Directed differentiation of human embryonic stem cells into conjunctival epithelial cells

Severe conjunctival damage can lead to extensive ocular cicatrisation, fornix shortening, and even ocular surface failure, resulting in significant vision impairment. Conjunctival reconstruction is the primary therapeutic strategy for these clinical conjunctival diseases. However, there have been limited studies on induced differentiation of conjunctival epithelial cells derived from stem cells. In this study, we established a chemical defined differentiation protocol from human embryonic stem cells (hESCs) into conjunctival epithelial cells. hES cell line H1 was used for differentiation, and RT-qPCR, immunofluorescence staining, Periodic-acid-Schiff staining (PAS), and transcriptome analysis were employed to identify the differentiated cells. Here, to imitate the development of the vertebrate conjunctiva, hESCs were induced using a three-step process involving first chetomin was used to induce ocular surface ectoderm, then nicotinamide was used to induce ocular surface epithelial progenitor cells, and finally epidermal growth factor, keratinocyte growth factor and other factors were used to differentiate mature conjunctival epithelial cells. hESC-derived conjunctival epithelial cells expressed mature conjunctival epithelial lineage markers (including PAX6, P63, K13). The presence of goblet cells was confirmed by positive PAS. Transcriptome analysis revealed that hESC-derived conjunctival epithelial cells possessed a more naïve phenotype, and exhibited greater proliferation capacity compared to mature human conjunctival epithelial cells, suggesting their potential as alternative seed cells for conjunctival reconstruction.

Transcription Factor ATF3 Participates in DeltaNp63-Mediated Proliferation of Corneal Epithelial Cells

Understanding the regulatory mechanisms underlying corneal epithelial cell (CEC) proliferation in vitro may provide the means to boost CEC production in cell therapy for ocular disorders. The transcription factor ΔNp63 plays a crucial role in the proliferation of CECs, but the underlying mechanisms is yet to be elucidated. TP63 and ΔNp63 are encoded by the TP63 gene via alternative promoters. We previously reported that both ΔNp63 and activating transcription factor (ATF3) are substantially expressed in cultured CECs, but the regulatory relationship between ΔNp63 and ATF3 is unknown. In the present study, we found that ΔNp63 increased ATF3 expression and ATF3 promoter activity in cultured CECs. The deletion of the p63 binding core site reduced ATF3 promoter activity. CECs overexpressing ATF3 exhibited significantly greater proliferation than control CECs. ATF3 knockdown suppressed the ΔNp63-induced increase in cell proliferation. Overexpression of ATF3 in CECs significantly elevated protein and mRNA levels of cyclin D. The protein levels of keratin 3/14, integrin β1, and involucrin did not differ between ATF3-overexpressing CECs, ATF3-downregulated CECs, and control cells. In conclusion, our results suggest that ΔNp63 increases CEC proliferation via the ΔNp63/ATF3/CDK pathway.

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.

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

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

The role of the PI3K/AKT signalling pathway in the corneal epithelium: recent updates

Phosphatidylinositol 3 kinase (PI3K)/AKT (also called protein kinase B, PKB) signalling regulates various cellular processes, such as apoptosis, cell proliferation, the cell cycle, protein synthesis, glucose metabolism, and telomere activity. Corneal epithelial cells (CECs) are the outermost cells of the cornea; they maintain good optical performance and act as a physical and immune barrier. Various growth factors, including epidermal growth factor receptor (EGFR) ligands, insulin-like growth factor 1 (IGF1), neurokinin 1 (NK-1), and insulin activate the PI3K/AKT signalling pathway by binding their receptors and promote antiapoptotic, anti-inflammatory, proliferative, and migratory functions and wound healing in the corneal epithelium (CE). Reactive oxygen species (ROS) regulate apoptosis and inflammation in CECs in a concentration-dependent manner. Extreme environments induce excess ROS accumulation, inhibit PI3K/AKT, and cause apoptosis and inflammation in CECs. However, at low or moderate levels, ROS activate PI3K/AKT signalling, inhibiting apoptosis and stimulating proliferation of healthy CECs. Diabetes-associated hyperglycaemia directly inhibit PI3K/AKT signalling by increasing ROS and endoplasmic reticulum (ER) stress levels or suppressing the expression of growth factors receptors and cause diabetic keratopathy (DK) in CECs. Similarly, hyperosmolarity and ROS accumulation suppress PI3K/AKT signalling in dry eye disease (DED). However, significant overactivation of the PI3K/AKT signalling pathway, which mediates inflammation in CECs, is observed in both infectious and noninfectious keratitis. Overall, upon activation by growth factors and NK-1, PI3K/AKT signalling promotes the proliferation, migration, and anti-apoptosis of CECs, and these processes can be regulated by ROS in a concentration-dependent manner. Moreover, PI3K/AKT signalling pathway is inhibited in CECs from individuals with DK and DED, but is overactivated by keratitis.

Single cell transcriptomics reveals the heterogeneity of the human cornea to identify novel markers of the limbus and stroma

The cornea is the clear window that lets light into the eye. It is composed of five layers: epithelium, Bowman's layer, stroma, Descemet's membrane and endothelium. The maintenance of its structure and transparency are determined by the functions of the different cell types populating each layer. Attempts to regenerate corneal tissue and understand disease conditions requires knowledge of how cell profiles vary across this heterogeneous tissue. We performed a single cell transcriptomic profiling of 19,472 cells isolated from eight healthy donor corneas. Our analysis delineates the heterogeneity of the corneal layers by identifying cell populations and revealing cell states that contribute in preserving corneal homeostasis. We identified expression of CAV1, HOMER3 and CPVL in the corneal epithelial limbal stem cell niche, CKS2, STMN1 and UBE2C were exclusively expressed in highly proliferative transit amplifying cells, CXCL14 was expressed exclusively in the suprabasal/superficial limbus, and NNMT was exclusively expressed by stromal keratocytes. Overall, this research provides a basis to improve current primary cell expansion protocols, for future profiling of corneal disease states, to help guide pluripotent stem cells into different corneal lineages, and to understand how engineered substrates affect corneal cells to improve regenerative therapies.

Corneal Epithelial Stem Cells-Physiology, Pathophysiology and Therapeutic Options

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

The foggy world(s) of p63 isoform regulation in normal cells and cancer

The p53 family member p63 exists as two major protein variants (TAp63 and ΔNp63) with distinct expression patterns and functional properties. Whilst downstream target genes of p63 have been studied intensively, how p63 variants are themselves controlled has been relatively neglected. Here, we review advances in understanding ΔNp63 and TAp63 regulation, highlighting their distinct pathways. TAp63 has roles in senescence and metabolism, and in germ cell genome maintenance, where it is activated post-transcriptionally by phosphorylation cascades after DNA damage. The function and regulation of TAp63 in mesenchymal and haematopoietic cells is less clear but may involve epigenetic control through DNA methylation. ΔNp63 functions to maintain stem/progenitor cells in various epithelia and is overexpressed in squamous and certain other cancers. ΔNp63 is transcriptionally regulated through multiple enhancers in concert with chromatin modifying proteins. Many signalling pathways including growth factors, morphogens, inflammation, and the extracellular matrix influence ΔNp63 levels, with inconsistent results reported. There is also evidence for reciprocal regulation, including ΔNp63 activating its own transcription. ΔNp63 is downregulated during cell differentiation through transcriptional regulation, while post-transcriptional events cause proteasomal degradation. Throughout the review, we identify knowledge gaps and highlight discordances, providing potential explanations including cell-context and cell-matrix interactions. Identifying individual p63 variants has roles in differential diagnosis and prognosis, and understanding their regulation suggests clinically approved agents for targeting p63 that may be useful combination therapies for selected cancer patients. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

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.

ΔNp63 is upregulated during salivary gland regeneration following duct ligation and irradiation in mice

The transcription factor p63, a component of the p53 family, has important functions in development, homeostasis, and regeneration of epithelial tissues. However, the role of p63 in the regeneration of exocrine glands, including the salivary glands (SGs), has not been fully investigated. We investigated p63 expression in SG regeneration induced by duct ligation and irradiation. The expression of ΔNp63, a p63 isoform, increased and was colocalized with keratin 5 positive cells were myoepithelial cells. Furthermore, ΔNp63 expression was regulated by FGF7 stimulation via p38 MAPK phosphorylation and affected SG morphogenesis. These results suggest that ΔNp63 is essential for SG regeneration and may be a new target for regenerative treatment.

Effects of HGF and KGF gene silencing on vascular endothelial growth factor and its receptors in rat ultraviolet radiation‑induced corneal neovascularization

Hepatocyte growth factor (HGF) and keratinocyte growth factor (KGF), two paracrine growth factors, modulate corneal epithelial cell metabolism, apoptosis and survival. Vascular endothelial growth factor (VEGF) serves as a proangiogenic factor in corneal neovascularization (CNV), which is a major cause of vision impairment and corneal blindness. The aim of the present study was to evaluate the ability of HGF and KGF to influence VEGF and its receptor, kinase insert domain receptor (Flk‑1) in corneal injury and CNV in rats induced by ultraviolet radiation (UVR). An UVR‑induced corneal injury rat model was successfully established to characterize the expression patterns of KGF, HGF, VEGF and Flk‑1 in corneal tissues. Corneal epithelial cells were extracted and treated with small interfering RNAs (siRNAs) targeting KGF, HGF or both (si‑KGF, si‑HGF or si‑HGF/KGF). The effects of HGF and KGF were examined through detection of the expression of KGF, HGF, VEGF and Flk‑1, and the evaluation of cell proliferation, cell cycle and cell apoptosis. The expression levels of KGF, HGF, VEGF and Flk‑1 in corneal tissues were increased in the rat model. In the cell experiments, the transfection of si‑HGF/KGF resulted in reductions in VEGF, Flk‑1, KGF and HGF. In addition, decreased cell proliferation and elevated cell apoptosis were found in the corneal epithelial cells from the rat model following KGF and HGF gene silencing. Taken together, these findings suggest that HGF and KGF gene silencing inhibits UVR‑induced corneal epithelial proliferation and CNV and may function as novel targets for corneal wound healing.

Keratinocyte Growth Factor (KGF) Modulates Epidermal Progenitor Cell Kinetics through Activation of p63 in Middle Ear Cholesteatoma

The basal stem/progenitor cell maintains homeostasis of the epidermis. Progressive disturbance of this homeostasis has been implicated as a possible cause in the pathogenesis of epithelial disease, such as middle ear cholesteatoma. In many cases of stem/progenitor cell regulation, the importance of extracellular signals provided by the surrounding cells is well-recognized. Keratinocyte growth factor (KGF) is a mesenchymal-cell-derived paracrine growth factor that specifically participates in skin homeostasis; however, the overexpression of KGF induces middle ear cholesteatoma. In this study, two kinds of thymidine analogs were transferred at different time points and we investigated the effects of overexpressed KGF on the cell kinetics of stem/progenitor cells in vivo. As a result, BrdU(+)EdU(+) cells (stem/progenitor cells) were detected in the thickened epithelium of KGF-transfected specimens. The use of a high-resolution microscope enabled us to analyze the phosphorylated level of p63 in individual nuclei, and the results clearly demonstrated that BrdU(+)EdU(+) cells are regarded as progenitor cells. In the overexpression of KGF, the stimulation of progenitor cell proliferation was inhibited by SU5402, an inhibitor for tyrosine kinase of KGFR. These findings indicate that KGF overexpression may increase stem/progenitor cell proliferation and block terminal differentiation, resulting in epithelial hyperplasia, which is typical in middle ear cholesteatoma.

2A-DUB/Mysm1 Regulates Epidermal Development in Part by Suppressing p53-Mediated Programs

Development and homeostasis of the epidermis are governed by a complex network of sequence-specific transcription factors and epigenetic modifiers cooperatively regulating the subtle balance of progenitor cell self-renewal and terminal differentiation. To investigate the role of histone H2A deubiquitinase 2A-DUB/Mysm1 in the skin, we systematically analyzed expression, developmental functions, and potential interactions of this epigenetic regulator using Mysm1-deficient mice and skin-derived epidermal cells. Morphologically, skin of newborn and young adult Mysm1-deficient mice was atrophic with reduced thickness and cellularity of epidermis, dermis, and subcutis, in context with altered barrier function. Skin atrophy correlated with reduced proliferation rates in Mysm1^-/- epidermis and hair follicles, and increased apoptosis compared with wild-type controls, along with increases in DNA-damage marker γH2AX. In accordance with diminished α6-Integrin^high+CD34⁺ epidermal stem cells, reduced colony formation of Mysm1^-/- epidermal progenitors was detectable in vitro. On the molecular level, we identified p53 as potential mediator of the defective Mysm1-deficient epidermal compartment, resulting in increased pro-apoptotic and anti-proliferative gene expression. In Mysm1^-/-p53^-/- double-deficient mice, significant recovery of skin atrophy was observed. Functional properties of Mysm1^-/- developing epidermis were assessed by quantifying the transepidermal water loss. In summary, this investigation uncovers a role for 2A-DUB/Mysm1 in suppression of p53-mediated inhibitory programs during epidermal development.

Role of Corneal Stromal Cells on Epithelial Cell Function during Wound Healing

Following injury, corneal stromal keratocytes transform into repair-phenotype of activated stromal fibroblasts (SFs) and participate in wound repair. Simultaneously, ongoing bi-directional communications between corneal stromal-epithelial cells also play a vital role in mediating the process of wound healing. Factors produced by stromal cells are known to induce proliferation, differentiation, and motility of corneal epithelial cells, which are also subsequently the main processes that occur during wound healing. In this context, the present study aims to investigate the effect of SFs conditioned medium (SFCM) on corneal epithelial cell function along with substance P (SP). Antibody microarrays were employed to profile differentially expressed cell surface markers and cytokines in the presence of SFCM and SP. Antibody microarray data revealed enhanced expression of the ITGB1 in corneal epithelial cells following stimulation with SP whereas SFCM induced abundant expression of IL-8, ITGB1, PD1L1, PECA1, IL-15, BDNF, ICAM1, CD8A, CD44 and NTF4. All these proteins have either direct or indirect roles in epithelial cell growth, movement and adhesion related signaling cascades during tissue regeneration. We also observed activation of MAPK signaling pathway along with increased expression of focal adhesion kinase (FAK), paxillin, vimentin, β-catenin and vasodilator-stimulated phosphoprotein (VASP) phosphorylation. Additionally, epithelial-to-mesenchymal transition (EMT) regulating transcription factors Slug and ZEB1 expression were enhanced in the presence of SFCM. SP enriched the expression of integrin subunits α4, α5, αV, β1 and β3 whereas SFCM increased α4, α5, αV, β1 and β5 integrin subunits. We also observed increased expression of Serpin E1 following SP and SFCM treatment. Wound healing scratch assay revealed enhanced migration of epithelial cells following the addition of SFCM. Taken together, we conclude that SFCM-mediated sustained activation of ZEB1, Slug in combination with upregulated migration-associated integrins and ERK (Extracellular signal-regulated kinase)-FAK-paxillin axis, may lead to induce type 2 EMT-like changes during corneal epithelial wound healing.

Over-expression of ΔNp63α facilitates rat corneal wound healing in vivo

To investigate the roles of ΔNp63α during corneal wound healing and the genes regulated by ΔNp63α in limbal epithelial cells. Adenovirus or shRNA targeting ΔNp63α were pre-injected into the anterior chamber of rat eyeballs and the central corneal epithelium was then wounded with NaOH. The effects of ΔNp63α expression during wound healing were observed by propidium iodide staining. In addition, limbal epithelial cells were cultured and ectopically expressed ΔNp63α by transfecting Ad-ΔNp63α. Total RNA was extracted from transfected epithelial cells and subjected to a gene expression microarray assay. The results showed that over-expression of ΔNp63α accelerated the process of corneal wound healing while knockdown of ΔNp63α impaired the process. ΔNp63α positively up-regulated several cell growth promoter genes and could be referred as a positive regulator of limbal epithelial cell proliferation. It might also inhibit cell differentiation and cell death by differential target gene regulation.

Concise Review: Stem Cells for Corneal Wound Healing

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

Fibroblast Growth Factor Receptor 2 (FGFR2) Is Required for Meibomian Gland Homeostasis in the Adult Mouse

Purpose

Little is known about the signaling mechanisms controlling meibomian gland (MG) homeostasis and the pathogenic processes leading to MG atrophy and dysfunction in dry eye disease (DED). We investigated the role of fibroblast growth factor receptor 2 (FGFR2) in the MG homeostasis of adult mice.

Methods

A triple transgenic mouse strain (Krt14-rtTA; tetO-Cre; Fgfr2flox/flox), referred to as Fgfr2CKO mice, was generated in which the Fgfr2 gene is ablated by Cre recombinase in keratin 14 (Krt14)-expressing epithelial cells on doxycycline (Dox) induction. FGFR2 expression in normal human and mouse MGs was evaluated by immunohistochemistry. Pathologic MG changes in transgenic mice with conditional deletion of FGFR2 were examined by lipid staining, histology, and immunostaining.

Results

FGFR2 was highly expressed in normal human MGs and adult mouse MGs. Two-month-old Fgfr2CKO mice fed Dox-containing chow for 2 weeks developed severe MG atrophy. MG acinar atrophy in the Fgfr2CKO mice was associated with reduced lipid (meibum) production and the development of clinical findings similar to those in humans with evaporative DED related to MG dysfunction (MGD). Immunohistochemical analyses showed that FGFR2 deletion severely affected proliferation and differentiation of MG acinar cells but affected MG ductal cells to a lesser extent.

Conclusions

FGFR2 deletion results in significant MG acinar atrophy and clinical manifestations of MGD in Fgfr2CKO mice, suggesting that MG homeostasis is FGFR2 dependent. The Fgfr2CKO mice with inducible MG atrophy can serve as a valuable animal model for investigating the pathogenesis of MGD and developing novel therapeutic strategies for MGD-related DED.

Induced pluripotent stem cell-derived limbal epithelial cells (LiPSC) as a cellular alternative for in vitro ocular toxicity testing

Induced pluripotent stem cells hold great potential to produce unlimited amount of differentiated cells as cellular source for regenerative medicine but also for in vitro drug screening and cytotoxicity tests. Ocular toxicity testing is mandatory to evaluate the risks of drugs and cosmetic products before their application to human patients by preventing eye irritation or insult. Since the global ban to use animals, many human-derived alternatives have been proposed, from ex-vivo enucleated postmortem cornea, primary corneal cell culture and immortalized corneal epithelial cell lines. All of them share limitations for their routine use. Using an improved protocol, we derived limbal epithelial cells from human induced pluripotent stem cells, named LiPSC, that are able to be passaged and differentiate further into corneal epithelial cells. Comparative RT-qPCR, immunofluorescence staining, flow cytometry analysis and zymography assays demonstrate that LiPSC are morphologically and molecularly similar to the adult stem cells. Moreover, contrary to HCE, LiPSC and primary limbal cells display similarly sensitive to cytotoxicity treatment among passages. Our data strongly suggest that LiPSC could become a powerful alternative cellular model for cosmetic and drug tests.

High-resolution promoter map of human limbal epithelial cells cultured with keratinocyte growth factor and rho kinase inhibitor

An in vitro model of corneal epithelial cells (CECs) has been developed to study and treat corneal disorders. Nevertheless, conventional CEC culture supplemented with epidermal growth factor (EGF) results in a loss of CEC characteristics. It has recently been reported that limbal epithelial cells (LECs) cultured with keratinocyte growth factor (KGF) and the rho kinase inhibitor Y-27632 could maintain the expression of several CEC-specific markers. However, the molecular mechanism underlying the effect of culture media on LECs remains to be elucidated. To elucidate this mechanism, we performed comprehensive gene expression analysis of human LECs cultured with EGF or KGF/Y-27632, by cap analysis of gene expression (CAGE). Here, we found that LECs cultured with KGF and Y-27632 presented a gene expression profile highly similar to that of CECs in vivo. In contrast, LECs cultured with EGF lost the characteristic CEC gene expression profile. We further discovered that CEC-specific PAX6 promoters are highly activated in LECs cultured with KGF and Y-27632. Our results provide strong evidence that LECs cultured with KGF and Y-27632 would be an improved in vitro model in the context of gene expression. These findings will accelerate basic studies of CECs and clinical applications in regenerative medicine.

Long-term homeostasis and wound healing in an in vitro epithelial stem cell niche model

Cultures of epithelial cells are limited by the proliferative capacity of primary cells and cell senescence. Herein we show that primary human epithelial cell sheets cultured without dermal equivalents maintained homeostasis in vitro for at least 1 year. Transparency of these sheets enabled live observation of pigmented melanocytes and Fluorescent Ubiquitination-based Cell Cycle Indicator (FUCCI) labeled epithelial cells during wound healing. Cell turn over and KRT15 expression pattern stabilized within 3 months, when KRT15 bright clusters often associated with niche-like melanocytes became apparent. EdU labels were retained in a subset of epithelial cells and melanocytes after 6 months chasing, suggesting their slow cell cycling property. FUCCI-labeling demonstrated robust cell migration and proliferation following wounding. Transparency and long-term (1 year) homeostasis of this model will be a powerful tool for the study of wound healing and cell linage tracing.

Optimization of Corneal Epithelial Progenitor Cell Growth on Bombyx mori Silk Fibroin Membranes

Scaffolds prepared from silk fibroin derived from cocoons of the domesticated silkworm moth Bombyx mori have demonstrated potential to support the attachment and growth of human limbal epithelial (HLE) cells in vitro. In this study, we attempted to further optimize protocols to promote the expansion of HLE cells on B. mori silk fibroin- (BMSF-) based scaffolds. BMSF films were initially coated with different extracellular matrix proteins and then analysed for their impact on corneal epithelial cell adhesion, cell morphology, and culture confluency. Results showed that collagen I, collagen III, and collagen IV consistently improved HCE-T cell adherence, promoted an elongated cell morphology, and increased culture confluency. By contrast, ECM coating had no significant effect on the performance of primary HLE cells cultured on BMSF films. In the second part of this study, primary HLE cells were grown on BMSF films in the presence of medium (SHEM) supplemented with keratinocyte growth factor (KGF) and the Rho kinase inhibitor, Y-27632. The results demonstrated that SHEM medium supplemented with KGF and Y-27632 dramatically increased expression of corneal differentiation markers, keratin 3 and keratin 12, whereas expression of the progenitor marker, p63, did not appear to be significantly influenced by the choice of culture medium.

Preservation of human limbal epithelial progenitor cells on carbodiimide cross-linked amniotic membrane via integrin-linked kinase-mediated Wnt activation

The Wnt pathway is a major signaling pathway that regulates corneal epithelial stem cells. However, little is known about how the ultrastructure of the limbal epithelial basement membrane (EBM) affects Wnt activity. Due to its enhanced matrix stability, the cross-linked amniotic membrane (AM) has gained increasing interest in the field of regenerative medicine. For the first time, we used EDC/NHS cross-linked denuded AM (CLDAM) as a simulated EBM substrate to investigate this mechanism. Human limbal epithelial (HLE) cells were cultured on dishes (HLE/dish), denuded AM (HLE/DAM) or CLDAM (HLE/CLDAM). Compared with HLE/dish or HLE/DAM cultures, HLE/CLDAM cultures showed greater BrdU retention and colony formation efficiency and expressed higher levels of p63, ABCG2, integrin β1, and integrin-linked kinase (ILK). Nuclear β-catenin and TCF-4 levels were higher in HLE/CLDAM cultures compared with HLE cells cultured on collagen IV, laminin, Matrigel, or DAM. Silencing of ILK in HLE/CLDAM cultures resulted in decreased levels of nuclear β-catenin, TCF-4 and deltaNp63α, whereas cytokeratin 12 expression increased. Over-expression of ILK in HLE/dish cultures had the opposite effects. Accordingly, we proposed that the CLDAM matrix, with its higher rigidity and rougher ultrastructure, better preserved HLE progenitor cells in vitro, possibly by activating integrin β1/ILK, which indirectly activated Wnt/β-catenin and subsequently deltaNp63α. Crosstalk between the integrin β1/ILK and Wnt/β-catenin pathways appears to play a crucial role in limbal progenitor cell survival on EBM.

STATEMENT OF SIGNIFICANCE

We demonstrated the superior capability of carbodiimide cross-linked denuded amniotic membrane (CLDAM) than natural DAM to preserve limbo-corneal epithelial progenitor cells in vitro, then we used CLDAM as a simulated epithelial basement membrane (EBM) to study how EBM maintains limbal epithelial stem cells (LESCs). We found that integrin-linked kinase (ILK) is an important mediator that transfers survival signals detected by integrin β1 to the Wnt/β-catenin pathway, which in turn up-regulates deltaNp63α, a master gene that regulates LESC function. The rougher surface of the limbal EBM suggests that the surface complexity of the LESC niche may be important in regulating LESC function, which is triggered by the recognition of topographic cues by integrin β1, followed by activation of the ILK/Wnt/β-catenin/p63 cascade.

Progress in corneal wound healing

Corneal wound healing is a complex process involving cell death, migration, proliferation, differentiation, and extracellular matrix remodeling. Many similarities are observed in the healing processes of corneal epithelial, stromal and endothelial cells, as well as cell-specific differences. Corneal epithelial healing largely depends on limbal stem cells and remodeling of the basement membrane. During stromal healing, keratocytes get transformed to motile and contractile myofibroblasts largely due to activation of transforming growth factor-β (TGF-β) system. Endothelial cells heal mostly by migration and spreading, with cell proliferation playing a secondary role. In the last decade, many aspects of wound healing process in different parts of the cornea have been elucidated, and some new therapeutic approaches have emerged. The concept of limbal stem cells received rigorous experimental corroboration, with new markers uncovered and new treatment options including gene and microRNA therapy tested in experimental systems. Transplantation of limbal stem cell-enriched cultures for efficient re-epithelialization in stem cell deficiency and corneal injuries has become reality in clinical setting. Mediators and course of events during stromal healing have been detailed, and new treatment regimens including gene (decorin) and stem cell therapy for excessive healing have been designed. This is a very important advance given the popularity of various refractive surgeries entailing stromal wound healing. Successful surgical ways of replacing the diseased endothelium have been clinically tested, and new approaches to accelerate endothelial healing and suppress endothelial-mesenchymal transformation have been proposed including Rho kinase (ROCK) inhibitor eye drops and gene therapy to activate TGF-β inhibitor SMAD7. Promising new technologies with potential for corneal wound healing manipulation including microRNA, induced pluripotent stem cells to generate corneal epithelium, and nanocarriers for corneal drug delivery are discussed. Attention is also paid to problems in wound healing understanding and treatment, such as lack of specific epithelial stem cell markers, reliable identification of stem cells, efficient prevention of haze and stromal scar formation, lack of data on wound regulating microRNAs in keratocytes and endothelial cells, as well as virtual lack of targeted systems for drug and gene delivery to select corneal cells.

1α, 25-Dihydroxyvitamin D₃ and the vitamin D receptor regulates ΔNp63α levels and keratinocyte proliferation

1α, 25-dihydroxyvitamin D₃ (VD₃), a secosteriod that has been explored as an anti-cancer agent, was also shown to promote cell survival. Its receptor, the Vitamin D Receptor (VDR), is a direct target of the proto-oncogene ΔNp63α, which is overexpressed in non-melanoma skin cancers. The interconnection between VDR/VD₃ signaling and ΔNp63α, led us to examine whether VDR/VD₃ signaling promotes keratinocyte proliferation by regulating ΔNp63α levels. Our data demonstrate that VDR regulates ΔNp63α expression at both the transcript and protein level. Interestingly, although low doses of VD₃ led to an increase in ΔNp63α protein levels and keratinocyte proliferation, high doses of VD₃ failed to increase ΔNp63α protein levels and resulted in reduced proliferation. Increased expression of ΔNp63α by low dose VD₃ was shown to be dependent on VDR and critical for the proliferative effects of VD₃. VD₃-mediated increases in ΔNp63α protein levels occur via activation of both p38 MAPK and Akt kinases. Finally, analysis of samples from patients with squamous cell carcinoma (SCC), basal cell carcinoma and precursors to invasive SCC demonstrated a significant correlation between p63 and VDR levels when compared with healthy normal skin control samples. Delineation of the mechanisms by which VD₃ exerts its effect on ΔNp63α and cell proliferation is critical for determining the future of VD₃ in cancer therapies.

Stromal-epithelial interaction study: The effect of corneal epithelial cells on growth factor expression in stromal cells using organotypic culture model

Interactions between stromal and epithelial cells play important roles in the development, homeostasis, and pathological conditions of the cornea. Soluble cytokines are critical factors in stromal-epithelial interactions, and growth factors secreted from corneal stromal cells contribute to the regulation of proliferation and differentiation of corneal epithelial cells (CECs). However, the manner in which the expression of growth factors is regulated in stromal cells has not been completely determined. To study stromal-epithelial cell interactions, we used an organotypic culture model. Human or rabbit CECs (HCECs or RCECs) were cultured on amniotic membranes placed on human corneal fibroblasts (HCFs) embedded in a collagen gel. The properties of the organotypic culture were examined by hematoxylin-eosin staining and immunofluorescence. In the organotypic culture, HCECs or RCECs were stratified into two-three layers after five days and five-seven layers after nine days. However, stratification was not observed when the HCECs were seeded on a collagen gel without fibroblasts. K3/K12 were expressed on day 9. The HCF-embedded collagen gels were collected on days 3, 5, or 9 after seeding the RCECs, and mRNA expression of growth factors FGF7, HGF, NGF, EGF, TGF-α, SCF, TGF-β1, TGF-β2, and TGF-β3 were quantified by real-time PCR. mRNA expression of the growth factors in HCFs cultured with RCECs were compared with those cultured without RCECs, as well as in monolayer cultures. mRNA expression of TGF-α was markedly increased in HCFs cultured with RCECs. However, mRNA expression of the TGF-β family was suppressed in HCFs cultured with RCECs. Principal component analysis revealed that mRNA expression of the growth factors in HCFs were generally similar when they were cultured with RCECs. In organotypic cultures, the morphological changes in the CECs and the expression patterns of the growth factors in the stromal cells clearly demonstrated stromal-epithelial cell interactions, and the results suggest that stromal cells and epithelial cells may act in concert in the cornea.

Pathway Regulation of p63, a Director of Epithelial Cell Fate

The p53-related gene p63 is required for epithelial cell establishment and its expression is often altered in tumor cells. Great strides have been made in understanding the pathways and mechanisms that regulate p63 levels, such as the Wnt, Hedgehog, Notch, and EGFR pathways. We discuss here the multiple signaling pathways that control p63 expression as well as transcription factors and post-transcriptional mechanisms that regulate p63 levels. While a unified picture has not emerged, it is clear that the fine-tuning of p63 has evolved to carefully control epithelial cell differentiation and fate.

Normalization of wound healing and stem cell marker patterns in organ-cultured human diabetic corneas by gene therapy of limbal cells

Overexpression of c-met and suppression of matrix metalloproteinase-10 (MMP-10) and cathepsin F genes was previously shown to normalize wound healing, epithelial and stem cell marker patterns in organ-cultured human diabetic corneas. We now examined if gene therapy of limbal cells only would produce similar effects. Eight pairs of organ-cultured autopsy human diabetic corneas were used. One cornea of each pair was treated for 48 h with adenoviruses (Ad) harboring full-length c-met mRNA or a mixture (combo) of Ad with c-met and shRNA to MMP-10 and cathepsin F genes. Medium was kept at the limbal level to avoid transduction of central corneal epithelium. Fellow corneas received control Ad with EGFP gene. After additional 5 (c-met) or 10 days (combo) incubation, central corneal epithelial debridement with n-heptanol was performed, and wound healing times were determined microscopically. Corneal cryostat sections were immunostained for diabetic and putative limbal stem cell markers, α3β1 integrin, nidogen-1, fibronectin, laminin γ3 chain, ΔNp63α, keratins 14, 15, and 17, as well as for activated signaling intermediates, phosphorylated EGFR, Akt, and p38. Limbal c-met overexpression significantly accelerated healing of 8.5-mm epithelial wounds over EGFP controls (6.3 days vs. 9.5 days, p < 0.02). Combo treatment produced a similar result (6.75 days vs. 13.5 days, p < 0.03). Increased immunostaining vs. EGFP controls for most markers and signaling intermediates accompanied c-met gene or combo transduction. Gene therapy of limbal epithelial stem cell compartment has a beneficial effect on the diabetic corneal wound healing and on diabetic and stem cell marker expression, and shows potential for alleviating symptoms of diabetic keratopathy.

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

Differentiation of human limbal-derived induced pluripotent stem cells into limbal-like epithelium

Limbal epithelial stem cell (LESC) deficiency (LSCD) leads to corneal abnormalities resulting in compromised vision and blindness. LSCD can be potentially treated by transplantation of appropriate cells, which should be easily expandable and bankable. Induced pluripotent stem cells (iPSCs) are a promising source of transplantable LESCs. The purpose of this study was to generate human iPSCs and direct them to limbal differentiation by maintaining them on natural substrata mimicking the native LESC niche, including feederless denuded human amniotic membrane (HAM) and de-epithelialized corneas. These iPSCs were generated with nonintegrating vectors from human primary limbal epithelial cells. This choice of parent cells was supposed to enhance limbal cell differentiation from iPSCs by partial retention of parental epigenetic signatures in iPSCs. When the gene methylation patterns were compared in iPSCs to parental LESCs using Illumina global methylation arrays, limbal-derived iPSCs had fewer unique methylation changes than fibroblast-derived iPSCs, suggesting retention of epigenetic memory during reprogramming. Limbal iPSCs cultured for 2 weeks on HAM developed markedly higher expression of putative LESC markers ABCG2, ΔNp63α, keratins 14, 15, and 17, N-cadherin, and TrkA than did fibroblast iPSCs. On HAM culture, the methylation profiles of select limbal iPSC genes (including NTRK1, coding for TrkA protein) became closer to the parental cells, but fibroblast iPSCs remained closer to parental fibroblasts. On denuded air-lifted corneas, limbal iPSCs even upregulated differentiated corneal keratins 3 and 12. These data emphasize the importance of the natural niche and limbal tissue of origin in generating iPSCs as a LESC source with translational potential for LSCD treatment.

Suitability of human Tenon's fibroblasts as feeder cells for culturing human limbal epithelial stem cells

Corneal epithelial regeneration through ex vivo expansion of limbal stem cells (LSCs) on 3T3-J2 fibroblasts has revealed some limitations mainly due to the corneal microenvironment not being properly replicated, thus affecting long term results. Insights into the feeder cells that are used to expand LSCs and the mechanisms underlying the effects of human feeder cells have yet to be fully elucidated. We recently developed a standardized methodology to expand human Tenon's fibroblasts (TFs). Here we aimed to investigate whether TFs can be employed as feeder cells for LSCs, characterizing the phenotype of the co-cultures and assessing what human soluble factors are secreted. The hypothesis that TFs could be employed as alternative human feeder layer has not been explored yet. LSCs were isolated from superior limbus biopsies, co-cultured on TFs, 3T3-J2 or dermal fibroblasts (DFs), then analyzed by immunofluorescence (p63α), colony-forming efficiency (CFE) assay and qPCR for a panel of putative stem cell and epithelial corneal differentiation markers (KRT3). Co-cultures supernatants were screened for a set of soluble factors. Results showed that the percentage of p63α(+)LSCs co-cultured onto TFs was significantly higher than those on DFs (p = 0.032) and 3T3-J2 (p = 0.047). Interestingly, LSCs co-cultures on TFs exhibited both significantly higher CFE and mRNA expression levels of ΔNp63α than on 3T3-J2 and DFs (p < 0.0001), showing also significantly greater levels of soluble factors (IL-6, HGF, b-FGF, G-CSF, TGF-β3) than LSCs on DFs. Therefore, TFs could represent an alternative feeder layer to both 3T3-J2 and DFs, potentially providing a suitable microenvironment for LSCs culture.

PEDF promotes self-renewal of limbal stem cell and accelerates corneal epithelial wound healing

Limbal epithelial stem cell (LSC) transplantation is a prevalent therapeutic method for patients with LSC deficiency. The maintenance of stem cell characteristics in the process of culture expansion is critical for the success of ocular surface reconstruction. Pigment epithelial-derived factor (PEDF) increased the numbers of holoclone in LSC monolayer culture and preserved the stemness of LSC in suspension culture by evidence of ΔNp63α, Bmi-1, and ABCG2 expression. BrdU pulse-labeling assay also demonstrated that PEDF stimulated LSCs proliferation. In air-lift culture of limbal equivalent, PEDF was capable of increasing the numbers of ΔNp63α-positive cells. The mitogenic effect of PEDF was found to be mediated by the phosphorylations of p38 MAPK and STAT3 in LSCs. Synthetic 44-mer PEDF (residues 78-121) was as effective as the full length PEDF in LSC expansion in suspension culture and limbal equivalent formation, as well as the activation of p38 MAPK and STAT3. In mice subjecting to mechanical removal of cornea epithelium, 44-mer PEDF facilitated corneal wound healing. Microscopically, 44-mer PEDF advanced the early proliferative response in limbus, increased the proliferation of ΔNp63α-positive cells both in limbus and in epithelial healing front, and assisted the repopulation of limbus in the late phase of wound healing. In conclusion, the capability of expanding LSC in cell culture and in animal indicates the potential of PEDF and its fragment (e.g., 44-mer PEDF) in ameliorating limbal stem cell deficiency; and their uses as therapeutics for treating corneal wound.

Keratinocyte growth factor-2 and autologous serum potentiate the regenerative effect of mesenchymal stem cells in cornea damage in rats

AIM

To investigate the healing process after severe corneal epithelial damage in rats treated with mesenchymal stem cells (MSCs) cultured with or without keratinocyte growth factor (KGF-2) and autologous serum (AS) on amniotic membrane (AM). Many patients are blind and devastated by severe ocular surface diseases due to limbal stem cell deficiency. Bone marrow-derived MSCs are potential sources for cell-based tissue engineering to repair or replace the corneal tissue, having the potential to differentiate to epithelial cells.

METHODS

The study included 5 groups each including 10 female "Sprague Dawley" rats in addition to 20 male rats used as bone marrow donors. Group I rats received AM+MSCs, Group II rats AM+MSCs cultured with KGF-2, Group III rats AM+MSCs cultured with KGF-2+AS, Group IV rats only AM and Group V rats, none. AS was derived from blood drawn from male rats and bone marrow was obtained from the femur and tibia bones of the same animals. Therapeutic effect was evaluated with clinical, histopathological and immunohistochemical assessment. MSC engraftment was demonstrated via detection of donor genotype (Y+) in the recipient tissue (X) with polymerase chain reaction.

RESULTS

Corneal healing was significantly better in Groups I-III rats treated with MSC transplantation compared to Group IV and Group V rats with supportive treatment only. The best results were obtained in Group III rats with 90% transparency, 70% lack of neovascularization, and 100% epithelium damage limited to less than 1/4 of cornea.

CONCLUSION

We suggest that culture of MSCs with KGF-2 and AS on AM is effective in corneal repair in case of irreversible damage to limbal stem cells.

Enhanced wound healing, kinase and stem cell marker expression in diabetic organ-cultured human corneas upon MMP-10 and cathepsin F gene silencing

PURPOSE

Diabetic corneas overexpress proteinases including matrix metalloproteinase-10 (M10) and cathepsin F (CF). Our purpose was to assess if silencing M10 and CF in organ-cultured diabetic corneas using recombinant adenovirus (rAV)-driven small hairpin RNA (rAV-sh) would normalize slow wound healing, and diabetic and stem cell marker expression.

METHODS

Sixteen pairs of organ-cultured autopsy human diabetic corneas (four per group) were treated with rAV-sh. Proteinase genes were silenced either separately, together, or both, in combination (Combo) with rAV-driven c-met gene overexpression. Fellow control corneas received rAV-EGFP. Quantitative RT-PCR confirmed small hairpin RNA (shRNA) silencing effect. Ten days after transfection, 5-mm epithelial wounds were made with n-heptanol and healing time recorded. Diabetic, signaling, and putative stem cell markers were studied by immunofluorescence of corneal cryostat sections.

RESULTS

Proteinase silencing reduced epithelial wound healing time versus rAV-enhanced green fluorescent protein (EGFP) control (23% for rAV-shM10, 31% for rAV-shCF, and 36% for rAV-shM10 + rAV-shCF). Combo treatment was even more efficient (55% reduction). Staining patterns of diabetic markers (α₃β₁ integrin and nidogen-1), and of activated epidermal growth factor receptor and its signaling target activated Akt were normalized upon rAV-sh treatment. Combo treatment also restored normal staining for activated p38. All treatments, especially the combined ones, increased diabetes-altered staining for putative limbal stem cell markers, ΔNp63α, ABCG2, keratins 15 and 17, and laminin γ3 chain.

CONCLUSIONS

Small hairpin RNA silencing of proteinases overexpressed in diabetic corneas enhanced corneal epithelial and stem cell marker staining and accelerated wound healing. Combined therapy with c-met overexpression was even more efficient. Specific corneal gene therapy has a potential for treating diabetic keratopathy.

Long-term maintenance of limbal epithelial progenitor cells using rho kinase inhibitor and keratinocyte growth factor

Corneal epithelial stem cells are located in the limbus, the junction between the cornea and the conjunctiva. A limbal epithelium model in vitro would be useful for the study of epithelial stem cells, as well as improving the quality of cultivated epithelial sheets for the treatment of limbal stem cell deficiency. In this study, we succeeded in constructing a limbal epithelium-like structure that could be maintained for at least 5 months in vitro. We modified conventional medium by replacing epidermal growth factor with keratinocyte growth factor (KGF) and adding Y-27632, a rho kinase inhibitor. Using this medium, epithelial cells freshly isolated from human limbus were cocultured with human mesenchymal stem cell-derived feeder cells. Cells formed a stratified layer without air exposure, and both basal and suprabasal layers maintained their unique morphologies for up to 5 months. Basal layers expressed the progenitor marker p63 uniformly and K15 heterogeneously. Expressions of PAX6, K3, and K12 indicated that cell sheets underwent normal differentiation in the corneal epithelium lineage. Although medium was changed daily after day 7, cell debris was observed every day, suggesting that cell sheets underwent turnover. Furthermore, secondary colonies were observed from cells dissociated from 1-month and 3-month cultured sheets. In conclusion, human limbal epithelial cell sheet cultures with KGF and Y-27632 maintained stratification, high expression of both stem/progenitor markers and differentiation markers, and colony-forming cells long-term. This protocol may be useful as an in vitro limbal epithelial model for basic studies.

Transcriptional regulators of the ΔNp63: their role in limbal epithelial cell proliferation

The surface cells of corneal epithelium are regularly shed off and replaced by new cells that are derived from limbal epithelial stem cells (LESC). LESC are believed to reside in the basal layer of the limbal epithelium and are characterized with high expression levels of ΔNp63, a transcription factor (TF) which is believed to play roles in the regulation of LESC proliferation. In this study, we examined the transcriptional regulation of ΔNp63 in limbal epithelial cell. We employed DNA pull down assay followed by LC/MS analysis and cDNA microarray analysis to identify the TFs that were capable of binding to ΔNp63 promoter or were expressed at higher levels in limbus over cornea. The TFs thus selected were further examined for their in vivo ΔNp63 promoter binding by chromatin immunoprecipitation assay. We identified six putative TFs (PAX6, EGR1, CEBPB, JUN, ATF3, and ARID5B) through the aforementioned approaches. Among them, PAX6 and EGR1 were shown to promote the transcription of ΔNp63 and led to increased cell proliferation. In contrast, CEBPB and ATF3 appeared to exert little or no effect on ΔNp63 expression, however, their silencing suppressed cell proliferation. Although JUN exhibited low promoter-binding specificity, however, it affected ΔNp63 expression and limbal epithelial cell proliferation in ways similar to that of PAX6 and EGR1. Intriguingly, ARID5B was highly expressed in the limbal epithelial cell, however, its silencing by siRNA did not obviously affect the expression of ΔNp63, nor did it reduce cell proliferation of the limbal epithelial cell.

The key role of insulin-like growth factor I in limbal stem cell differentiation and the corneal wound-healing process

Limbal stem cells (LSC), which reside in the basal layer of the limbus, are thought to be responsible for corneal epithelial healing after injury. When the cornea is damaged, LSC start to proliferate, differentiate, and migrate to the site of injury. To characterize the signaling molecules ensuring communication between the cornea and LSC, we established a mouse model of mechanical corneal damage. The central cornea or limbal tissue was excised at different time intervals after injury, and the expression of genes in the explants was determined. It was observed that a number of genes for growth and differentiation factors were significantly upregulated in the cornea rapidly after injury. The ability of these factors to regulate the differentiation and proliferation of limbal cells was tested. It was found that the insulin-like growth factor-I (IGF-I), which is rapidly overexpressed after injury, enhances the expression of IGF receptor in limbal cells and induces the differentiation of LSC into cells expressing the corneal cell marker, cytokeratin K12, without any effect on limbal cell proliferation. In contrast, the epidermal growth factor (EGF) and fibroblast growth factor-β (FGF-β), which are also produced by the damaged corneal epithelium, supported limbal cell proliferation without any effect on their differentiation. Other factors did not affect limbal cell differentiation or proliferation. Thus, IGF-I was identified as the main factor stimulating the expression of IGF receptors in limbal cells and inducing the differentiation of LSC into cells expressing corneal epithelial cell markers. The proliferation of these cells was supported by EGF and FGF.

Changes in corneal basal epithelial phenotypes in an altered basement membrane

Background

To examine the corneal epithelial phenotype in an altered basement membrane.

Methodology/Principal Findings

Corneas from 9 patients with symptoms of continuous unstable corneal curvature (CUCC) were harvested by penetrating keratoplasty and subjected to histology examination and immunohistochemical staining with transactivating and N-terminally truncated pP63 transcript (ΔNp63), cytokeratin 3 (Krt3), ATP-binding cassette sub-family G member 2 (ABCG2), connexin 43 (CX43), p38 mitogen-activated protein kinases (p38MAPK), activating protein 2 (TFAP2), and extracellular signal-regulated kinase (Erk1/2) monoclonal antibodies. Positive immunostaining with ABCG2, p38MAPK, and TFAP2 monoclonal antibodies was observed in the basal epithelial cells of CUCC patients, and CX43 and ΔNp63 were detected in the full-thickness epithelial cells of CUCC patients.

Conclusions/Significance

Our results indicate that alteration of the corneal basement membrane induces a de-differentiation-like phenotype in corneal basal epithelial cells.

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

Asymmetrical cell division in mammalian corneal epithelia is not widely accepted and it is not well characterized. Although some data led to propose that asymmetrical division occurs along the entire corneal epithelium, evidence from different laboratories implies that asymmetrical cell division in adult individuals could be confined to limbal epithelium, as suggested by the location of the corneal epithelial stem cells and the distribution of some molecular markers involved in regulation of stratification and cell differentiation. Nevertheless, most evidence sustaining the participation of asymmetric mitosis in corneal development and differentiation is merely an inference. Recent results based in cell culture experiments suggest that asymmetric division is part of the differentiation program; in such case, mitotic spindle orientation would be regulated by the structure, composition and active signaling pathways at the stem cell niche. Together, the results support the view that in adults, asymmetric division might be confined to limbus, and hence, the process takes place both in apico-basal oriented cells and in cells in which the mitotic spindle is horizontally oriented. In contrast, during development, asymmetrical divisions would be determined by intrinsic mechanisms involving cell polarity, predominantly occurring in apico-basal oriented cells. Future studies should be oriented to understand the regulation of the asymmetrical/symmetrical division, and the control of cell fate by the niche.

Resveratrol-induced p53-independent apoptosis of human nasopharyngeal carcinoma cells is correlated with the downregulation of ΔNp63

ΔNp63, the N-terminal truncated isoform of p63, has been found to be overexpressed in several human epithelial cancers, including nasopharyngeal carcinomas (NPCs), suggesting a function in carcinogenesis. Trans-resveratrol (RSV) has been shown to exert proapoptotic activities through a p53-dependent or p53-independent pathway in various cancer cells. However, the effects of RSV on NPC are still unexplored. In this study, we investigated the apoptotic effects of RSV on ΔNp63-overexpressing NPC cell lines. We showed that RSV (12-100 μ) induced dose-dependent growth suppression, cell-cycle arrest in the S phase and caspase-dependent apoptosis in NPC-TW076 and NPC-TW039 cells. The RSV effect was accompanied by the downregulation of ΔNp63 and the upregulation of p53 protein in a dose-dependent manner. By using small-interfering RNA (siRNA) technology, we found that the targeted silencing of ΔNp63 induced apoptosis and sensitized the NPC cells to RSV-induced apoptosis through caspase-3 activation, whereas suppression of p53 by siRNA did not inhibit RSV-induced apoptosis. Furthermore, transfection with p53 siRNA or pretreatment with caspase inhibitors (Z-VAD-fmk or Z-DEVD-fmk) had no influence on the RSV downregulation of ΔNp63. Interestingly, ecoptic expression of ΔNp63 did not significantly block RSV-induced cell death and was also downregulated after RSV treatment. Downregulation of ΔNp63 by RSV was shown to occur at the mRNA transcript and post-translational levels. Importantly, RSV enhanced chemotheraptic drug-induced apoptosis in NPC and two human carcinoma cell lines, HT1376 and Hep3B cells. These results suggested that ΔNp63, but not p53, is a molecular target of RSV-induced apoptosis and the regulation of ΔNp63 expression by RSV may provide a therapeutic effect of RSV in human NPC.