Challenges to the study of asymmetric cell division in corneal and limbal epithelia

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.

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.

The Impaired Wound Healing Process Is a Major Factor in Remodeling of the Corneal Epithelium in Adult and Adolescent Patients With Keratoconus

Purpose

Keratoconus (KTCN) is the most common corneal ectasia, characterized by pathological cone formation. Here, to provide an insight into the remodeling of the corneal epithelium (CE) during the course of the disease, we evaluated topographic regions of the CE of adult and adolescent patients with KTCN.

Methods

The CE samples from 17 adult and 6 adolescent patients with KTCN, and 5 control CE samples were obtained during the CXL and PRK procedures, respectively. Three topographic regions, central, middle, and peripheral, were separated toward RNA sequencing and MALDI-TOF/TOF Tandem Mass Spectrometry. Data from transcriptomic and proteomic investigations were consolidated with the morphological and clinical findings.

Results

The critical elements of the wound healing process, epithelial-mesenchymal transition, cell-cell communications, and cell-extracellular matrix interactions were altered in the particular corneal topographic regions. Abnormalities in pathways of neutrophils degranulation, extracellular matrix processing, apical junctions, IL, and IFN signaling were revealed to cooperatively disorganize the epithelial healing. Deregulation of the epithelial healing, G2M checkpoints, apoptosis, and DNA repair pathways in the middle CE topographic region in KTCN explains the presence of morphological changes in the corresponding doughnut pattern (a thin cone center surrounded by a thickened annulus). Despite similar morphological characteristics of CE samples in adolescents and adults with KTCN, their transcriptomic features were different. Values of the posterior corneal elevation differentiated adults with KTCN from adolescents with KTCN and correlated with the expression of TCHP, SPATA13, CNOT3, WNK1, TGFB2, and KRT12 genes.

Conclusions

Identified molecular, morphological, and clinical features indicate the effect of impaired wound healing on corneal remodeling in KTCN CE.

Limbal BCAM expression identifies a proliferative progenitor population capable of holoclone formation and corneal differentiation

The corneal epithelium is renowned for high regenerative potential, which is dependent on the coordinated function of its diverse progenitor subpopulations. However, the molecular pathways governing corneal epithelial progenitor differentiation are incompletely understood. Here, we identify a highly proliferative limbal epithelial progenitor subpopulation characterized by expression of basal cell adhesion molecule (BCAM) that is capable of holocone formation and corneal epithelial sheet generation. BCAM-positive cells can be found among ABCB5-positive limbal stem cells (LSCs) as well as among ABCB5-negative limbal epithelial cell populations. Mechanistically, we show that BCAM is functionally required for cellular migration and differentiation and that its expression is regulated by the transcription factor p63. In aggregate, our study identifies limbal BCAM expression as a marker of highly proliferative corneal epithelial progenitor cells and defines the role of BCAM as a critical molecular mediator of corneal epithelial differentiation.

Using scRNA sequencing of ABCB5-positive human limbal stem cells, Sasamoto et al. identify a BCAM-positive highly proliferative limbal epithelial progenitor subpopulation that is capable of holocone formation and corneal epithelial sheet generation. BCAM regulated by the stem cell transcription factor p63 is functionally required for corneal cell migration and differentiation.

Interleukin-13 increases the stemness of limbal epithelial stem cells cultures

This study aimed to determine the effect of interleukin-13 (IL13) on the stemness, differentiation, proliferation, clonogenicity, and morphology of cultured limbal epithelial cells (LECs). Human limbal explants were used to culture LECs up to the second passage (P0-P2) with or without IL13 (IL13+ and IL13-, respectively). Cells were analyzed by qPCR (for the expression of ΔNp63α, BMI-1, keratin (K) 3, K7, K12, K14, K17, mucin 4, and MKI67) and immunofluorescence staining for p63α. The clonogenic ability was determined by colony-forming assay (CFA), and their metabolic activity was measured by WST-1 assay. The results of the CFA showed a significantly increased clonogenic ability in P1 and P2 cultures when LECs were cultured with IL13. In addition, the expression of putative stem cell markers (ΔNp63α, K14, and K17) was significantly higher in all IL13+ cultures compared to IL13-. Similarly, immunofluorescence analysis showed a significantly higher percentage of p63α positive cells in P2 cultures with IL13 than without it. LECs cultures without IL13 lost their cuboidal morphology with a high nucleocytoplasmic ratio after P1. The use of IL13 also led to significantly higher proliferation in P2, which can be reflected by a higher ability to reach confluence in P2 cultures. On the other hand, IL13 had no effect on corneal epithelial cell differentiation (K3 and K12 expression), and the expression of the conjunctival marker K7 significantly increased in all IL13+ cultures compared to the respective cell culture without IL13. This study showed that IL13 enhanced the stemness of LECs by increasing the clonogenicity and the expression of putative stem cell markers of LECs while maintaining their stem cell morphology. We established IL13 as a culture supplement for LESCs, which increases their stemness potential in culture, even after the second passage, and may lead to the greater success of LESCs transplantation in patients with LSCD.

In Vivo Labeling and Tracking of Proliferating Corneal Endothelial Cells by 5-Ethynyl-2'-Deoxyuridine in Rabbits

Purpose

To develop a method to label proliferating corneal endothelial cells (ECs) in rabbits in vivo and track their migration over time.

Methods

We compared intraperitoneal (IP) and intracameral (IC) administration of 5-ethynyl-2'-deoxyuridine (EdU) in two experiments: (1) six rabbits received IP or IC EdU. Blood and aqueous humor (AH) samples were incubated with HL-60 cells. Flow cytometry detected the EdU incorporation, representing the bioavailability of EdU. (2) In vivo EdU labeling was investigated in pulse-chase study: 48 rabbits received EdU IP or IC. The corneas were flat-mounted after 1, 2, 5, or 40 days and imaged using fluorescence microscopy. EdU+ and Ki67+ ECs were quantified and their distance from the peripheral endothelial edge was measured.

Results

EdU was bioavailable in the AH up to 4 hours after IC injection. No EdU was detected in the blood or the AH after IP injection. High quality EdU labeling of EC was obtained only after IC injection, achieving 2047 ± 702 labeled ECs. Proliferating ECs were located exclusively in the periphery within 1458 ± 146 µm from the endothelial edge. After 40 days, 1490 ± 397 label-retaining ECs (LRCs) were detected, reaching 2219 ± 141 µm from the edge, indicating that LRCs migrated centripetally.

Conclusions

IC EdU injection enables the labeling and tracking of proliferating ECs. LRCs seem to be involved in endothelial homeostasis, yet it remains to be investigated whether they represent endothelial progenitor cells.

Translational Relevance

EdU labeling in animal models can aid the search for progenitor cells and the development of cell therapy for corneal endothelial dysfunction.

KLF4 Coordinates Corneal Epithelial Apical-Basal Polarity and Plane of Cell Division and Is Downregulated in Ocular Surface Squamous Neoplasia

Purpose

Previously, we demonstrated that Krüppel-like factor 4 (KLF4) promotes corneal epithelial (CE) homeostasis by suppressing epithelial-mesenchymal transition (EMT) and TGF-β signaling. As TGF-β affects epithelial apicobasal polarity (ABP) and plane of division, we investigated the role of KLF4 in these processes.

Methods

Klf4 was ablated in adult ternary transgenic Klf4^Δ/ΔCE (Klf4^LoxP/LoxP/Krt12^rtTA/rtTA/Tet-O-Cre) mouse CE using doxycycline chow. ABP and plane of division markers’ expression in Klf4^Δ/ΔCE and human ocular surface squamous neoplasia (OSSN) tissues relative to controls was evaluated by quantitative PCR, immunoblots, and/or immunofluorescent staining.

Results

Klf4^Δ/ΔCE CE cells displayed downregulation of apical Pals1 and Crumbs1, apicolateral Par3, and basolateral Scribble, as well as upregulation of Rho family GTPase Cdc42, suggesting disruption of ABP. Phalloidin staining revealed that the Klf4^Δ/ΔCE CE actin cytoskeleton is disrupted. Klf4^Δ/ΔCE cells favored vertical plane of division within 67.5° to 90° of the CE basement membrane (39% and 47% of the dividing cells relative to 23% and 26% in the control based on phospho-histone-H3 and survivin, respectively), resulting in more dividing cells within the Klf4^Δ/ΔCE CE as reported previously. KLF4 was downregulated in human OSSN tissues that displayed EMT and downregulation of PAR3, PALS1, and SCRIB, consistent with a protective role for KLF4.

Conclusions

By demonstrating that Klf4 ablation affects CE expression of ABP markers and Cdc42, cytoskeletal actin organization, and the plane of cell division and that KLF4 is downregulated in OSSN tissues that display EMT and lack ABP, these results elucidate the key integrative role of KLF4 in coordinating CE cell polarity and plane of division, loss of which results in OSSN.

Peripheral (not central) corneal epithelia contribute to the closure of an annular debridement injury

The well-accepted proposition that central corneal epithelia have limited self-renewal and therefore poor regenerative capacity has recently been challenged. However, methods for real-time monitoring to identify which cells take part in this process are scant. In this study, we visualized and quantified the contribution of central versus peripheral/limbal epithelia during annular wound healing by intravital imaging, through an organ culture system, and via computational modeling. Our results verify the contribution of K14⁺ limbal-derived stem cells and their early progeny in playing a vital role in this process, while central corneal epithelia contribute minimally to wound closure.

Corneal epithelia have limited self-renewal and therefore reparative capacity. They are continuously replaced by transient amplifying cells which spawn from stem cells and migrate from the periphery. Because this view has recently been challenged, our goal was to resolve the conflict by giving mice annular injuries in different locations within the corneolimbal epithelium, then spatiotemporally fate-mapping cell behavior during healing. Under these conditions, elevated proliferation was observed in the periphery but not the center, and wounds predominantly resolved by centripetally migrating limbal epithelia. After wound closure, the central corneal epithelium was completely replaced by K14⁺ limbal-derived clones, an observation supported by high-resolution fluorescence imaging of genetically marked cells in organ-cultured corneas and via computational modeling. These results solidify the essential role of K14⁺ limbal epithelial stem cells for wound healing and refute the notion that stem cells exist within the central cornea and that their progeny have the capacity to migrate centrifugally.

Visualizing the Contribution of Keratin-14+ Limbal Epithelial Precursors in Corneal Wound Healing

It is thought that corneal epithelial injuries resolve by leading-edge cells “sliding” or “rolling” into the wound bed. Here, we challenge this notion and show by real-time imaging that corneal wounds initially heal by “basal cell migration.” The K14CreER^T2-Confetti multi-colored reporter mouse was employed to spatially and temporally fate-map cellular behavior during corneal wound healing. Keratin-14⁺ basal epithelia are forced into the wound bed by increased population pressure gradient from the limbus to the wound edge. As the defect resolves, centripetally migrating epithelia decelerate and replication in the periphery is reduced. With time, keratin-14⁺-derived clones diminish in number concomitant with their expansion, indicative that clonal evolution aligns with neutral drifting. These findings have important implications for the involvement of stem cells in acute tissue regeneration, in key sensory tissues such as the cornea.

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Basal limbal epithelial cell proliferation is increased following a corneal injury

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Corneal epithelial wounds initially heal by K14⁺ basal cell migration

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STICS accurately measures clonal dynamics during wound closure

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Computational modeling confirms the pivotal role of LESCs in wound repair

Characterization and comparison of human limbal explant cultures grown under defined and xeno-free conditions

Human limbal epithelial cells (LECs) intended for treatment of limbal stem cell deficiency are commonly cultivated on a 3T3 feeder layer with complex culture medium supplemented with fetal bovine serum (FBS). However, FBS is a xenogeneic component containing poorly characterised constituents and exhibits quantitative and qualitative lot-to-lot variations. Human limbal explants were plated on untreated or fibrin coated plastic plates and cultured in two non-xenogeneic media (supplemented with either human serum or platelet lysate only). Our aim was to find out whether the characteristics of harvested LEC cultures are comparable to those of LEC cultivated in the gold standard - FBS-supplemented complex medium. The growth kinetics, cell proliferation, differentiation, stemness maintenance, apoptosis and contamination by other cell types were evaluated and compared among these conditions. In all of them LECs were successfully cultivated. Stemness was preserved in both xeno-free media. However, cells cultured with human serum on the fibrin-coated plates had the highest growth rate and cell proliferation and very low fibroblast-like cell contamination. These data suggest that xeno-free cell culture conditions can replace the traditional FBS-supplemented medium and thereby provide a safer protocol for ex vivo cultured limbal stem cell transplants.

The Role of Limbal Epithelial Stem Cells in Regulating Corneal (Lymph)angiogenic Privilege and the Micromilieu of the Limbal Niche following UV Exposure

The cornea is a clear structure, void of blood, and lymphatic vessels, functioning as our window to the world. Limbal epithelial stem cells, occupying the area between avascular cornea and vascularized conjunctiva, have been implicated in tissue border maintenance, preventing conjunctivalisation and propagation of blood and lymphatic vessels into the cornea. Defects in limbal epithelial stem cells are linked to corneal neovascularisation, including lymphangiogenesis, chronic inflammation, conjunctivalisation, epithelial abnormalities including the presence of goblet cells, breaks in Bowman's membrane, persistent epithelial defects and ulceration, ocular surface squamous neoplasia, lipid keratopathy, pain, discomfort, and compromised vision. It has been postulated that pterygium is an example of focal limbal deficiency. Previous reports showing changes occurring in limbal epithelium during pterygium pathogenesis suggest that there is a link to stem cell damage. In this light, pterygium can serve as a model disease of UV-induced stem cell damage also characterised by corneal blood and lymphangiogenesis. This review focuses on the role of corneal and limbal epithelial cells and the stem cell niche in maintaining corneal avascularity and corneal immune privilege and how this may be deregulated following UV exposure. We present an overview of the PUBMED literature in the field as well as recent work from our laboratories.

Tracing the fate of limbal epithelial progenitor cells in the murine cornea

Stem cell (SC) division, deployment, and differentiation are processes that contribute to corneal epithelial renewal. Until now studying the destiny of these cells in a living mammal has not been possible. However, the advent of inducible multicolor genetic tagging and powerful imaging technologies has rendered this achievable in the translucent and readily accessible murine cornea. K14CreER(T2)-Confetti mice that harbor two copies of the Brainbow 2.1 cassette, yielding up to 10 colors from the stochastic recombination of fluorescent proteins, were used to monitor K-14(+) progenitor cell dynamics within the corneal epithelium in live animals. Multicolored columns of cells emerged from the basal limbal epithelium as they expanded and migrated linearly at a rate of 10.8 µm/day toward the central cornea. Moreover, the permanent expression of fluorophores, passed on from progenitor to progeny, assisted in discriminating individual clones as spectrally distinct streaks containing more than 1,000 cells within the illuminated area. The centripetal clonal expansion is suggestive that a single progenitor cell is responsible for maintaining a narrow corridor of corneal epithelial cells. Our data are in agreement with the limbus as the repository for SC as opposed to SC being distributed throughout the central cornea. This is the first report describing stem/progenitor cell fate determination in the murine cornea using multicolor genetic tracing. This model represents a powerful new resource to monitor SC kinetics and fate choice under homeostatic conditions, and may assist in assessing clonal evolution during corneal development, aging, wound-healing, disease, and following transplantation.

Evaluating alternative stem cell hypotheses for adult corneal epithelial maintenance

In this review we evaluate evidence for three different hypotheses that explain how the corneal epithelium is maintained. The limbal epithelial stem cell (LESC) hypothesis is most widely accepted. This proposes that stem cells in the basal layer of the limbal epithelium, at the periphery of the cornea, maintain themselves and also produce transient (or transit) amplifying cells (TACs). TACs then move centripetally to the centre of the cornea in the basal layer of the corneal epithelium and also replenish cells in the overlying suprabasal layers. The LESCs maintain the corneal epithelium during normal homeostasis and become more active to repair significant wounds. Second, the corneal epithelial stem cell (CESC) hypothesis postulates that, during normal homeostasis, stem cells distributed throughout the basal corneal epithelium, maintain the tissue. According to this hypothesis, LESCs are present in the limbus but are only active during wound healing. We also consider a third possibility, that the corneal epithelium is maintained during normal homeostasis by proliferation of basal corneal epithelial cells without any input from stem cells. After reviewing the published evidence, we conclude that the LESC and CESC hypotheses are consistent with more of the evidence than the third hypothesis, so we do not consider this further. The LESC and CESC hypotheses each have difficulty accounting for one main type of evidence so we evaluate the two key lines of evidence that discriminate between them. Finally, we discuss how lineage-tracing experiments have begun to resolve the debate in favour of the LESC hypothesis. Nevertheless, it also seems likely that some basal corneal epithelial cells can act as long-term progenitors if limbal stem cell function is compromised. Thus, this aspect of the CESC hypothesis may have a lasting impact on our understanding of corneal epithelial maintenance, even if it is eventually shown that stem cells are restricted to the limbus as proposed by the LESC hypothesis.

Limbal stem cells: Central concepts of corneal epithelial homeostasis

A strong cohort of evidence exists that supports the localisation of corneal stem cells at the limbus. The distinguishing characteristics of limbal cells as stem cells include slow cycling properties, high proliferative potential when required, clonogenicity, absence of differentiation marker expression coupled with positive expression of progenitor markers, multipotency, centripetal migration, requirement for a distinct niche environment and the ability of transplanted limbal cells to regenerate the entire corneal epithelium. The existence of limbal stem cells supports the prevailing theory of corneal homeostasis, known as the XYZ hypothesis where X represents proliferation and stratification of limbal basal cells, Y centripetal migration of basal cells and Z desquamation of superficial cells. To maintain the mass of cornea, the sum of X and Y must equal Z and very elegant cell tracking experiments provide strong evidence in support of this theory. However, several recent studies have suggested the existence of oligopotent stem cells capable of corneal maintenance outside of the limbus. This review presents a summary of data which led to the current concepts of corneal epithelial homeostasis and discusses areas of controversy surrounding the existence of a secondary stem cell reservoir on the corneal surface

Comparison of upstream regulators in human ex vivo cultured cornea limbal epithelial stem cells and differentiated corneal epithelial cells

Background

The surface of the human eye is covered by corneal epithelial cells (CECs) which regenerate from a small population of limbal epithelial stem cells (LESCs). Cell therapy with LESCs is a non-penetrating treatment for preventing blindness due to LESC deficiency or dysfunction. Our aim was to identify new putative molecular markers and upstream regulators in the LESCs and associated molecular pathways.

Results

Genome-wide microarray transcriptional profiling was used to compare LESCs to differentiated human CECs. Ingenuity-based pathway analysis was applied to identify upstream regulators and pathways specific to LESCs. ELISA and flow cytometry were used to measure secreted and surface expressed proteins, respectively. More than 2 fold increase and decrease in expression could be found in 1830 genes between the two cell types. A number of molecules functioning in cellular movement (381), proliferation (567), development (552), death and survival (520), and cell-to-cell signaling (290) were detected having top biological functions in LESCs and several of these were confirmed by flow cytometric surface protein analysis. Custom-selected gene groups related to stemness, differentiation, cell adhesion, cytokines and growth factors as well as angiogenesis could be analyzed. The results show that LESCs play a key role not only in epithelial differentiation and tissue repair, but also in controlling angiogenesis and extracellular matrix integrity. Some pro-inflammatory cytokines were found to be important in stemness-, differentiation- and angiogenesis-related biological functions: IL-6 and IL-8 participated in most of these biological pathways as validated by their secretion from LESC cultures.

Conclusions

The gene and molecular pathways may provide a more specific understanding of the signaling molecules associated with LESCs, therefore, help better identify and use these cells in the treatment of ocular surface diseases.

Phenotypic alteration of limbal niche-associated limbal epithelial stem cell deficiency by ultraviolet-B exposure-induced phototoxicity in mice

Good vision requires a healthy cornea, and a healthy cornea needs healthy stem cells. Limbal epithelial stem cells (LESCs) are a traditional source of corneal epithelial cells and are recruited for the continuous production of epithelium without seizing throughout an animal's life, which maintains corneal transparency. Like the maintenance of other adult somatic stem cells, the maintenance of LESCs depends on the specific microenvironmental niche in which they reside. The purpose of this study was to determine the microenvironmental damage associated with LESCs fate due to ultraviolet (UV)-B exposure in a mouse model. Structural alteration and deregulation of the stem cell and its neighboring niche components were observed by using clinical, morphological, explant culture study, and flowcytometric analysis, which demonstrated that the limbal microenvironment plays an important role in cornea-related disease development. In UV-exposed mice, overexpression of vascular endothelial growth factor receptor 2 indicated neovascularization, decreased CD38 expression signified the alteration of limbal epithelial superficial cells, and the loss of limbal stem cell marker p63 indicated limbal stem cell deficiency in the limbal vicinity. We concluded that LESC deficiency diseases (LESCDDs) are associated with pathophysiological changes in the LESC niche, with some inhibitory interception such as UV-B irradiation, which results in corneal defects.

Limbal epithelial stem cells: role of the niche microenvironment

The cornea contains a reservoir of self-regenerating epithelial cells that are essential for maintaining its transparency and good vision. The study of stem cells in this functionally important organ has grown over the past four decades, partly due to the ease with which this tissue is visualized, its accessibility with minimally invasive instruments, and the fact that its stem cells are segregated within a transitional zone between two functionally diverse epithelia. While human, animal, and ex vivo models have been instrumental in progressing the corneal stem cell field, there is still much to be discovered about this exquisitely sensitive window for sight. This review will provide an overview of the human cornea, where its stem cells reside and how components of the microenvironment including extracellular matrix proteins and their integrin receptors are thought to govern corneal stem cell homeostasis.