Transcription factor TCF4 maintains the properties of human corneal epithelial stem cells

Sox9 marks limbal stem cells and is required for asymmetric cell fate switch in the corneal epithelium

Adult tissues with high cellular turnover require a balance between stem cell renewal and differentiation, yet the mechanisms underlying this equilibrium are unclear. The cornea exhibits a polarized lateral flow of progenitors from the peripheral stem cell niche to the center; attributed to differences in cellular fate. To identify genes that are critical for regulating the asymmetric fates of limbal stem cells and their transient amplified progeny in the central cornea, we utilized an in vivo cell cycle reporter to isolate proliferating basal cells across the anterior ocular surface epithelium and performed single-cell transcriptional analysis. This strategy greatly increased the resolution and revealed distinct basal cell identities with unique expression profiles of structural genes and transcription factors. We focused on Sox9; a transcription factor implicated in stem cell regulation across various organs. Sox9 was found to be differentially expressed between limbal stem cells and their progeny in the central corneal. Lineage tracing analysis confirmed that Sox9 marks long-lived limbal stem cells and conditional deletion led to abnormal differentiation and squamous metaplasia in the central cornea. These data suggest a requirement for Sox9 for the switch to asymmetric fate and commitment toward differentiation, as transient cells exit the limbal niche. By inhibiting terminal differentiation of corneal progenitors and forcing them into perpetual symmetric divisions, we replicated the Sox9 loss-of-function phenotype. Our findings reveal an essential role for Sox9 for the spatial regulation of asymmetric fate in the corneal epithelium that is required to sustain tissue homeostasis.

Ectoine protects corneal epithelial survival and barrier from hyperosmotic stress by promoting anti-inflammatory cytokine IL-37

PURPOSE

To explore novel role and molecular mechanism of a natural osmoprotectant ectoine in protecting corneal epithelial cell survival and barrier from hyperosmotic stress.

METHODS

Primary human corneal epithelial cells (HCECs) were established from donor limbus. The confluent cultures in isosmolar medium were switched to hyperosmotic media (400-500 mOsM), with or without ectoine or rhIL-37 for different time periods. Cell viability and proliferation were evaluated by MTT or WST assay. The integrity of barrier proteins and the expression of cytokines and cathepsin S were evaluated by RT-qPCR, ELISA, and immunostaining with confocal microscopy.

RESULTS

HCECs survived well in 450mOsM but partially damaged in 500mOsM medium. Ectoine well protected HCEC survival and proliferation at 500mOsM. The integrity of epithelial barrier was significantly disrupted in HCECs exposed to 450mOsM, as shown by 2D and 3D confocal immunofluorescent images of tight junction proteins ZO-1 and occludin. Ectoine at 5-20 mM well protected these barrier proteins under hyperosmotic stress. The expression of TNF-α, IL-1β, IL-6 and IL-8 were dramatically stimulated by hyperosmolarity but significantly suppressed by Ectoine at 5-40 mM. Cathepsin S, which was stimulated by hyperosmolarity, directly disrupted epithelial barrier. Interestingly, anti-inflammatory cytokine IL-37 was suppressed by hyperosmolarity, but restored by ectoine at mRNA and protein levels. Furthermore, rhIL-37 suppressed cathepsin S and rescued cell survival and barrier in HCECs exposed to hyperosmolarity.

CONCLUSION

Our findings demonstrate that ectoine protects HCEC survival and barrier from hyperosmotic stress by promoting IL-37. This provides new insight into pathogenesis and therapeutic potential for dry eye disease.

Corneal epithelial development and homeostasis

The corneal epithelium (CE), the most anterior cellular structure of the eye, is a self-renewing stratified squamous tissue that protects the rest of the eye from external elements. Each cell in this exquisite three-dimensional structure needs to have proper polarity and positional awareness for the CE to serve as a transparent, refractive, and protective tissue. Recent studies have begun to elucidate the molecular and cellular events involved in the embryonic development, post-natal maturation, and homeostasis of the CE, and how they are regulated by a well-coordinated network of transcription factors. This review summarizes the status of related knowledge and aims to provide insight into the pathophysiology of disorders caused by disruption of CE development, and/or homeostasis.

[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).

Regulation of Limbal Epithelial Stem Cells: Importance of the Niche

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

Human melanocyte development and melanoma dedifferentiation at single-cell resolution

In humans, epidermal melanocytes are responsible for skin pigmentation, defence against ultraviolet radiation and the deadliest common skin cancer, melanoma. Although there is substantial overlap in melanocyte development pathways between different model organisms, species-dependent differences are frequent and the conservation of these processes in human skin remains unresolved. Here, we used a single-cell enrichment and RNA-sequencing pipeline to study human epidermal melanocytes directly from the skin, capturing transcriptomes across different anatomical sites, developmental age, sexes and multiple skin tones. We uncovered subpopulations of melanocytes that exhibit anatomical site-specific enrichment that occurs during gestation and persists through adulthood. The transcriptional signature of the volar-enriched subpopulation is retained in acral melanomas. Furthermore, we identified human melanocyte differentiation transcriptional programs that are distinct from gene signatures generated from model systems. Finally, we used these programs to define patterns of dedifferentiation that are predictive of melanoma prognosis and response to immune checkpoint inhibitor therapy.

Single-Cell Transcriptomics Identifies a Unique Entity and Signature Markers of Transit-Amplifying Cells in Human Corneal Limbus

Purpose

Differentiated from adult stem cells (ASCs), transit-amplifying cells (TACs) play an important role in tissue homeostasis, development, and regeneration. This study aimed to characterize the gene expression profile of a candidate TAC population in limbal basal epithelial cells using single-cell RNA sequencing (scRNA-seq).

Methods

Single cells isolated from the basal corneal limbus were subjected to scRNA-seq using the 10x Genomics platform. Cell types were clustered by graph-based visualization methods and unbiased computational analysis. BrdU proliferation assays, immunofluorescent staining, and real-time reverse transcription quantitative polymerase chain reaction were performed using multiple culture models of primary human limbal epithelial cells to characterize the TAC pool.

Results

Single-cell transcriptomics of 16,360 limbal basal cells revealed 12 cell clusters. A unique cluster (3.21% of total cells) was identified as a TAC entity, based on its less differentiated progenitor status and enriched exclusive proliferation marker genes, with 98.1% cells in S and G2/M phases. The cell cycle-dependent genes were revealed to be largely enriched by the TAC population. The top genes were characterized morphologically and functionally at protein and mRNA levels. The specific expression patterns of RRM2, TK1, CENPF, NUSAP1, UBE2C, and CDC20 were well correlated in a time- and cycle-dependent manner with proliferation stages in the cell growth and regeneration models.

Conclusions

For the first time, to the best of our knowledge, we have identified a unique TAC entity and uncovered a group of cell cycle-dependent genes that serve as TAC signature markers. The findings provide insight into ASCs and TACs and lay the foundation for understanding corneal homeostasis and diseases.

EHF is essential for epidermal and colonic epithelial homeostasis, and suppresses Apc-initiated colonic tumorigenesis

Ets homologous factor (EHF) is a member of the epithelial-specific Ets (ESE) family of transcription factors. To investigate its role in development and epithelial homeostasis, we generated a series of novel mouse strains in which the Ets DNA-binding domain of Ehf was deleted in all tissues (Ehf-/-) or specifically in the gut epithelium. Ehf-/- mice were born at the expected Mendelian ratio, but showed reduced body weight gain, and developed a series of pathologies requiring most Ehf-/- mice to reach an ethical endpoint before reaching 1 year of age. These included papillomas in the facial skin, abscesses in the preputial glands (males) or vulvae (females), and corneal ulcers. Ehf-/-mice also displayed increased susceptibility to experimentally induced colitis, which was confirmed in intestinal-specific Ehf knockout mice. Gut-specific Ehf deletion also impaired goblet cell differentiation, induced extensive transcriptional reprogramming in the colonic epithelium and enhanced Apc-initiated adenoma development. The Ets DNA-binding domain of EHF is therefore essential for postnatal homeostasis of the epidermis and colonic epithelium, and its loss promotes colonic tumour development.

miR-133a-5p suppresses gastric cancer through TCF4 down-regulation

Background

The effect of microRNAs (miRNA) on cancer regulations has received a considerable amount of attention recently. MiR-133a-5p has been identified as an anti-tumor miRNA in several types of cancers. However, the effect of miR-133a-5p on gastric cancer (GC) have not been uncovered. In this study, we sought to evaluate the regulation of TCF4 expression by miR-133-5p and the role of the miR-25-3p/TCF4 axis in the progression of GC, with the aim of identifying a potential therapeutic target for GC.

Methods

TCGA (The Cancer Genome Atlas), GTEx (The Genotype-Tissue Expression) and GEO (Gene Expression Omnibus) database were used to analyze the expression and prognosis. We performed MTT and EdU assays to elucidate the effect on cell replication. Apoptotic cells were stained with annexin V-fluorescein isothiocyanate and propidium iodide to stain, and then analyzed by flow cytometry. The effect on cell metastasis was investigated in wound healing and transwell assays. A dual-luciferase reporter assay was used to check for the direct targeting of TCF4 by miR-133a-5p. Bioinformatic analysis of the relationship of TCF4 with tumor microenvironment and the signaling cascade of TCF4 was finally performed.

Results

We found that the level of miR-133a-5p was decreased in both tumor tissues and GC cell lines. MiR-133a-5p inhibited cell growth and metastasis, but promoted cell apoptosis. MiR-133a-5p directly targeted TCF4 and downregulated its expression. TCF4 was highly expressed in tumor and higher level of TCF4 indicated poorer prognosis. Moreover, TCF4 overexpression reversed the aforementioned anti-tumor activity of miR-133a-5p. The expression level of TCF4 was significantly correlated with tumor-infiltrating immune cells.

Conclusions

Our findings altogether reveal that miR-133a-5p can serve as a tumor suppressor in gastric cancer via the miR-133a-5p/TCF4 pathway.

Understanding cornea epithelial stem cells and stem cell deficiency: Lessons learned using vertebrate model systems

Animal models have contributed greatly to our understanding of human diseases. Here, we focus on cornea epithelial stem cell (CESC) deficiency (commonly called limbal stem cell deficiency, LSCD). Corneal development, homeostasis and wound healing are supported by specific stem cells, that include the CESCs. Damage to or loss of these cells results in blindness and other debilitating ocular conditions. Here we describe the contributions from several vertebrate models toward understanding CESCs and LSCD treatments. These include both mammalian models, as well as two aquatic models, Zebrafish and the amphibian, Xenopus. Pioneering developments have been made using stem cell transplants to restore normal vision in patients with LSCD, but questions still remain about the basic biology of CESCs, including their precise cell lineages and behavior in the cornea. We describe various cell lineage tracing studies to follow their patterns of division, and the fates of their progeny during development, homeostasis, and wound healing. In addition, we present some preliminary results using the Xenopus model system. Ultimately, a more thorough understanding of these cornea cells will advance our knowledge of stem cell biology and lead to better cornea disease therapeutics.

Single-cell transcriptomics identifies limbal stem cell population and cell types mapping its differentiation trajectory in limbal basal epithelium of human cornea

PURPOSE

This study aimed to uncover novel cell types in heterogenous basal limbus of human cornea for identifying LSC at single cell resolution.

METHODS

Single cells of human limbal basal epithelium were isolated from young donor corneas. Single-cell RNA-Sequencing was performed using 10x Genomics platform, followed by clustering cell types through the graph-based visualization method UMAP and unbiased computational informatic analysis. Tissue RNA in situ hybridization with RNAscope, immunofluorescent staining and multiple functional assays were performed using human corneas and limbal epithelial culture models.

RESULTS

Single-cell transcriptomics of 16,360 limbal basal cells revealed 12 cell clusters belonging to three lineages. A smallest cluster (0.4% of total cells) was identified as LSCs based on their quiescent and undifferentiated states with enriched marker genes for putative epithelial stem cells. TSPAN7 and SOX17 are discovered and validated as new LSC markers based on their exclusive expression pattern and spatial localization in limbal basal epithelium by RNAscope and immunostaining, and functional role in cell growth and tissue regeneration models with RNA interference in cultures. Interestingly, five cell types/states mapping a developmental trajectory of LSC from quiescence to proliferation and differentiation are uncovered by Monocle3 and CytoTRACE pseudotime analysis. The transcription factor networks linking novel signaling pathways are revealed to maintain LSC stemness.

CONCLUSIONS

This human corneal scRNA-Seq identifies the LSC population and uncovers novel cell types mapping the differentiation trajectory in heterogenous limbal basal epithelium. The findings provide insight into LSC concept and lay the foundation for understanding the corneal homeostasis and diseases.

Dehydroxyhispolon Methyl Ether, A Hispolon Derivative, Inhibits WNT/β-Catenin Signaling to Elicit Human Colorectal Carcinoma Cell Apoptosis

Colorectal cancer (CRC) is the fourth leading cause of cancer mortality worldwide. Aberrant activation of WNT/β-catenin signaling present in the vast majority of CRC cases is indispensable for CRC initiation and progression, and thus is a promising target for CRC therapeutics. Hispolon is a fungal-derived polyphenol with a pronounced anticancer effect. Several hispolon derivatives, including dehydroxyhispolon methyl ether (DHME), have been chemically synthesized for developing lead molecules with stronger anticancer activity. Herein, a DHME-elicited anti-CRC effect with the underlying mechanism is reported for the first time. Specifically, DHME was found to be more cytotoxic than hispolon against a panel of human CRC cell lines, while exerting limited toxicity to normal human colon cell line CCD 841 CoN. Additionally, the cytotoxic effect of DHME appeared to rely on inducing apoptosis. This notion was evidenced by DHME-elicited upregulation of poly (ADP-ribose) polymerase (PARP) cleavage and a cell population positively stained by annexin V, alongside the downregulation of antiapoptotic B-cell lymphoma 2 (BCL-2), whereas the blockade of apoptosis by the pan-caspase inhibitor z-VAD-fmk attenuated DHME-induced cytotoxicity. Further mechanistic inquiry revealed the inhibitory action of DHME on β-catenin-mediated, T-cell factor (TCF)-dependent transcription activity, suggesting that DHME thwarted the aberrantly active WNT/β-catenin signaling in CRC cells. Notably, ectopic expression of a dominant–active β-catenin mutant (∆N90-β-catenin) abolished DHME-induced apoptosis while also restoring BCL-2 expression. Collectively, we identified DHME as a selective proapoptotic agent against CRC cells, exerting more potent cytotoxicity than hispolon, and provoking CRC cell apoptosis via suppression of the WNT/β-catenin signaling axis.

Human Regulatory T Cells From Umbilical Cord Blood Display Increased Repertoire Diversity and Lineage Stability Relative to Adult Peripheral Blood

The human T lymphocyte compartment is highly dynamic over the course of a lifetime. Of the many changes, perhaps most notable is the transition from a predominantly naïve T cell state at birth to the acquisition of antigen-experienced memory and effector subsets following environmental exposures. These phenotypic changes, including the induction of T cell exhaustion and senescence, have the potential to negatively impact efficacy of adoptive T cell therapies (ACT). When considering ACT with CD4+CD25+CD127-/lo regulatory T cells (Tregs) for the induction of immune tolerance, we previously reported ex vivo expanded umbilical cord blood (CB) Tregs remained more naïve, suppressed responder T cells equivalently, and exhibited a more diverse T cell receptor (TCR) repertoire compared to expanded adult peripheral blood (APB) Tregs. Herein, we hypothesized that upon further characterization, we would observe increased lineage heterogeneity and phenotypic diversity in APB Tregs that might negatively impact lineage stability, engraftment capacity, and the potential for Tregs to home to sites of tissue inflammation following ACT. We compared the phenotypic profiles of human Tregs isolated from CB versus the more traditional source, APB. We conducted analysis of fresh and ex vivo expanded Treg subsets at both the single cell (scRNA-seq and flow cytometry) and bulk (microarray and cytokine profiling) levels. Single cell transcriptional profiles of pre-expansion APB Tregs highlighted a cluster of cells that showed increased expression of genes associated with effector and pro-inflammatory phenotypes (CCL5, GZMK, CXCR3, LYAR, and NKG7) with low expression of Treg markers (FOXP3 and IKZF2). CB Tregs were more diverse in TCR repertoire and homogenous in phenotype, and contained fewer effector-like cells in contrast with APB Tregs. Interestingly, expression of canonical Treg markers, such as FOXP3, TIGIT, and IKZF2, were increased in CB CD4+CD127+ conventional T cells (Tconv) compared to APB Tconv, post-expansion, implying perinatal T cells may adopt a default regulatory program. Collectively, these data identify surface markers (namely CXCR3) that could be depleted to improve purity and stability of APB Tregs, and support the use of expanded CB Tregs as a potentially optimal ACT modality for the treatment of autoimmune and inflammatory diseases.

Selecting Appropriate Reference Genes for Quantitative Real-Time Polymerase Chain Reaction Studies in Isolated and Cultured Ocular Surface Epithelia

The introduction of tissue engineering has allowed scientists to push the boundaries and treat seriously damaged ocular surface epithelia. They have managed to do this through the development of biological substitutes that restore, maintain or improve tissue function. To ensure the generation of a therapeutically safe and effective graft, knowledge on the transcriptional profile of native and cultured ocular surface epithelia is of undeniable value. Gene expression studies are, however, only as reliable as their proper selection of internal reaction controls or reference genes. In this study, we determined the expression stability of a number of reference genes: 18s rRNA, ACTB, ATP5B, CyC1, EIF4A2, GAPDH, RPL13A, SDHA, TOP1, UBC, and YWHAZ in primary isolates as well as in ex vivo cultured ocular surface epithelia explants (day 0 and/or day 14). Expression stability of the reference genes was assessed with both the geNorm and NormFinder software that use a pairwise comparison and a model-based approach, respectively. Our results extend the general recommendation of using multiple reference genes for normalization purposes to our model systems and provide an overview of several references genes that are likely to be stable in similar culture protocols.

Rescue of human corneal epithelial cells after alkaline insult using renalase derived peptide, RP-220

AIM

To study the effect of renalase peptide, RP-220, on cell viability of human corneal epithelial cells after alkali insult.

METHODS

A dose-response relationship between cell viability and exposure to NaOH solution were characterized using cultured human corneal epithelial cells. Viability of corneal epithelial cells was determined using commercially available MTT and CyQUANT^® assays.

RESULTS

At a concentration of 6 mmol/L, insult with NaOH leads to reduced corneal epithelial cell viability by approximately 30%. This reduced viability was prevented by treating the cells after initial insult with the 20-amino acid renalase derived peptide (RP-220).

CONCLUSION

RP-220 has a pro-survival role for RP-220 following alkaline insult to corneal epithelial cells.

Sostdc1 is expressed in all major compartments of developing and adult mammalian eyes

PURPOSE

This study was conducted in order to study Sostdc1 expression in rat and human developing and adult eyes.

METHODS

Using the yeast signal sequence trap screening method, we identified the Sostdc1 cDNA encoding a protein secreted by the adult rat retinal pigment epithelium. We determined by in situ hybridization, RT-PCR, immunohistochemistry, and western blot analysis Sostdc1 gene and protein expression in developing and postnatal rat ocular tissue sections. We also investigated Sostdc1 immunohistolocalization in developing and adult human ocular tissues.

RESULTS

We demonstrated a prominent Sostdc1 gene expression in the developing rat central nervous system (CNS) and eyes at early developmental stages from E10.5 days postconception (dpc) to E13 dpc. Specific Sostdc1 immunostaining was also detected in most adult cells of rat ocular tissue sections. We also identified the rat ocular embryonic compartments characterized by a specific Sostdc1 immunohistostaining and specific Pax6, Sox2, Otx2, and Vsx2 immunohistostaining from embryonic stages E10.5 to E13 dpc. Furthermore, we determined the localization of SOSTDC1 immunoreactivity in ocular tissue sections of developing and adult human eyes. Indeed, we detected SOSTDC1 immunostaining in developing and adult human retinal pigment epithelium (RPE) and neural retina (NR) as well as in several developing and adult human ocular compartments, including the walls of choroidal and scleral vessels. Of utmost importance, we observed a strong SOSTDC1 expression in a pathological ocular specimen of type 2 Peters' anomaly complicated by retinal neovascularization as well in the walls ofother pathological extra-ocular vessels.  CONCLUSION: As rat Sostdc1 and human SOSTDC1 are dual antagonists of the Wnt/β-catenin and BMP signaling pathways, these results underscore the potential crucial roles of these pathways and their antagonists, such as Sostdc1 and SOSTDC1, in developing and adult mammalian normal eyes as well as in syndromic and nonsyndromic congenital eye diseases.

Knockdown of long non-coding RNA TINCR decreases radioresistance in colorectal cancer cells

An increasing number of studies have revealed the role of long non-coding RNAs in cancer. However, the mechanisms of action and functional utility in colorectal cancer (CRC) have not been fully elucidated. Here we describe the functional role and potential mechanism of TINCR (terminal differentiation-induced non-coding RNA) in CRC. Firstly, TINCR was selected using sequencing analyses and the starBase database. Cell Counting Kit-8, scratch wound healing, and transwell assays revealed that TINCR inhibited proliferation and migration in SW620 and HTC116 cells. Intriguingly, TINCR expression was up-regulated in a radioresistant CRC cell line (SW620R). Although TINCR had no significant effects on SW620R cell proliferation or migration, knockdown of TINCR reduced the radioresistance, and its overexpression had opposite effects. We then focused on transcription factor 4 (TCF4) as it is downregulated in CRC and associated with increased stemness in tumors. We found that TINCR and TCF4 levels were positively related in SW620R cells. TINCR knockdown reduced sphere formation ability in SW620R cells. TINCR also suppressed the OCT4 and SOX2 stemness genes, despite having no effect on NANOG. The expression levels of these genes were substantially higher in SW620R than in SW620 cells. To further explore the mechanism of TINCR and radioresistance, miR-137 was analyzed as it targets TCF4. We firstly confirmed that TCF4 is a target of miR-137. We then identified that TINCR knockdown enhanced miR-137 expression in SW620R cells. Collectively, these findings suggest that TINCR knockdown inhibits TCF4 by regulating miR-137 expression.

Understanding cornea homeostasis and wound healing using a novel model of stem cell deficiency in Xenopus

Limbal Stem Cell Deficiency (LSCD) is a painful and debilitating disease that results from damage or loss of the Corneal Epithelial Stem Cells (CESCs). Therapies have been developed to treat LSCD by utilizing epithelial stem cell transplants. However, effective repair and recovery depends on many factors, such as the source and concentration of donor stem cells, and the proper conditions to support these transplanted cells. We do not yet fully understand how CESCs heal wounds or how transplanted CESCs are able to restore transparency in LSCD patients. A major hurdle has been the lack of vertebrate models to study CESCs. Here we utilized a short treatment with Psoralen AMT (a DNA cross-linker), immediately followed by UV treatment (PUV treatment), to establish a novel frog model that recapitulates the characteristics of cornea stem cell deficiency, such as pigment cell invasion from the periphery, corneal opacity, and neovascularization. These PUV treated whole corneas do not regain transparency. Moreover, PUV treatment leads to appearance of the Tcf7l2 labeled subset of apical skin cells in the cornea region. PUV treatment also results in increased cell death, immediately following treatment, with pyknosis as a primary mechanism. Furthermore, we show that PUV treatment causes depletion of p63 expressing basal epithelial cells, and can stimulate mitosis in the remaining cells in the cornea region. To study the response of CESCs, we created localized PUV damage by focusing the UV radiation on one half of the cornea. These cases initially develop localized stem cell deficiency characteristics on the treated side. The localized PUV treatment is also capable of stimulating some mitosis in the untreated (control) half of those corneas. Unlike the whole treated corneas, the treated half is ultimately able to recover and corneal transparency is restored. Our study provides insight into the response of cornea cells following stem cell depletion, and establishes Xenopus as a suitable model for studying CESCs, stem cell deficiency, and other cornea diseases. This model will also be valuable for understanding the nature of transplanted CESCs, which will lead to progress in the development of therapeutics for LSCD.

Molecular markers for corneal epithelial cells in larval vs. adult Xenopus frogs

Corneal Epithelial Stem Cells (CESCs) and their proliferative progeny, the Transit Amplifying Cells (TACs), are responsible for maintaining the integrity and transparency of the cornea. These stem cells (SCs) are widely used in corneal transplants and ocular surface reconstruction. Molecular markers are essential to identify, isolate and enrich for these cells, yet no definitive CESC marker has been established. An extensive literature survey shows variability in the expression of putative CESC markers among vertebrates; being attributed to species-specific variations, or other differences in developmental stages of these animals, approaches used in these studies and marker specificity. Here, we expanded the search for CESC markers using the amphibian model Xenopus laevis. In previous studies we found that long-term label retaining cells (suggestive of CESCs and TACs) are present throughout the larval basal corneal epithelium. In adult frogs, these cells become concentrated in the peripheral cornea (limbal region). Here, we used immunofluorescence to characterize the expression of nine proteins in the corneas of both Xenopus larvae and adults (post-metamorphic). We found that localization of some markers change between larval and adult stages. Markers such as p63, Keratin 19, and β1-integrin are restricted to basal corneal epithelial cells of the larvae. After metamorphosis their expression is found in basal and intermediate layer cells of the adult frog corneal epithelium. Another protein, Pax6 was expressed in the larval corneas, but surprisingly it was not detected in the adult corneal epithelium. For the first time we report that Tcf7l2 can be used as a marker to differentiate cornea vs. skin in frogs. Tcf7l2 is present only in the frog skin, which differs from reports indicating that the protein is expressed in the human cornea. Furthermore, we identified the transition between the inner, and the outer surface of the adult frog eyelid as a key boundary in terms of marker expression. Although these markers are useful to identify different regions and cellular layers of the frog corneal epithelium, none is unique to CESCs or TACs. Our results confirm that there is no single conserved CESC marker in vertebrates. This molecular characterization of the Xenopus cornea facilitates its use as a vertebrate model to understand the functions of key proteins in corneal homeostasis and wound repair.

Transcription factor profiling identifies Sox9 as regulator of proliferation and differentiation in corneal epithelial stem/progenitor cells

Understanding transcription factor (TF) regulation of limbal epithelial stem/progenitor cells (LEPCs) may aid in using non-ocular cells to regenerate the corneal surface. This study aimed to identify and characterize TF genes expressed specifically in LEPCs isolated from human donor eyes by laser capture microdissection. Using a profiling approach, preferential limbal expression was found for SoxE and SoxF genes, particularly for Sox9, which showed predominantly cytoplasmic localization in basal LEPCs and nuclear localization in suprabasal and corneal epithelial cells, indicating nucleocytoplasmic translocation and activation during LEPC proliferation and differentiation. Increased nuclear localization of Sox9 was also observed in activated LEPCs following clonal expansion and corneal epithelial wound healing. Knockdown of SOX9 expression in cultured LEPCs by RNAi led to reduced expression of progenitor cell markers, e.g. keratin 15, and increased expression of differentiation markers, e.g. keratin 3. Furthermore, SOX9 silencing significantly suppressed the proliferative capacity of LEPCs and reduced levels of glycogen synthase kinase 3 beta (GSK-3ß), a negative regulator of Wnt/ß-catenin signaling. Sox9 expression, in turn, was significantly suppressed by treatment of LEPCs with exogenous GSK-3ß inhibitors and enhanced by small molecule inhibitors of Wnt signaling. Our results suggest that Sox9 and Wnt/ß-catenin signaling cooperate in mutually repressive interactions to achieve a balance between quiescence, proliferation and differentiation of LEPCs in the limbal niche. Future molecular dissection of Sox9-Wnt interaction and mechanisms of nucleocytoplasmic shuttling of Sox9 may aid in improving the regenerative potential of LEPCs and the reprogramming of non-ocular cells for corneal surface regeneration.

Repairing the corneal epithelium using limbal stem cells or alternative cell-based therapies

INTRODUCTION

The corneal epithelium is maintained by limbal stem cells (LSCs) that reside in the basal epithelial layer of the tissue surrounding the cornea termed the limbus. Loss of LSCs results in limbal stem cell deficiency (LSCD) that can cause severe visual impairment. Patients with partial LSCD may respond to conservative therapies designed to rehabilitate the remaining LSCs. However, if these conservative approaches fail or, if complete loss of LSCs occurs, transplantation of LSCs or their alternatives is the only option. While a number of clinical studies utilizing diverse surgical and cell culture techniques have shown favorable results, a universal cure for LSCD is still not available. Knowledge of the potential risks and benefits of current approaches, and development of new technologies, is essential for further improvement of LSCD therapies.

AREAS COVERED

This review focuses on cell-based LSCD treatment approaches ranging from current available clinical therapies to preclinical studies of novel promising applications.

EXPERT OPINION

Improved understanding of LSC identity and development of LSC expansion methods will influence the evolution of successful LSCD therapies. Ultimately, future controlled clinical studies enabling direct comparison of the diverse employed approaches will help to identify the most effective treatment strategies.

Human induced pluripotent stem cell differentiation and direct transdifferentiation into corneal epithelial-like cells

The corneal epithelium is maintained by a small pool of tissue stem cells located at the limbus. Through certain injuries or diseases this pool of stem cells may get depleted. This leads to visual impairment. Standard treatment options include autologous or allogeneic limbal stem cell (LSC) transplantation, however graft rejection and chronic inflammation lowers the success rate over long time. Induced pluripotent stem (iPS) cells have opened new possibilities for treating various diseases with patient specific cells, eliminating the risk of immune rejection. In recent years, several protocols have been developed, aimed at the differentiation of iPS cells into the corneal epithelial lineage by mimicking the environmental niche of limbal stem cells. However, the risk of teratoma formation associated with the use of iPS cells hinders most applications from lab into clinics. Here we show that the differentiation of iPS cells into corneal epithelial cells results in the expression of corneal epithelial markers showing a successful differentiation, but the process is long and the level of gene expression for the pluripotency markers does not vanish completely. Therefore we set out to determine a direct transdifferentiation approach to circumvent the intermediate state of pluripotency (iPS-stage). The resulting cells, obtained by direct transdifferentiation of fibroblasts into limbal cells, exhibited corneal epithelial cell morphology and expressed corneal epithelial markers. Hence we shows for the first time a direct transdifferentiation of human dermal fibroblasts into the corneal epithelial lineage that may serve as source for corneal epithelial cells for transplantation approaches.

Divergent effects of Wnt/β-catenin signaling modifiers on the preservation of human limbal epithelial progenitors according to culture condition

Wnt signaling plays an important role in the regulation of self-renewal in stem cells. Here we investigated the effect of CHIR99021, the primary transducer of the Wnt signaling canonical pathway, and IWP2, a wide action Wnt signal blocker, on the growth and differentiation of the limbal epithelial progenitor cells when these cells are cultured in two different, common culture approaches, outgrowth from limbal biopsy explants and isolated cell seeded in low calcium medium. Consistent with their expected effects, irrespective of the culture system, IWP2 decreased total β-catenin while CHIR99021 increased it in nuclear localization. However, IWP2 increased stem/progenitor cell marker (p63α and ABCG2) content and clonogenic capacity in the explants but had opposite effects on isolated cells. CHIR99021 reduced the growth rate, stem/progenitor cell marker content and clonogenic capacity in the explants but also had the opposite effect on the isolated cells. These results show that the outcome of Wnt/β-catenin signaling modification is dependent on the culture systems. Transplantation of limbal epithelial sheets from explant cultures is one of the standard treatments of limbal stem cell deficiency. Our study shows that Wnt-associated activity has a strong negative impact on stem/progenitor cell preservation in limbal explant cultures.

Limbal stem cells: identity, developmental origin, and therapeutic potential

The cornea is our window to the world and our vision is critically dependent on corneal clarity and integrity. Its epithelium represents one of the most rapidly regenerating mammalian tissues, undergoing full-turnover over the course of approximately 1-2 weeks. This robust and efficient regenerative capacity is dependent on the function of stem cells residing in the limbus, a structure marking the border between the cornea and the conjunctiva. Limbal stem cells (LSC) represent a quiescent cell population with proliferative capacity residing in the basal epithelial layer of the limbus within a cellular niche. In addition to LSC, this niche consists of various cell populations such as limbal stromal fibroblasts, melanocytes and immune cells as well as a basement membrane, all of which are essential for LSC maintenance and LSC-driven regeneration. The LSC niche's components are of diverse developmental origin, a fact that had, until recently, prevented precise identification of molecularly defined LSC. The recent success in prospective LSC isolation based on ABCB5 expression and the capacity of this LSC population for long-term corneal restoration following transplantation in preclinical in vivo models of LSC deficiency underline the considerable potential of pure LSC formulations for clinical therapy. Additional studies, including genetic lineage tracing of the developmental origin of LSC will further improve our understanding of this critical cell population and its niche, with important implications for regenerative medicine. WIREs Dev Biol 2018, 7:e303. doi: 10.1002/wdev.303 This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Disease Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration.

Identification for Differential Localization of Putative Corneal Epithelial Stem Cells in Mouse and Human

Human Corneal epithelial stem cells (CESCs) have been identified to reside in limbus for more than 2 decades. However, the precise location of CESCs in other mammalian remains elusive. This study was to identify differential localization of putative CESCs in mice. Through a series of murine corneal cross-sections from different directions, we identified that anatomically and morphologically the murine limbus is composed of the thinnest epithelium and the thinnest stroma without any palisades of Vogt-like niche structure. The cells expressing five of stem/progenitor cell markers are localized in basal layer of entire murine corneal epithelium. BrdU label-retaining cells, a key characteristic of epithelial stem cells, are detected in both limbal and central cornea of mouse eye. Functionally, corneal epithelium can be regenerated in cultures from central and limbal explants of murine cornea. Such a distribution of mouse CESCs is different from human cornea, where limbal stem cell concept has been well established and accepted. We are aware that some new evidence supports limbal stem cell concept in mouse recently. However, it is important to know that central cornea may provide an alternative source of stem cells when one utilizes mice as animal model for corneal research.

Study of corneal epithelial progenitor origin and the Yap1 requirement using keratin 12 lineage tracing transgenic mice

Key issues in corneal epithelium biology are the mechanism for corneal epithelium stem cells to maintain the corneal epithelial homeostasis and wound healing responses, and what are the regulatory molecular pathways involved. There are apparent discrepancies about the locations of the progenitor populations responsible for corneal epithelial self-renewal. We have developed a genetic mouse model to trace the corneal epithelial progenitor lineages during adult corneal epithelial homeostasis and wound healing response. Our data revealed that the early corneal epithelial progenitor cells expressing keratin-12 originated from limbus, and gave rise to the transit amplifying cells that migrated centripetally to differentiate into corneal epithelial cells. Our results support a model that both corneal epithelial homeostasis and wound healing are mainly maintained by the activated limbal stem cells originating form limbus, but not from the corneal basal epithelial layer. In the present study, we further demonstrated the nuclear expression of transcriptional coactivator YAP1 in the limbal and corneal basal epithelial cells and its essential role for maintaining the high proliferative potential of those corneal epithelial progenitor cells in vivo.

Tcf7l2 localization of putative stem/progenitor cells in mouse conjunctiva

Conjunctival integrity and preservation is indispensable for vision. The self-renewing capacity of conjunctival cells controls conjunctival homeostasis and regeneration; however, the source of conjunctival self-renewal and the underlying mechanism is currently unclear. Here, we characterize the biochemical phenotype and proliferative potential of conjunctival epithelial cells in adult mouse by detecting proliferation-related signatures and conducting clonal analysis. Further, we show that transcription factor 7-like 2 (T-cell-specific transcription factor 4), a DNA binding protein expressed in multiple types of adult stem cells, is highly correlated with proliferative signatures in basal conjunctival epithelia. Clonal studies demonstrated that Transcription factor 7-like 2 (Tcf7l2) was coexpressed with p63α and proliferating cell nuclear antigen (PCNA) in propagative colonies. Furthermore, Tcf7l2 was actively transcribed concurrently with conjunctival epithelial proliferation in vitro. Collectively, we suggest that Tcf7l2 may be involved in maintenance of stem/progenitor cells properties of conjunctival epithelial stem/progenitor cells, and with the fornix as the optimal site to isolate highly proliferative conjunctival epithelial cells in adult mice.

Ciliary neurotrophic factor promotes the activation of corneal epithelial stem/progenitor cells and accelerates corneal epithelial wound healing

Ciliary neurotrophic factor (CNTF), a well-known neuroprotective cytokine, has been found to play an important role in neurogenesis and functional regulations of neural stem cells. As one of the most innervated tissue, however, the role of CNTF in cornea epithelium remains unclear. This study was to explore the roles and mechanisms of CNTF in the activation of corneal epithelial stem/progenitor cells and wound healing of both normal and diabetic mouse corneal epithelium. In mice subjecting to mechanical removal of corneal epithelium, the corneal epithelial stem/progenitor cell activation and wound healing were promoted by exogenous CNTF application, while delayed by CNTF neutralizing antibody. In cultured corneal epithelial stem/progenitor cells, CNTF enhanced the colony-forming efficiency, stimulated the mitogenic proliferation, and upregulated the expression levels of corneal epithelial stem/progenitor cell-associated transcription factors. Furthermore, the promotion of CNTF on the corneal epithelial stem/progenitor cell activation and wound healing was mediated by the activation of STAT3. Moreover, in diabetic mice, the content of CNTF in corneal epithelium decreased significantly when compared with that of normal mice, and the supplement of CNTF promoted the diabetic corneal epithelial wound healing, accompanied with the advanced activation of corneal epithelial stem/progenitor cells and the regeneration of corneal nerve fibers. Thus, the capability of expanding corneal epithelial stem/progenitor cells and promoting corneal epithelial wound healing and nerve regeneration indicates the potential application of CNTF in ameliorating limbal stem cell deficiency and treating diabetic keratopathy.

Transcription factor E2-2 inhibits the proliferation of endothelial progenitor cells by suppressing autophagy

Endothelial progenitor cells (EPCs) play a key role in repairing the injured vascular endothelium by differentiating into mature endothelial cells (ECs) or secreting cytokines in a paracrine manner to promote proliferation of existing ECs. However, the mechanisms underlying the proliferation of EPCs were not fully understood. In order to investigate the mechanisms of EPC proliferation, we isolated EPCs from mononuclear cells of mouse spleens. By manipulating E2-2 expression in vitro, we observed that E2-2 negatively regulated the proliferation of EPCs. Moreover, we noted that E2-2 negatively regulated the autophagy of EPCs by studying the expression of LC3II and p62. We also demonstrated that an autophagy inhibitor chloroquine (CQ) decreased the proliferation of EPCs in a concentration-dependent manner. Interestingly, CQ reversed the increase in cell proliferation and autophagy in the E2-2 knockdown group. Furthermore, we detected the expression of autophagy‑related protein ATG7 in EPCs which had been transfected with small interfering (siRNA)‑E2-2 and siRNA‑autophagy related 7 (ATG7) or were untransfected. Our study revealed that E2-2 regulated EPC autophagy via mediating ATG7 expression. We conclude that E2-2 inhibited EPC proliferation via suppressing their autophagy, and E2-2 regulated EPC autophagy by mediating the expression of ATG7.

Mesenchymal Stem Cell-Derived Extracellular Vesicles Promote Angiogenesis: Potencial Clinical Application

Mesenchymal stem cells (MSCs) are adult multipotent stem cells that are able to differentiate into multiple specialized cell types including osteocytes, adipocytes, and chondrocytes. MSCs exert different functions in the body and have recently been predicted to have a major clinical/therapeutic potential. However, the mechanisms of self-renewal and tissue regeneration are not completely understood. It has been shown that the biological effect depends mainly on its paracrine action. Furthermore, it has been reported that the secretion of soluble factors and the release of extracellular vesicles, such as exosomes, could mediate the cellular communication to induce cell-differentiation/self-renewal. This review provides an overview of MSC-derived exosomes in promoting angiogenicity and of the clinical relevance in a therapeutic approach.

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.

Lens regeneration from the cornea requires suppression of Wnt/β-catenin signaling

The frog, Xenopus laevis, possesses a high capacity to regenerate various larval tissues, including the lens, which is capable of complete regeneration from the cornea epithelium. However, the molecular signaling mechanisms of cornea-lens regeneration are not fully understood. Previous work has implicated the involvement of the Wnt signaling pathway, but molecular studies have been very limited. Iris-derived lens regeneration in the newt (Wolffian lens regeneration) has shown a necessity for active Wnt signaling in order to regenerate a new lens. Here we provide evidence that the Wnt signaling pathway plays a different role in the context of cornea-lens regeneration in Xenopus. We examined the expression of frizzled receptors and wnt ligands in the frog cornea epithelium. Numerous frizzled receptors (fzd1, fzd2, fzd3, fzd4, fzd6, fzd7, fzd8, and fzd10) and wnt ligands (wnt2b.a, wnt3a, wnt4, wnt5a, wnt5b, wnt6, wnt7b, wnt10a, wnt11, and wnt11b) are expressed in the cornea epithelium, demonstrating that this tissue is transcribing many of the ligands and receptors of the Wnt signaling pathway. When compared to flank epithelium, which is lens regeneration incompetent, only wnt11 and wnt11b are different (present only in the cornea epithelium), identifying them as potential regulators of cornea-lens regeneration. To detect changes in canonical Wnt/β-catenin signaling occurring within the cornea epithelium, axin2 expression was measured over the course of regeneration. axin2 is a well-established reporter of active Wnt/β-catenin signaling, and its expression shows a significant decrease at 24 h post-lentectomy. This decrease recovers to normal endogenous levels by 48 h. To test whether this signaling decrease was necessary for lens regeneration to occur, regenerating eyes were treated with either 6-bromoindirubin-3'-oxime (BIO) or 1-azakenpaullone - both activators of Wnt signaling - resulting in a significant reduction in the percentage of cases with successful regeneration. In contrast, inhibition of Wnt signaling using either the small molecule IWR-1, treatment with recombinant human Dickkopf-1 (rhDKK1) protein, or transgenic expression of Xenopus DKK1, did not significantly affect the percentage of successful regeneration. Together, these results suggest a model where Wnt/β-catenin signaling is active in the cornea epithelium and needs to be suppressed during early lens regeneration in order for these cornea cells to give rise to a new lentoid. While this finding differs from what has been described in the newt, it closely resembles the role of Wnt signaling during the initial formation of the lens placode from the surface ectoderm during early embryogenesis.

Frizzled 7 maintains the undifferentiated state of human limbal stem/progenitor cells

Wnt signaling pathway plays an important role in the regulation of human limbal stem/progenitor cells (LSCs). To examine the possible function of Frizzled (Fz) receptors in LSCs, the expression of 10 Fz receptors was profiled in the limbus and cornea. Only Fz7 had preferential expression in the basal limbal epithelium which contains the LSCs. The expression of Fz7 was colocalized with the putative LSC markers including p63α, N-cadherin and keratin (K) 14, and was minimum in cells expressing the corneal maturation marker K12. The expression of Fz7 was higher in the enriched LSCs population and decreased in cultured LSCs when there was a loss of progenitor phenotype. When the Fz7 was knocked down (Fz(KD)) using shRNA in primary LSCs, the expression of putative LSC markers ABCG2, ΔNp63α, and K14 was decreased significantly. The colony forming efficiency of the Fz7(KD) LSCs was significantly decreased in the subsequent passage 1 and 2 compared to the control. Our finding suggests that Wnt signaling is one of the factors of LSC niche, and Fz7 helps to maintain the undifferentiated state of LSCs.

Unique expression pattern and functional role of periostin in human limbal stem cells

Periostin is a non-structural matricellular protein. Little is known about periostin in human limbal stem cells (LSCs). This study was to explore the unique expression pattern and functional role of periostin in maintaining the properties of human LSCs. Fresh donor corneal tissues were used to make cryosections for evaluation of periostin expression on ex vivo tissues. Primary human limbal epithelial cells (HLECs) were generated from limbal explant culture. In vitro culture models for proliferation and epithelial regeneration were performed to explore functional role of periostin in LSCs. The mRNA expression was determined by reverse transcription and quantitative real-time PCR (RT-qPCR), and the protein production and localization were detected by immunofluorescent staining and Western blot analysis. Periostin protein was found to be exclusively immunolocalized in the basal layer of human limbal epithelium. Periostin localization was well matched with nuclear factor p63, but not with corneal epithelial differentiation marker Keratin 3. Periostin transcripts was also highly expressed in limbal than corneal epithelium. In primary HLECs, periostin expression at mRNA and protein levels was significantly higher in 50% and 70% confluent cultures at exponential growth stage than in 100% confluent cultures at slow growth or quiescent condition. This expression pattern was similar to other stem/progenitor cell markers (p63, integrin β1 and TCF4). Periostin expression at transcripts, protein and immunoreactivity levels increased significantly during epithelial regeneration in wound healing process, especially in 16-24 hours at wound edge, which was accompanied by similar upregulation and activation of p63, integrin β1 and TCF4. Our findings demonstrated that periostin is exclusively produced by limbal basal epithelium and co-localized with p63, where limbal stem cells reside. Periostin promotes HLEC proliferation and regeneration with accompanied activation of stem/progenitor cell markers p63, integrin β1 and TCF4, suggesting its novel role in maintaining the phenotype and functional properties of LSC.

Signature microRNAs in human cornea limbal epithelium

This study was aimed to identify the signature microRNAs, which regulate the biological processes of corneal epithelial progenitor cell (CEPC) homeostasis and regulation through characterizing the differential expression profile of microRNAs in human limbal epithelium containing adult CEPC versus central corneal epithelium without CEPC. MicroRNA microarray had identified 37 microRNAs enriched in human corneal epithelium. Among them, nine were significantly upregulated in limbal epithelium and one in central corneal epithelium after validation by TaqMan® real-time polymerase chain reaction. In addition to our previous finding of miR-143 and 145, the expression of miR-10b, 126, and 155 was localized in limbal epithelium (LE) (predominantly basal layers) by using locked nucleic acid-based in situ hybridization. Potential target genes were predicted by TargetScan Human v6.0 and compared to the reported human cornea epithelial gene profile GSE5543. Analyzed by web-based Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and DAVID Functional Annotation Bioinformatics Resources v6.7, the downregulated genes were involved in pathways of immune response and cellular protection, apoptosis, and cell movement whereas upregulated genes with cell survival, cell-matrix interaction, and cell-cell adhesion. We found a constant occurrence of miR-143, 145, and 155 in all KEGG pathways regulating limbal epithelial events. By Ingenuity Systems (IPA®) analysis, these microRNAs could cooperatively regulate cell growth and apoptosis via tumor necrosis factor activation and MYC repression. Our findings thus suggest a unique microRNA signature existing in human limbal epithelium and participating in CEPC homeostasis.

MicroRNA and mRNA cargo of extracellular vesicles from porcine adipose tissue-derived mesenchymal stem cells

Mesenchymal stromal/stem cells (MSCs) are clinically useful for cell-based therapy, but concerns regarding their ability to replicate limit their human application. MSCs release extracellular vesicles (EVs) that mediate at least in part the paracrine effects of the parental cells. To understand the molecular basis of their biological properties, we characterized the RNA cargo of EVs from porcine adipose-tissue derived MSCs. Comprehensive characterization of mRNA and miRNA gene expression using high-throughput RNA sequencing (RNA-seq) revealed that EVs are selectively enriched for distinct classes of RNAs. For example, EVs preferentially express mRNA for transcription factors (e.g. MDFIC, POU3F1, NRIP1) and genes involved in angiogenesis (e.g. HGF, HES1, TCF4) and adipogenesis (e.g. CEBPA, KLF7). EVs also express Golgi apparatus genes (ARRB1, GOLGA4) and genes involved in TGF-β signaling. In contrast, mitochondrial, calcium signaling, and cytoskeleton genes are selectively excluded from EVs, possibly because these genes remain sequestered in organelles or intracellular compartments. RNA-seq generated reads for at least 386 annotated miRNAs, but only miR148a, miR532-5p, miR378, and let-7f were enriched in EVs compared to MSCs. Gene ontology analysis indicates that these miRNAs target transcription factors and genes that participate in several cellular pathways, including angiogenesis, cellular transport, apoptosis, and proteolysis. Our data suggest that EVs transport gene regulatory information to modulate angiogenesis, adipogenesis, and other cell pathways in recipient cells. These observations may contribute to development of regenerative strategies using EVs to overcome potential complications of cell-based therapy.

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.

Wnt inhibitory factor 1 suppresses cancer stemness and induces cellular senescence

Hyperactivation of the Wingless-type (Wnt)/β-catenin pathway promotes tumor initiation, tumor growth and metastasis in various tissues. Although there is evidence for the involvement of Wnt/β-catenin pathway activation in salivary gland tumors, the precise mechanisms are unknown. Here we report for the first time that downregulation of the Wnt inhibitory factor 1 (WIF1) is a widespread event in salivary gland carcinoma ex-pleomorphic adenoma (CaExPA). We also show that WIF1 downregulation occurs in the CaExPA precursor lesion pleomorphic adenoma (PA) and indicates a higher risk of progression from benign to malignant tumor. Our results demonstrate that diverse mechanisms including WIF1 promoter hypermethylation and loss of heterozygosity contribute to WIF1 downregulation in human salivary gland tumors. In accordance with a crucial role in suppressing salivary gland tumor progression, WIF1 re-expression in salivary gland tumor cells inhibited cell proliferation, induced more differentiated phenotype and promoted cellular senescence, possibly through upregulation of tumor-suppressor genes, such as p53 and p21. Most importantly, WIF1 significantly diminished the number of salivary gland cancer stem cells and the anchorage-independent cell growth. Consistent with this observation, WIF1 caused a reduction in the expression of pluripotency and stemness markers (OCT4 and c-MYC), as well as adult stem cell self-renewal and multi-lineage differentiation markers, such as WNT3A, TCF4, c-KIT and MYB. Furthermore, WIF1 significantly increased the expression of microRNAs pri-let-7a and pri-miR-200c, negative regulators of stemness and cancer progression. In addition, we show that WIF1 functions as a positive regulator of miR-200c, leading to downregulation of BMI1, ZEB1 and ZEB2, with a consequent increase in downstream targets such as E-cadherin. Our study emphasizes the prognostic and therapeutic potential of WIF1 in human salivary gland CaExPA. Moreover, our findings demonstrate a novel mechanism by which WIF1 regulates cancer stemness and senescence, which might have major implications in the field of cancer biology.

The Ets transcription factor EHF as a regulator of cornea epithelial cell identity

The cornea is the clear, outermost portion of the eye composed of three layers: an epithelium that provides a protective barrier while allowing transmission of light into the eye, a collagen-rich stroma, and an endothelium monolayer. How cornea development and aging is controlled is poorly understood. Here we characterize the mouse cornea transcriptome from early embryogenesis through aging and compare it with transcriptomes of other epithelial tissues, identifying cornea-enriched genes, pathways, and transcriptional regulators. Additionally, we profiled cornea epithelium and stroma, defining genes enriched in these layers. Over 10,000 genes are differentially regulated in the mouse cornea across the time course, showing dynamic expression during development and modest expression changes in fewer genes during aging. A striking transition time point for gene expression between postnatal days 14 and 28 corresponds with completion of cornea development at the transcriptional level. Clustering classifies co-expressed, and potentially co-regulated, genes into biologically informative categories, including groups that exhibit epithelial or stromal enriched expression. Based on these findings, and through loss of function studies and ChIP-seq, we show that the Ets transcription factor EHF promotes cornea epithelial fate through complementary gene activating and repressing activities. Furthermore, we identify potential interactions between EHF, KLF4, and KLF5 in promoting cornea epithelial differentiation. These data provide insights into the mechanisms underlying epithelial development and aging, identifying EHF as a regulator of cornea epithelial identity and pointing to interactions between Ets and KLF factors in promoting epithelial fate. Furthermore, this comprehensive gene expression data set for the cornea is a powerful tool for discovery of novel cornea regulators and pathways.

Ocular surface development and gene expression

The ocular surface-a continuous epithelial surface with regional specializations including the surface and glandular epithelia of the cornea, conjunctiva, and lacrimal and meibomian glands connected by the overlying tear film-plays a central role in vision. Molecular and cellular events involved in embryonic development, postnatal maturation, and maintenance of the ocular surface are precisely regulated at the level of gene expression by a well-coordinated network of transcription factors. A thorough appreciation of the biological characteristics of the ocular surface in terms of its gene expression profiles and their regulation provides us with a valuable insight into the pathophysiology of various blinding disorders that disrupt the normal development, maturation, and/or maintenance of the ocular surface. This paper summarizes the current status of our knowledge related to the ocular surface development and gene expression and the contribution of different transcription factors to this process.

TCF4 (e2-2; ITF2): a schizophrenia-associated gene with pleiotropic effects on human disease

Common SNPs in the transcription factor 4 (TCF4; ITF2, E2-2, SEF-2) gene, which encodes a basic Helix-Loop-Helix (bHLH) transcription factor, are associated with schizophrenia, conferring a small increase in risk. Other common SNPs in the gene are associated with the common eye disorder Fuch's corneal dystrophy, while rare, mostly de novo inactivating mutations cause Pitt-Hopkins syndrome. In this review, we present a systematic bioinformatics and literature review of the genomics, biological function and interactome of TCF4 in the context of schizophrenia. The TCF4 gene is present in all vertebrates, and although protein length varies, there is high conservation of primary sequence, including the DNA binding domain. Humans have a unique leucine-rich nuclear export signal. There are two main isoforms (A and B), as well as complex splicing generating many possible N-terminal amino acid sequences. TCF4 is highly expressed in the brain, where plays a role in neurodevelopment, interacting with class II bHLH transcription factors Math1, HASH1, and neuroD2. The Ca(2+) sensor protein calmodulin interacts with the DNA binding domain of TCF4, inhibiting transcriptional activation. It is also the target of microRNAs, including mir137, which is implicated in schizophrenia. The schizophrenia-associated SNPs are in linkage disequilibrium with common variants within putative DNA regulatory elements, suggesting that regulation of expression may underlie association with schizophrenia. Combined gene co-expression analyses and curated protein-protein interaction data provide a network involving TCF4 and other putative schizophrenia susceptibility genes. These findings suggest new opportunities for understanding the molecular basis of schizophrenia and other mental disorders.

A native-like corneal construct using donor corneal stroma for tissue engineering

Tissue engineering holds great promise for corneal transplantation to treat blinding diseases. This study was to explore the use of natural corneal stroma as an optimal substrate to construct a native like corneal equivalent. Human corneal epithelium was cultivated from donor limbal explants on corneal stromal discs prepared by FDA approved Horizon Epikeratome system. The morphology, phenotype, regenerative capacity and transplantation potential were evaluated by hematoxylin eosin and immunofluorescent staining, a wound healing model, and the xeno-transplantation of the corneal constructs to nude mice. An optically transparent and stratified epithelium was rapidly generated on donor corneal stromal substrate and displayed native-like morphology and structure. The cells were polygonal in the basal layer and became flattened in superficial layers. The epithelium displayed a phenotype similar to human corneal epithelium in vivo. The differentiation markers, keratin 3, involucrin and connexin 43, were expressed in full or superficial layers. Interestingly, certain basal cells were immunopositive to antibodies against limbal stem/progenitor cell markers ABCG2 and p63, which are usually negative in corneal epithelium in vivo. It suggests that this bioengineered corneal epithelium shared some characteristics of human limbal epithelium in vivo. This engineered epithelium was able to regenerate in 4 days following from a 4mm-diameter wound created by a filter paper soaked with 1 N NaOH. This corneal construct survived well after xeno-transplantation to the back of a nude mouse. The transplanted epithelium remained multilayer and became thicker with a phenotype similar to human corneal epithelium. Our findings demonstrate that natural corneal stroma is an optimal substrate for tissue bioengineering, and a native-like corneal construct has been created with epithelium containing limbal stem cells. This construct may have great potential for clinical use in corneal reconstruction.

Differing roles for TCF4 and COL8A2 in central corneal thickness and fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) is the most common late-onset, vision-threatening corneal dystrophy in the United States, affecting about 4% of the population. Advanced FECD involves a thickening of the cornea from stromal edema and changes in Descemet membrane. To understand the relationship between FECD and central corneal thickness (CCT), we characterized common genetic variation in COL8A2 and TCF4, genes previously implicated in CCT and/or FECD. Other genes previously associated with FECD (PITX2, ZEB1, SLC4A11), and genes only known to affect CCT (COL5A1, FOXO1, AVGR8, ZNF469) were also interrogated. FECD probands, relatives and controls were recruited from 32 clinical sites; a total of 532 cases and 204 controls were genotyped and tested for association of FECD case/control status, a 7-step FECD severity scale and CCT, adjusting for age and sex. Association of FECD grade with TCF4 was highly significant (OR  = 6.01 at rs613872; p = 4.8×10⁻²⁵), and remained significant when adjusted for changes in CCT (OR  = 4.84; p = 2.2×10⁻¹⁶). Association of CCT with TCF4 was also significant (p = 6.1×10⁻⁷), but was abolished with adjustment for FECD grade (p = 0.92). After adjusting for FECD grade, markers in other genes examined were modestly associated (p ∼ 0.001) with FECD and/or CCT. Thus, common variants in TCF4 appear to influence FECD directly, and CCT secondarily via FECD. Additionally, changes in corneal thickness due to the effect of other loci may modify disease severity, age-at-onset, or other biomechanical characteristics.