Plasticity of ocular surface epithelia: Using a murine model of limbal stem cell deficiency to delineate metaplasia and transdifferentiation

Navigating the path to corneal healing success and challenges: a comprehensive overview

The cornea serves to protect the eye from external insults and refracts light to the retina. Maintaining ocular homeostasis requires constant epithelial renewal and an efficient healing process following injury. Corneal wound healing is a dynamic process involving several key cell populations and molecular pathways. Immediately after a large corneal epithelial injury involving limbal stem cells, conjunctival epithelial cells migrate toward the center of the wound guided by the newly formed electrical field (EF). Proliferation and transdifferentiation play a critical role in corneal epithelial regeneration. Corneal nerve endings migrate through the EF, connect with the migrating epithelial cells, and provide them with multiple growth factors. Finally, the migrated epithelial cells undergo differentiation, which is also regulated by corneal nerve endings. All these processes require energy and effective cellular cross-talk between different cell lines and extracellular matrix molecules. We provide an overview of the roles and interactions between corneal wound regeneration components that may help develop fascinating new targeted therapeutic strategies to enhance corneal wound healing with less injury-related corneal opacity and neovascularization.

Transdifferentiation of rat keratinocyte progenitors to corneal epithelial cells by limbal niche via the STAT3/PI3K/AKT signaling pathway

PURPOSE

To develop a method for enriching keratinocyte progenitor cells (KPCs) and establish a limbal niche (LN)-mediated transdifferentiation protocol of KPCs into corneal epithelial cells.

METHODS

Limbal niche cells (LNCs) were isolated from limbal tissues through enzymatic digestion and characterized. Conditioned medium from LNCs cultures was collected. KPCs were enriched by rapid adhesion of Matrigel and subsequently cultured in either an LNCs-conditioned medium supplemented with KSFM (LN-KS) or SHEM (LN-SH) for 14 days. Corneal-specific marker expression was assessed to evaluate transdifferentiation efficiency. Key transcription factors and signaling pathways involved in the transdifferentiation process were identified through single-cell and RNA sequencing, and were validated by western blot and quantitative real-time PCR.

RESULTS

Both LN-KS and LN-SH protocols successfully induced corneal epithelial cell transdifferentiation from KPCs, with LN-KS demonstrating higher efficiency in generating CK12 + and p63 + cells (p < 0.001). RNA sequencing analysis and western blot have revealed significant activation of STAT3 and PI3K/AKT signaling pathways. Inhibition of STAT3 blocked the activation of PI3K/AKT signaling pathway and impaired corneal epithelial cell transdifferentiation.

CONCLUSIONS

This study demonstrates the ability of LN to promote KPCs transdifferentiation into corneal epithelial cells in vitro, and this process is partially mediated by the STAT3/PI3K/AKT signaling pathway.

Induced pluripotent stem-cell-derived corneal epithelium for transplant surgery: a single-arm, open-label, first-in-human interventional study in Japan

BACKGROUND

The loss of corneal epithelial stem cells from the limbus at the edge of the cornea has severe consequences for vision, with the pathological manifestations of a limbal stem-cell deficiency (LSCD) difficult to treat. Here, to the best of our knowledge, we report the world's first use of corneal epithelial cell sheets derived from human induced pluripotent stem cells (iPSCs) to treat LSCD.

METHODS

This non-randomised, single-arm, clinical study involved four eyes of four patients with LSCD at the Department of Ophthalmology, Osaka University Hospital. They comprised a woman aged 44 years with idiopathic LSCD (patient 1), a man aged 66 years with ocular mucous membrane pemphigoid (patient 2), a man aged 72 years with idiopathic LSCD (patient 3), and a woman aged 39 years with toxic epidermal necrosis (patient 4). Allogeneic human iPSC-derived corneal epithelial cell sheets (iCEPSs) were transplanted onto affected eyes. This was done sequentially in two sets of HLA-mismatched surgeries, with patients 1 and 2 receiving low-dose cyclosporin and patients 3 and 4 not. The primary outcome measure was safety, ascertained by adverse events. These were monitored continuously throughout the 52-week follow-up period, and during an additional 1-year safety monitoring period. Secondary outcomes, reflective of efficacy, were also recorded. This study is registered with UMIN, UMIN000036539 and is complete.

FINDINGS

Patients were enrolled between June 17, 2019 and Nov 16, 2020. We had 26 adverse events during the 52-week follow-up period (consisting of 18 mild and one moderate event in treated eyes, and seven mild non-ocular events), with nine recorded in the additional 1-year safety monitoring period. No serious adverse events, such as tumourigenesis or clinical rejection, occurred during the whole 2-year observational period. At 52 weeks, secondary measures of efficacy showed that the disease stage had improved, corrected distance visual acuity was enhanced, and corneal opacification had diminished in all treated eyes. Corneal epithelial defects, subjective symptoms, quality-of-life questionnaire scores and corneal neovascularisation mostly improved or were unchanged. Overall, the beneficial efficacy outcomes achieved for patients 1 and 2 were better than those achieved for patients 3 and 4.

INTERPRETATION

iCEPS transplantation for LSCD was found to be safe throughout the study period. A larger clinical trial is planned to further investigate the efficacy of the procedure.

FUNDING

The Japan Agency for Medical Research and Development, the Ministry of Education, Culture, Sports, Science, and Technology-Japan, and the UK Biotechnology and Biological Sciences Research Council.

De-Differentiation of Corneal Epithelial Cells Into Functional Limbal Epithelial Stem Cells After the Ablation of Innate Stem Cells

Purpose

Regeneration after tissue injury is often associated with cell fate plasticity, which restores damaged or lost cells. Here, we examined the de-differentiation of corneal epithelial cells (CECs) into functional limbal epithelial stem cells (LESCs) after the ablation of innate stem cells.

Methods

The regeneration of LESCs after the ablation of innate LESCs was identified by a set of markers: ApoE+/Cx43low/CK12-. CK14-CreERT2 or Slc1a3-CreERT mice were crossed with reporter mice to trace the fate of CECs. YAP-TEAD inhibitor verteporfin (VTP) and LATS inhibitor TRULI were used to examine the role of Hippo/YAP pathway in the de-differentiation of CECs.

Results

LESCs-ablation cornea showed to be functionally normal, including the maintenance of corneal transparency, prevention of conjunctivalization, and wound healing rate equivalent to that of normal cornea. ApoE+/Cx43low/CK12- LESCs regenerated at the limbus at 6 days after the ablation of innate stem cells, and maintained for at least 6 months. Corneal epithelial lineage tracing showed that CECs migrated back to the limbus after the ablation of innate stem cells, and de-differentiated into active and quiescent LESCs (aLESCs and qLESCs), which participated in corneal epithelial homeostasis and wound healing, respectively, like their innate counterparts. However, when the limbus niche was destroyed by NaOH (1 M, 5 seconds), CECs that occupied the limbus could not de-differentiate into ApoE+/Cx43low/CK12- LESCs and cornea developed into conjunctivalization. In addition, the protein level and activity of YAP increased at the early stage (1-2 days) after the ablation of limbal epithelium, and decreased when the de-differentiation occurred. The YAP-TEAD inhibitor VTP promoted the de-differentiation, whereas LATS inhibitor TRULI inhibited the de-differentiation of CECs. However, the persistent activation of YAP prevented the de-differentiation of CECs after an additional NaOH burn to the limbal stroma, and VTP could not rescue the capacity of CECs to de-differentiate into LESCs.

Conclusions

These results reveal the de-differentiation of CECs into functional LESCs after the ablation of innate stem cells, and suggest potential role of Hippo/YAP pathway in the de-differentiation of CECs in vivo.

A detailed survey of the murine limbus, its stem cell distribution, and its boundaries with the cornea and conjunctiva

The narrow intersection between the cornea and conjunctiva, otherwise known as the limbus, is purported to harbor stem cells (SCs) that replenish the ocular surface epithelium throughout life. Damage to this site or depletion of its SCs can have dire consequences for eye health and vision. To date, various SC and keratin proteins have been used to identify the limbus, however, none could definitively mark its boundaries. Herein, we use the mouse as a model system to investigate whether structural and phenotypic features can be used to define the limbus and its boundaries with adjacent tissues. We demonstrate that differentially aligned blood and lymphatic vessels, intraepithelial nerves, and basal epithelial cellular and nuclei dimensions can be used as structural landmarks of the limbus. Identification of these features enabled approximation of the limbal expanse, which varied across distinct ocular surface quadrants, with the superior nasal and inferior temporal limbus being the widest and narrowest, respectively. Moreover, label-retaining SCs were unevenly distributed across the ocular circumference, with increased numbers in the superior temporal and inferior temporal moieties. These findings will heighten our current understanding of the SC niche, be beneficial for accurately predicting SC distribution to improve their isolation and devising efficacious cell therapies, and importantly, aid the ongoing search for novel SC markers.

Development and characterization of a preclinical mouse model of alkali-induced limbal stem cell deficiency

PURPOSE

Limbal stem cell deficiency (LSCD) secondary to ocular surface alkali burn is a blinding condition that features corneal conjunctivalization. Mechanistic insights into its pathophysiology are lacking. Here, we developed a mouse model that recapitulates human disease to comprehensively delineate the clinicopathological features of a conjunctivalized cornea.

METHODS

LSCD was induced in the right eyes of 6-8-week-old C57BL/6 male and female mice (n = 151) by topical administration of 0.25N sodium hydroxide on the cornea. Uninjured left eyes served as controls. Clinical, histological, phenotypic, molecular, and immunological assessments were performed at multiple time-points over 6-months.

RESULTS

Clinically, alkali burn caused persistent corneal opacity (p = 0.0014), increased punctate staining (p = 0.0002), and reduced epithelial thickness (p = 0.0082) compared to controls. Total LSCD was confirmed in corneal whole mounts by loss of K12 protein (p < 0.0001) and mRNA expression (p = 0.0090). Instead, K8+, K13+, K15+ and MUC5AC+ conjunctival epithelia prevailed. 20 % of injured corneas developed islands of K12+ epithelia, suggesting epithelial transdifferentiation. Squamous metaplasia was detected in 50 % of injured corneas. Goblet cell density peaked early post-injury but decreased over time (p = 0.0047). Intraepithelial corneal basal nerve density remained reduced even at 6-months post-injury (p = 0.0487).

CONCLUSIONS

We developed and comprehensively characterized a preclinical mouse model of alkali-induced LSCD. Understanding the pathophysiological processes that transpire on the ocular surface in LSCD is key to discovering, testing, and advancing biological and pharmacological interventions that can be dispensed prior to or in conjunction with stem cell therapy to rehabilitate the cornea and restore vision.

Biologicals and Biomaterials for Corneal Regeneration and Vision Restoration in Limbal Stem Cell Deficiency

The mammalian cornea is decorated with stem cells bestowed with the life-long task of renewing the epithelium, provided they remain healthy, functional, and in sufficient numbers. If not, a debilitating disease known as limbal stem cell deficiency (LSCD) can develop causing blindness. Decades after the first stem cell (SC) therapy is devised to treat this condition, patients continue to suffer unacceptable failures. During this time, improvements to therapeutics have included identifying better markers to isolate robust SC populations and nurturing them on crudely modified biological or biomaterial scaffolds including human amniotic membrane, fibrin, and contact lenses, prior to their delivery. Researchers are now gathering information about the biomolecular and biomechanical properties of the corneal SC niche to decipher what biological and/or synthetic materials can be incorporated into these carriers. Advances in biomedical engineering including electrospinning and 3D bioprinting with surface functionalization and micropatterning, and self-assembly models, have generated a wealth of biocompatible, biodegradable, integrating scaffolds to choose from, some of which are being tested for their SC delivery capacity in the hope of improving clinical outcomes for patients with LSCD.

Cell therapy in the cornea: The emerging role of microenvironment

The cornea is an ideal testing field for cell therapies. Its highly ordered structure, where specific cell populations are sequestered in different layers, together with its accessibility, has allowed the development of the first stem cell-based therapy approved by the European Medicine Agency. Today, different techniques have been proposed for autologous and allogeneic limbal and non-limbal cell transplantation. Cell replacement has also been attempted in cases of endothelial cell decompensation as it occurs in Fuchs dystrophy: injection of cultivated allogeneic endothelial cells is now in advanced phases of clinical development. Recently, stromal substitutes have been developed with excellent integration capability and transparency. Finally, cell-derived products, such as exosomes obtained from different sources, have been investigated for the treatment of severe corneal diseases with encouraging results. Optimization of the success rate of cell therapies obviously requires high-quality cultured cells/products, but the role of the surrounding microenvironment is equally important to allow engraftment of transplanted cells, to preserve their functions and, ultimately, lead to restoration of tissue integrity and transparency of the cornea.

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.

The Potential Reversible Transition between Stem Cells and Transient-Amplifying Cells: The Limbal Epithelial Stem Cell Perspective

Stem cells (SCs) undergo asymmetric division, producing transit-amplifying cells (TACs) with increased proliferative potential that move into tissues and ultimately differentiate into a specialized cell type. Thus, TACs represent an intermediary state between stem cells and differentiated cells. In the cornea, a population of stem cells resides in the limbal region, named the limbal epithelial stem cells (LESCs). As LESCs proliferate, they generate TACs that move centripetally into the cornea and differentiate into corneal epithelial cells. Upon limbal injury, research suggests a population of progenitor-like cells that exists within the cornea can move centrifugally into the limbus, where they dedifferentiate into LESCs. Herein, we summarize recent advances made in understanding the mechanism that governs the differentiation of LESCs into TACs, and thereafter, into corneal epithelial cells. We also outline the evidence in support of the existence of progenitor-like cells in the cornea and whether TACs could represent a population of cells with progenitor-like capabilities within the cornea. Furthermore, to gain further insights into the dynamics of TACs in the cornea, we outline the most recent findings in other organ systems that support the hypothesis that TACs can dedifferentiate into SCs.

Ozone exposure affects corneal epithelial fate by promoting mtDNA leakage and cGAS/STING activation

Ozone is a common air pollutant associated with various human diseases. The human ocular surface is frequently exposed to ozone in the troposphere, but the mechanisms by which ozone affects the ocular surface health remain unclear. This study aimed to establish a mouse model to investigate the effects of ozone exposure on the ocular surface and the corneal epithelium. The findings revealed that ozone exposure disrupted corneal epithelial homeostasis and differentiation, resulting in corneal squamous metaplasia. Further, ozone exposure induced oxidative damage and cytoplasmic leakage of mitochondrial DNA (mtDNA), thereby activating the cGAS/STING signaling pathway. The activation of the cGAS/STING signaling pathway triggered the activation of downstream NF-κB and TRAF6 signaling pathways, causing corneal inflammation, thereby promoting corneal inflammation and squamous metaplasia. Finally, C-176, a selective STING inhibitor, effectively prevented and treated corneal inflammation and squamous metaplasia caused by ozone exposure. This study revealed the role of mtDNA leakage-mediated cGAS/STING activation in corneal squamous epithelial metaplasia caused by ozone exposure. It also depicted the abnormal expression pattern of corneal epithelial keratin using three-dimensional images, providing new targets and strategies for preventing and treating corneal squamous metaplasia and other ocular surface diseases.

Eyes open on stem cells

Recently, the murine cornea has reemerged as a robust stem cell (SC) model, allowing individual SC tracing in living animals. The cornea has pioneered seminal discoveries in SC biology and regenerative medicine, from the first corneal transplantation in 1905 to the identification of limbal SCs and their transplantation to successfully restore vision in the early 1990s. Recent experiments have exposed unexpected properties attributed to SCs and progenitors and revealed flexibility in the differentiation program and a key role for the SC niche. Here, we discuss the limbal SC model and its broader relevance to other tissues, disease, and therapy.

Regenerated Corneal Epithelium Expresses More βIII-Tubulin After Chemical Injuries Compared to Mechanical Injuries

Purpose

Defining the regenerative response following various types of corneal chemical and mechanical injuries is important for understanding the pathophysiology of the injury and evaluating the effectiveness of the therapies. This study characterizes corneal epithelial healing in a murine chemical and mechanical injury model.

Methods

Four groups of 10 mice each received complete corneolimbal injuries by AlgerBrush, AlgerBrush/thermal, NaOH (0.5 N), or ethanol. Slit-lamp and optical coherence tomography examinations were performed daily for 14 days. Corneal opacity (CO) and neovascularization (NV) were evaluated. The origin of the regenerated epithelium was illustrated by anti-cytokeratin 12 (K12) and anti-K13. The height of regenerated corneal epithelium and intraepithelial free nerve endings (FNEs) stained with anti-βIII-tubulin were measured. The amount of fibrosis was measured by anti-α-smooth muscle actin (α-SMA) monoclonal antibody in the different groups. Statistical analysis was performed by ANOVA and t-test.

Results

Corneal opacity and neovascularization were markedly higher in the NaOH and AlgerBrush/thermal groups. Molecular studies revealed the following: Regenerated corneal epithelium thickness was less than normal in all groups, the AlgerBrush group had the shortest height of the regenerated epithelium, βIII-tubulin was expressed in the entire height of corneal epithelium in all groups except in the AlgerBrush group, and K12 was replaced by K13 in all groups.

Conclusions

Corneal wound healing is more effective following chemical injuries in terms of epithelial thickness. Inflammation may play an important role in the outcome.

Translational Relevance

Inflammation following different injuries may be redirected to be more effective in corneal regeneration and clarity.

Evaluation of the Algerbrush II rotating burr as a tool for inducing ocular surface failure in a mouse model

Purpose

The Algerbrush II has been widely used to induce corneal and limbal injuries in animal models. The extent of injury varies with the duration of exposure, pressure from the placement of the burr, and the size of the burr. However, no study has explored the correlation between the duration of exposure and the severity of injury in mouse model with corneal and limbal stem cell deficiency (LSCD) induced using the Algerbrush II. Therefore, this study aimed to evaluate the variations in the severity of corneal and limbal injury with different durations of the Algerbrush II application.

Methods

The entire cornea and limbus of C57BL/6 mice were injured for 30-45 s, 60-75 s, 90-120 s, and 3-4 min. Photography and slit-lamp examination was performed on days 0, 2, 4, and 7, followed by hematoxylin & eosin, periodic acid-Schiff, and immunohistochemical staining. Statistical analysis was performed using one way ANOVA analysis.

Results

A duration of 30-45 s of injury was found to be sufficient to induce superficial corneal and limbal epithelial debridement and re-epithelialization was completed in all eyes by day 7; however, clinical signs of LSCD were not observed in all mice. Increasing the exposure time to 90-120 s resulted in central 2+ corneal opacity with limbal and paracentral corneal neovascularization. All eyes injured for 3-4 min displayed clinical signs of LSCD, such as persistent epithelial defects on day 7 after the injury, central corneal neovascularization, and 2.2+ diffuse corneal opacity. Histological signs of LSCD, including goblet cell metaplasia and K13 expression on the corneal surface, were observed in all injured eyes.

Conclusions

Our findings suggest that the duration of injury is an important factor influencing the severity of LSCD in a murine model of injury. A 1-mm rotating burr was found to be more effective for keratectomy and pigment release, whereas a 0.5-mm burr was more suitable for corneal epithelial debridement.

Pathogenesis of Alkali Injury-Induced Limbal Stem Cell Deficiency: A Literature Survey of Animal Models

Limbal stem cell deficiency (LSCD) is a debilitating ocular surface disease that eventuates from a depleted or dysfunctional limbal epithelial stem cell (LESC) pool, resulting in corneal epithelial failure and blindness. The leading cause of LSCD is a chemical burn, with alkali substances being the most common inciting agents. Characteristic features of alkali-induced LSCD include corneal conjunctivalization, inflammation, neovascularization and fibrosis. Over the past decades, animal models of corneal alkali burn and alkali-induced LSCD have been instrumental in improving our understanding of the pathophysiological mechanisms responsible for disease development. Through these paradigms, important insights have been gained with regards to signaling pathways that drive inflammation, neovascularization and fibrosis, including NF-κB, ERK, p38 MAPK, JNK, STAT3, PI3K/AKT, mTOR and WNT/β-catenin cascades. Nonetheless, the molecular and cellular events that underpin re-epithelialization and those that govern long-term epithelial behavior are poorly understood. This review provides an overview of the current mechanistic insights into the pathophysiology of alkali-induced LSCD. Moreover, we highlight limitations regarding existing animal models and knowledge gaps which, if addressed, would facilitate development of more efficacious therapeutic strategies for patients with alkali-induced LSCD.