifies the differentially expressed genes related to the stemness of limbal stem cells in mice

Limbal stem cells (LSCs) reside in the basal layer of limbal epithelial cells (LECs). They are crucial for maintenance of corneal epithelium homeostasis and corneal wound healing. Their stemness is determined by their gene expression pattern. Despite of several positive identifiers have been reported, the unique biomarker for LSCs still remain elusive. Differentially expressed genes (DEGs) between stem cells and differentiated cells affect the fate of stem cells via specific signaling pathway. In order to understand the DEGs in the LSCs, RNA-seq was firstly conducted using a mouse model. A total of 1907 up-regulated DEGs and 395 down-regulated DEGs were identified in the limbus (L) compared to central cornea (CC) and conjunctiva (Cj). Reliability of the expression of genes from RNA-seq analysis was evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) and immunofluorescence staining. The expression pattern of putative biomarkers was considered to be age-related. In up-regulated DEGs GO analysis, 570 gene ontology (GO) terms were significantly enriched. Five groups of genes related with biological processes from these significantly enriched GO terms comprised ionic transport, regulation of tissue development, muscle contraction, visual perception, and cell adhesion, which were clustered as a weighted similar network. Whereas, in down-regulated DEGs GO analysis, 61 GO terms were significantly enriched and only one group of ATP biosynthesis and metabolic process were clustered. Furthermore, we identified 55 signaling pathways by the Kyoto Encyclopedia of Genes and Genomes (KEGG) database based on up-regulated genes and 14 KEGG pathways based on down-regulated genes. In this study, we provide a landscape of the expression of putative LSCs biomarkers and stemness-related signaling pathways in a mouse model. Our findings could aid in the identification of LSC niche factors that may be related to the stemness of the LSCs.

Spatial correlations constrain cellular lifespan and pattern formation in corneal epithelium homeostasis

Homeostasis in adult tissues relies on the replication dynamics of stem cells, their progenitors and the spatial balance between them. This spatial and kinetic coordination is crucial to the successful maintenance of tissue size and its replenishment with new cells. However, our understanding of the role of cellular replicative lifespan and spatial correlation between cells in shaping tissue integrity is still lacking. We developed a mathematical model for the stochastic spatial dynamics that underlie the rejuvenation of corneal epithelium. Our model takes into account different spatial correlations between cell replication and cell removal. We derive the tradeoffs between replicative lifespan, spatial correlation length, and tissue rejuvenation dynamics. We determine the conditions that allow homeostasis and are consistent with biological timescales, pattern formation, and mutants phenotypes. Our results can be extended to any cellular system in which spatial homeostasis is maintained through cell replication.

Chondroitin Sulfate as a Potential Modulator of the Stem Cell Niche in Cornea

Chondroitin sulfate (CS) is an important component of the extracellular matrix in multiple biological tissues. In cornea, the CS glycosaminoglycan (GAG) exists in hybrid form, whereby some of the repeating disaccharides are dermatan sulfate (DS). These CS/DS GAGs in cornea, through their presence on the proteoglycans, decorin and biglycan, help control collagen fibrillogenesis and organization. CS also acts as a regulatory ligand for a spectrum of signaling molecules, including morphogens, cytokines, chemokines, and enzymes during corneal growth and development. There is a growing body of evidence that precise expression of CS or CS/DS with specific sulfation motifs helps define the local extracellular compartment that contributes to maintenance of the stem cell phenotype. Indeed, recent evidence shows that CS sulfation motifs recognized by antibodies 4C3, 7D4, and 3B3 identify stem cell populations and their niches, along with activated progenitor cells and transitional areas of tissue development in the fetal human elbow. Various sulfation motifs identified by some CS antibodies are also specifically located in the limbal region at the edge of the mature cornea, which is widely accepted to represent the corneal epithelial stem cell niche. Emerging data also implicate developmental changes in the distribution of CS during corneal morphogenesis. This article will reflect upon the potential roles of CS and CS/DS in maintenance of the stem cell niche in cornea, and will contemplate the possible involvement of CS in the generation of eye-like tissues from human iPS (induced pluripotent stem) cells.

The progress in corneal translational medicine

Cornea tissue is in high demand by tissue donation centres globally, and thus tissue engineering cornea, which is the main topic of corneal translational medicine, can serve as a limitless alternative to a donated human cornea tissue. Tissue engineering aims to produce solutions to the challenges associated with conventional cornea tissue, including transplantation and use of human amniotic membrane (HAM), which have issues with storage and immune rejection in patients. Accordingly, by carefully selecting biomaterials and fabrication methods to produce these therapeutic tissues, the demand for cornea tissue can be met, with an improved healing outcome for recipients with less associated harmful risks. In this review paper, we aim to present the recent advancements in the research and clinical applications of cornea tissue, applications including biomaterial selection, fabrication methods, scaffold structure, cellular response to these scaffolds, and future advancements of these techniques.

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

Combined Epithelial, Stromal, and Anterior Chamber Ingrowth After Penetrating Keratoplasty

PURPOSE

To present a case of primary graft failure after penetrating keratoplasty found to have epithelial ingrowth into the host stroma on histopathologic analysis.

METHODS

This is a single observational case report.

RESULTS

We herein describe the clinical course of a case of primary graft failure after penetrating keratoplasty. The corneal button was sent for histopathologic analysis. Analysis of the patient's failed corneal button revealed circumferential epithelial full-thickness wound invasion and stromal epithelial invasion into corneal stroma.

CONCLUSIONS

Based on histopathologic analysis and this patient's presentation, the stromal ingrowth followed recipient epithelial invasion of the wound and stromal invasion through clefts in the donor corneal edges. Cases of primary graft failure should be assessed for histopathologic evidence of epithelial stromal ingrowth, despite its rarity. To our knowledge, epithelial ingrowth into the corneal donor stroma after penetrating keratoplasty has not been previously reported.

Expansion of Human Limbal Epithelial Stem/Progenitor Cells Using Different Human Sera: A Multivariate Statistical Analysis

Transplantation of human cultured limbal epithelial stem/progenitor cells (LESCs) has demonstrated to restore the integrity and functionality of the corneal surface in about 76% of patients with limbal stem cell deficiency. However, there are different protocols for the expansion of LESCs, and many of them use xenogeneic products, being a risk for the patients’ health. We compared the culture of limbal explants on the denuded amniotic membrane in the culture medium—supplemental hormone epithelial medium (SHEM)—supplemented with FBS or two differently produced human sera. Cell morphology, cell size, cell growth rate, and the expression level of differentiation and putative stem cell markers were examined. Several bioactive molecules were quantified in the human sera. In a novel approach, we performed a multivariate statistical analysis of data to investigate the culture factors, such as differently expressed molecules of human sera that specifically influence the cell phenotype. Our results showed that limbal cells cultured with human sera grew faster and contained similar amounts of small-sized cells, higher expression of the protein p63α, and lower of cytokeratin K12 than FBS cultures, thus, maintaining the stem/progenitor phenotype of LESCs. Furthermore, the multivariate analysis provided much data to better understand the obtaining of different cell phenotypes as a consequence of the use of different culture methodologies or different culture components.

The limbus: Structure and function

Limbal function is a key determinant of corneal epithelial integrity. Lineage tracing studies in mice have highlighted that the centripetal movement of epithelial progenitors from the limbus drives both the steady-state maintenance of the corneal epithelium and its regeneration following injury. It is well established that this is facilitated by a population of limbal epithelial stem cells within the limbus. It is becoming increasingly apparent that the behaviour of these stem cells and their ability to respond to the needs of the tissue are closely linked to their immediate microenvironment - the stem cell niche. Increasing understanding of the structural features of this niche and the signalling networks that they coordinate is required to enhance the therapeutic application of these cells in the treatment of limbal stem cell deficiency. Importantly, an improved characterisation of the hierarchy of limbal epithelial progenitors using both new and old putative markers will enable a greater appreciation for the effects of many of these limbal niche factors on stem cell fate.

A Small-Molecule Wnt Mimic Improves Human Limbal Stem Cell Ex Vivo Expansion

Ex vivo cultured limbal stem/progenitor cells is an effective alternative to other surgical treatments for limbal stem cell deficiency, but a standard xenobiotic-free method for culturing the LSCs in vitro needs to be optimized. Because Wnt ligands are required for LSC expansion and preservation in vitro, to create a small-molecule Wnt mimic, we created a consolidated compound by linking a Wnt inhibitor that binds to the Wnt co-receptor Frizzled to a peptide derived from the N-terminal Dickkopf-1 that binds to Lrp (low-density lipoprotein receptor-related protein) 5/6, another Wnt co-receptor. This Wnt mimic not only enhances cellular Wnt signaling activation, but also improves the progenitor cell phenotype of in vitro cultured limbal epithelial cells. As the maintenance of stem cell characteristics in the process of culture expansion is essential for the success of ocular surface reconstruction, the small molecules generated in this study may be helpful in the development of pharmaceutical reagents for treating corneal wounds.

A validated porcine corneal organ culture model to study the limbal response to corneal epithelial injury

Limbal epithelial stem cells are required for the maintenance and repair of the corneal epithelial surface. The difficulty in obtaining human corneal tissue for research purposes means that animal models for studying the corneal and limbal epithelium are extremely useful. Porcine corneal tissue represents an attractive experimental model, however, functional analysis of the limbal epithelial cell population is needed to validate the use of this tissue. Single cell clonal analysis revealed that holoclone-generating cells were enriched in the limbus as compared with the central cornea (38.3% vs 8.3%) and that label-retaining cells were also enriched in the limbus and compared with the central cornea (44.7 ± 6.4 vs 4.7 ± 1.5). Furthermore, it was demonstrated that in a 3D-printed organ culture system, porcine tissue was capable of maintaining and healing the corneal epithelium. Ki67 staining of corneal sections revealed that in response to central epithelial wounding, a greater proportion of progenitors in the basal limbal epithelium enter an actively dividing state. The authors present a comprehensively validated model system for studying the interactions between limbal niche factors and limbal epithelial stem cell fate.

Canine Corneal Stromal Cells Have Multipotent Mesenchymal Stromal Cell Properties In Vitro

The objective of this study was to determine whether corneal stromal cells (CSCs) from the limbal and central corneal stroma in dogs have multipotent mesenchymal stem/stromal cell (MSC) properties, and whether this cell population can be differentiated into keratocyte-like cells (KDCs). Normal, donated, mesocephalic dog corneas were used to isolate CSC in vitro. Immunohistochemistry demonstrated a distinct population of CD90 expressing cells in the anterior stroma throughout the limbal and central cornea. CSC could be cultured from both the limbal and central cornea and the culture kinetics showed a progenitor cell profile. The CSC expressed stem cell markers CD90, CD73, CD105, N-cadherin, and Pax6, while CD34 was negative. Limbal and central CSC differentiated into osteoblasts, chondrocytes, and adipocytes confirming their multipotency. Coculturing allogeneic peripheral blood mononuclear cells (PBMCs) with limbal CSCs did not affect baseline PBMC proliferation indicating a degree of innate immune privilege. Limbal CSC could be differentiated into KDCs that expressed Keratocan, Lumican, and ALDH1A3 and downregulated Pax6 and N-cadherin. In conclusion, canine CSCs have multipotent MSC properties similarly described in humans and could serve as a source of cells for cell therapy and studying corneal diseases.

Dynamic spatiotemporal expression pattern of limbal stem cell putative biomarkers during mouse development

Limbal stem cells (LSCs), a subpopulation of limbal epithelial basal cells, are crucial to the homeostasis and wound healing of corneal epithelium. The identification and isolation of LSCs remains a challenge due to lack of specific LSCs biomarkers. In this study, Haematoxylin-eosin (HE), 4', 6-diamidino-2-phenylindole (DAPI), and immunohistochemistry (IHC) stains were performed on the pre- and post-natal limbus tissues of mice which has the advantage of more controllable in term of sampling age relative to human origin. By morphological analysis, we supported that there is an absence of the Palisades of Vogt (POV) in the mouse. The development of prenatal and neonatal cornea was dominated by its stroma, whereas after eyelids opened at P14, the corneal epithelial cells (CECs) quickly go stratification in response to the liquid-air interface. Based on IHC staining, we found that the expression of LSCs putative biomarkers in limbal epithelial basal cells appeared in chronological order as follows: Vim = p63 > CK14 > CK15 (where = represents same time; > represents earlier), and in corneal epithelial basal cells were weakened in chronological order as follows: Vim > p63 > CK15 > CK14, which might also represent the stemness degree. Furthermore, the dynamic spatial expression of the examined LSCs putative biomarkers during mouse development also implied a temporal restriction. The expression of Vim in epithelial cells of mouse ocular surface occurred during E12-E19 only. The expression of CK15 was completely undetectable in CECs after P14, whereas the others putative molecular markers of LSCs, such as p63 and CK14, still remained weak expression, suggesting that CK15 was suitable to serve as the mouse LSCs biomarkers after P14. In this study, our data demonstrated the dynamic spatiotemporal expression pattern of LSCs putative biomarkers in mouse was age-related and revealed the time spectrum of the expression of LSCs in mouse, which adds in our knowledge by understanding the dynamic expression pattern of biomarkers of stem cells relate to maintenance of their stemness.

Single-Cell RNA Transcriptome Helps Define the Limbal/Corneal Epithelial Stem/Early Transit Amplifying Cells and How Autophagy Affects This Population

Purpose

Single-cell RNA-sequencing (scRNA-seq) was used to interrogate the relatively rare stem (SC) and early transit amplifying (TA) cell populations in limbal/corneal epithelia from wild-type and autophagy-compromised mice.

Methods

We conducted scRNA-seq on ocular anterior segmental tissue from wild-type and beclin 1–deficient (beclin1^+/−) mice, using a 10X Gemomics pipeline. Cell populations were distinguished by t-distributed stochastic neighbor embedding. Seurat analysis was conducted to compare gene expression profiles between these two groups of mice. Differential protein expression patterns were validated by immunofluorescence staining and immunoblotting.

Results

Unbiased clustering detected 10 distinct populations: three clusters of mesenchymal and seven clusters of epithelial cells, based on their unique molecular signatures. A discrete group of mesenchymal cells expressed genes associated with corneal stromal SCs. We identified three limbal/corneal epithelial cell subpopulations designated as stem/early TA, mature TA, and differentiated corneal epithelial cells. Thioredoxin-interacting protein and PDZ-binding kinase (PBK) were identified as novel regulators of stem/early TA cell quiescence. PBK arrested corneal epithelial cells in G2/M phase of the cell cycle. Beclin1^+/− mice displayed a decrease in proliferation-associated (Ki67, Lrig1) and stress-response (H2ax) genes. The most increased gene in beclin1^+/− mice was transcription factor ATF3, which negatively regulates limbal epithelial cell proliferation.

Conclusions

Establishment of a comprehensive atlas of genes expressed by stromal and epithelial cells from limbus and cornea forms the foundation for unraveling regulatory networks among these distinct tissues. Similarly, scRNA-seq profiling of the anterior segmental epithelia from wild-type and autophagy-deficient mice provides new insights into how autophagy influences proliferation in these tissues.

Long-term outcomes of conjunctival limbal autograft in patients with unilateral total limbal stem cell deficiency

PURPOSE

To investigate the long-term clinical outcomes of conjunctival limbal autograft (CLAU) in patients with unilateral total limbal stem cell deficiency (LSCD).

METHODS

In this retrospective interventional case series, the medical charts of patients with unilateral total LSCD were reviewed. Patients who underwent CLAU and no other allograft ocular stem cell transplantation with a minimum follow-up of 1 year were included. Main outcome measures were ocular surface stability, best-corrected visual acuity (BCVA), and postoperative complications.

RESULTS

27 eyes fulfilled the inclusion criteria with a mean follow-up period of 49.8 ± 36.6 months (4.15 years; range 12-186.72 months; 1-15.56 years). Ocular surface stability was achieved in 77.8% (n = 21) of eyes at last follow-up, while 22.2% (n = 6) developed partial surface failure. Optical penetrating or deep lamellar anterior keratoplasty was performed in 44.45% (n = 12). BCVA improved from 1.42 ± 0.95 mean LogMAR (equivalent to 20/400) preoperatively to 0.53 ± 0.47 mean LogMAR (equivalent to 20/70) at last follow-up (p < 0.001). BCVA ≥20/40 was achieved in 44.45% (n = 12) at last follow-up. Microbial keratitis occurred in 14.81% (n = 4). Ocular hypertension secondary to corticosteroid use developed in 25.9% (7/27) eyes. There were no other complications in the donor or recipient eyes.

CONCLUSIONS

CLAU can provide long-term ocular surface stability and successful visual outcomes in patients with unilateral LSCD.