Pax-6 is expressed early in the differentiation of a corneal epithelial model system

PAX6-WNK2 Axis Governs Corneal Epithelial Homeostasis

Purpose

Limbal stem/progenitor cells (LSCs) continuously proliferate and differentiate to replenish the corneal epithelium and play a vital role in corneal function and normal vision. A previous study revealed that paired box 6 (PAX6) is a master transcription factor involved in determining the fate of corneal epithelial cells (CECs). However, the molecular events downstream of PAX6 remain largely unknown. In this study, we aimed to clarify the regulation network of PAX6 in driving CEC differentiation.

Methods

An air-liquid culture system was used to differentiate LSCs into mature CECs. Specific targeting PAX6 short-hairpin RNAs were used to knock down PAX6 in LSC. RNA sequencing (RNA-seq) was used to analyze shPAX6-transfected CECs and CEC differentiation-associated genes to identify the potential downstream targets of PAX6. RNA-seq analysis, quantitative real-time PCR, and immunofluorescence staining were performed to clarify the function of WNK lysine deficient protein kinase 2 (WNK2), a downstream target of PAX6, and its relationship with corneal diseases.

Results

WNK2 expression increased during CEC differentiation and decreased upon PAX6 depletion. The distribution of WNK2 was specifically limited to the central corneal epithelium and suprabasal layer of the limbus. Knockdown of WNK2 impaired the expression of CEC-specific markers (KRT12, ALDH3A1, and CLU), disrupted the corneal differentiation process, and activated the terms of keratinization, inflammation, and cell proliferation, consistent with PAX6-depleted CEC and published microbial keratitis. Thus, aberrant expression of WNK2 was linked to corneal ulcers.

Conclusions

As a downstream target of PAX6, WNK2 plays an essential role in corneal epithelial cell differentiation and maintenance of corneal homeostasis.

Participation of the TBP-associated factors (TAFs) in cell differentiation

The understanding of the mechanisms that regulate gene expression to establish differentiation programs and determine cell lineages, is one of the major challenges in Developmental Biology. Besides the participation of tissue-specific transcription factors and epigenetic processes, the role of general transcription factors has been ignored. Only in recent years, there have been scarce studies that address this issue. Here, we review the studies on the biological activity of some TATA-box binding protein (TBP)-associated factors (TAFs) during the proliferation of stem/progenitor cells and their involvement in cell differentiation. Particularly, the accumulated evidence suggests that TAF4, TAF4b, TAF7L, TAF8, TAF9, and TAF10, among others, participate in nervous system development, adipogenesis, myogenesis, and epidermal differentiation; while TAF1, TAF7, TAF15 may be involved in the regulation of stem cell proliferative abilities and cell cycle progression. On the other hand, evidence suggests that TBP variants such as TBPL1 and TBPL2 might be regulating some developmental processes such as germ cell maturation and differentiation, myogenesis, or ventral specification during development. Our analysis shows that it is necessary to study in greater depth the biological function of these factors and its participation in the assembly of specific transcription complexes that contribute to the differential gene expression that gives rise to the great diversity of cell types existing in an organism. The understanding of TAFs' regulation might lead to the development of new therapies for patients which suffer from mutations, alterations, and dysregulation of these essential elements of the transcriptional machinery.

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.

Transcriptional profiles along cell programming into corneal epithelial differentiation

Using the rabbit corneal epithelial cell line RCE1(5T5) as a model, we analyzed three differentiation stages, distinguished on basis to the growth state of cultured cells and after studying the expression of transcription factors such as Oct4, Pax6 and ΔNp63α, selected differentiation markers, and signaling or epigenetic markers such as Notch receptors and Prdm3. Namely, proliferative non-differentiated cells, committed cells, and cells that constitute a stratified epithelium with a limbal epithelial-like structure. RNAseq based transcriptome analysis showed that 4891 genes were differentially expressed among these stages displaying distinctive gene signatures: proliferative cells had 1278 genes as gene signature, and seem to be early epithelial progenitors with an Oct4⁺, KLF4⁺, Myc⁺, ΔNp63α⁺, ABCG2⁺, Vimentin⁺, Zeb1⁺, VANGL1⁺, Krt3^-, Krt12^- phenotype. Committed cells had a gene signature with 417 genes and displayed markers indicative of the beginning of corneal differentiation, and genes characteristic of proliferative cells; we found the possible participation of Six3 and Six4 transcription factors along this stage. The third stage matches with a stratified corneal epithelium (gene signature comprising 979 genes) and is typified by an increase in the expression of WNT10A and NOTCH 2 and 3 signaling and Cux1 transcription factor, besides Pax6, KLF4 or Sox9. The differentiated cells express about 50% of the genes that belong to the Epidermal Differentiation Complex (EDC). Analysis of the differences between corneal epithelium and epidermis could be crucial to understand the regulatory mechanisms that lead to the expression of the differentiated phenotype.

(-)-Epigallocatechin-3-gallate, reduces corneal damage secondary from experimental grade II alkali burns in mice

BACKGROUND

Since recent reports have shown that (-)-Epigallocatechin-3-gallate (EGCG) could be used for treating proliferative and inflammatory disorders, we explored its use for the management of corneal chemical burns.

MATERIALS AND METHODS

Initially, EGCG was assayed on the rabbit corneal epithelial cell line RCE1(5T5) to establish the best testing conditions, and to avoid unwanted outcomes in the experimental animals. Then, we studied its effects on cell proliferation, cell cycle progression and cell differentiation. Afterwards, we instilled EGCG in experimental grade II corneal alkali burns in mice, three times a day up to 21days, and evaluated by slit lamp examination and histological sections of corneal epithelial, corneal endothelial and stromal edema, as well as the presence of inflammatory cells and neovascularization.

RESULTS

EGCG reduced cell growth and led to a decline in the proportion of proliferative cells in a concentration dependent manner. At 10μM, EGCG promoted cell differentiation, an effect not related with apoptosis or cytotoxicity. When 10μM EGCG was instilled in corneal alkali burns in mice three times a day up to 21days, EGCG significantly reduced corneal opacity and neovascularization. The improved clinical appearance of the cornea was associated to a controlled epithelial growth; epithelial morphology was similar to that observed in normal epithelium and contrasted with the hyperproliferative, desquamating epithelium observed in control burn wounds. EGCG reduced corneal, stromal and endothelial edema, and wound inflammation.

CONCLUSION

This work constitutes the first evidence for the use of EGCG in the acute phase of a corneal alkali burn, representing a possible novel alternative to improve patient outcomes as an add-on therapy.

Differentiation of rat adipose-derived mesenchymal stem cells into corneal-like epithelial cells driven by PAX6

Corneal integrity, transparency and vision acuity are maintained by corneal epithelial cells (CECs), which are continuously renewed by corneal limbal stem cells (LSCs). Deficiency of CECs and/or LSCs is associated with numerous ocular diseases. Paired box (PAX)6 is an eye development-associated transcription factor that is necessary for cell fate determination and differentiation of LSCs and CECs. In the present study, the PAX6 gene was introduced into adipose-derived rat mesenchymal stem cells (ADMSCs) to investigate whether PAX6-transfected cells were able to transdifferentiate into corneal-like epithelial cells and to further verify whether the cells were suitable as a cell source for corneal transplantation. The ADMSCs were isolated from the bilateral inguinal region of healthy Sprague Dawley rats. The characteristics of ADMSCs were identified using flow cytometric analysis. After subculture, ADMSCs underwent transfection with recombinant plasmid containing either PAX6-enhanced green fluorescent protein (EGFP) complementary (c)DNA or EGFP cDNA (blank plasmid group), followed by selection with G418 and determination of the transfection efficiency. Subsequently, the morphology of the ADMSCs and the expression profiles of corneal-specific markers CK3/12 and epithelial-specific adhesion protein were determined. E-cadherin was detected using immunofluorescence staining and western blot analysis at 21 days following transfection. An MTT cell proliferation and a colony formation assay were performed to assess the proliferative activity and clonogenicity of PAX6-transfected ADMSCs. Finally, the PAX6-expressing ADMSCs were transplanted onto the cornea of a rabbits with limbal stem cell deficiency (LSCD). At 21 days after transfection, the ADMSCs with PAX6 transfection exhibited a characteristic flagstone-like appearance with assembled corneal-like epithelial cells, and concomitant prominent expression of the corneal-specific markers cytokeratin 3/12 and E-cadherin. Furthermore, the proliferation and colony formation ability of PAX6-overexpressing ADMSCs was significantly retarded. The transplantation experiment indicated that PAX6-reprogramed ADMSCs attached to and replenished the damaged cornea via formation of stratified corneal epithelium. Taken together, these results suggested that conversion of ADMSCs into corneal-like epithelium may be driven by PAX6 transfection, which makes ADMSCs a promising cell candidate for the treatment of LSCD.

Novel Metabolic Pathway for N-Methylpyrrolidone Degradation in Alicycliphilus sp. Strain BQ1

The molecular mechanisms underlying the biodegradation of N-methylpyrrolidone (NMP), a widely used industrial solvent that produces skin irritation in humans and is teratogenic in rats, are unknown. Alicycliphilus sp. strain BQ1 degrades NMP. By studying a transposon-tagged mutant unable to degrade NMP, we identified a six-gene cluster (nmpABCDEF) that is transcribed as a polycistronic mRNA and encodes enzymes involved in NMP biodegradation. nmpA and the transposon-affected gene nmpB encode an N-methylhydantoin amidohydrolase that transforms NMP to γ-N-methylaminobutyric acid; this is metabolized by an amino acid oxidase (NMPC), either by demethylation to produce γ-aminobutyric acid (GABA) or by deamination to produce succinate semialdehyde (SSA). If GABA is produced, the activity of a GABA aminotransferase (GABA-AT), not encoded in the nmp gene cluster, is needed to generate SSA. SSA is transformed by a succinate semialdehyde dehydrogenase (SSDH) (NMPF) to succinate, which enters the Krebs cycle. The abilities to consume NMP and to utilize it for growth were complemented in the transposon-tagged mutant by use of the nmpABCD genes. Similarly, Escherichia coli MG1655, which has two SSDHs but is unable to grow in NMP, acquired these abilities after functional complementation with these genes. In wild-type (wt) BQ1 cells growing in NMP, GABA was not detected, but SSA was present at double the amount found in cells growing in Luria-Bertani medium (LB), suggesting that GABA is not an intermediate in this pathway. Moreover, E. coli GABA-AT deletion mutants complemented with nmpABCD genes retained the ability to grow in NMP, supporting the possibility that γ-N-methylaminobutyric acid is deaminated to SSA instead of being demethylated to GABA.IMPORTANCEN-Methylpyrrolidone is a cyclic amide reported to be biodegradable. However, the metabolic pathway and enzymatic activities for degrading NMP are unknown. By developing molecular biology techniques for Alicycliphilus sp. strain BQ1, an environmental bacterium able to grow in NMP, we identified a six-gene cluster encoding enzymatic activities involved in NMP degradation. These findings set the basis for the study of new enzymatic activities and for the development of biotechnological processes with potential applications in bioremediation.

TRPV4 Regulates Tight Junctions and Affects Differentiation in a Cell Culture Model of the Corneal Epithelium

TRPV4 (transient receptor potential vanilloid 4) is a cation channel activated by hypotonicity, moderate heat, or shear stress. We describe the expression of TRPV4 during the differentiation of a corneal epithelial cell model, RCE1(5T5) cells. TRPV4 is a late differentiation feature that is concentrated in the apical membrane of the outmost cell layer of the stratified epithelia. Ca²⁺ imaging experiments showed that TRPV4 activation with GSK1016790A produced an influx of calcium that was blunted by the specific TRPV4 blocker RN-1734. We analyzed the involvement of TRPV4 in RCE1(5T5) epithelial differentiation by measuring the development of transepithelial electrical resistance (TER) as an indicator of the tight junction (TJ) assembly. We showed that TRPV4 activity was necessary to establish the TJ. In differentiated epithelia, activation of TRPV4 increases the TER and the accumulation of claudin-4 in cell-cell contacts. Epidermal Growth Factor (EGF) up-regulates the TER of corneal epithelial cultures, and we show here that TRPV4 activation mimicked this EGF effect. Conversely, TRPV4 inhibition or knock down by specific shRNA prevented the increase in TER. Moreover, TRPP2, an EGF-activated channel that forms heteromeric complexes with TRPV4, is also concentrated in the outmost cell layer of differentiated RCE1(5T5) sheets. This suggests that the EGF regulation of the TJ may involve a heterotetrameric TRPV4-TRPP2 channel. These results demonstrated TRPV4 activity was necessary for the correct establishment of TJ in corneal epithelia and as well as the regulation of both the barrier function of TJ and its ability to respond to EGF. J. Cell. Physiol. 232: 1794-1807, 2017. © 2016 Wiley Periodicals, Inc.

Pterygium epithelium abnormal differentiation related to activation of extracellular signal-regulated kinase signaling pathway in vitro

AIM

To investigate whether the abnormal differentiation of the pterygium epithelium is related to the extracellular signal-regulated kinase (ERK) signaling pathway in vitro.

METHODS

The expression levels of phosphorylated ERK (P-ERK), keratin family members including K19 and K10 and the ocular master control gene Pax-6 were measured in 16 surgically excised pterygium tissues and 12 eye bank conjunctiva. In colony-forming cell assays, the differences in clone morphology and in K10, K19, P-ERK and Pax-6 expression between the head and body were investigated. When cocultured with the ERK signaling pathway inhibitor PD98059, the changes in clone morphology, colony-forming efficiency, differentiated marker K10, K19 and Pax-6 expression and P-ERK protein expression level were examined by immunoreactivity and Western blot analysis.

RESULTS

The expression of K19 and Pax-6 decreased in the pterygium, especially in the head. No staining of K10 was found in the normal conjunctiva epithelium, but it was found to be expressed in the superficial cells in the head of the pterygium. Characteristic upregulation of P-ERK was observed by immunohistochemistry. The clone from the head with more differentiated cells in the center expressed more K10, and the clone from the body expressed more K19. The P-ERK protein level increased in the pterygium epithelium compared with conjunctiva and decreased when cocultured with PD98059. The same medium with the ERK inhibitor PD98059 was more effective in promoting clonal growth than conventional medium with 3T3 murine feeder layers. It was observed that the epithelium clone co-cultured with the inhibitor had decreased K10 expression and increased K19 and Pax-6 expression.

CONCLUSION

We suggest ERK signaling pathway activation might play a role in the pterygium epithelium abnormal differentiation.

Differentiation of human amniotic epithelial cells into corneal epithelial-like cells in vitro

AIM

To explore the feasibility that human amniotic epithelial cells (hAECs) have the potential to differentiate into corneal epithelial-like cells under the microenvironment replicated by spontaneously immortalized human corneal epithelial cells (S-ihCECs).

METHODS

hAECs were isolated by enzyme digestion, and flow cytometry was used to analysis the expression of CD29/90/166/73/34 and HLA-DR. Recovered and cultured S-ihCECs, immunocytochemistry was used to detect the expression of CK3/12. The proliferation of S-ihCECs handled by different concentrations of mitomycin was detected by CCK-8. The proliferation of hAECs cultured by S-ihCECs culture media collected at different time was analyzed by CCK-8. After filtered out the optimal conditions, we collected S-ihCECs culture media for 5 days, then prepared conditioned medium to incubate hAECs, inverted phase contrast microscope and scanning electron microscope were used to observe the change of morphology in hAECs. Quantitative real-time reverse transcription-polymerase chain reaction (QRT-PCR) was carried out to evaluate the expression of Oct-4, NANOG, PAX6, and CK12 in the differentiation period. Immunocytochemistry and western bloting were used to detect the expression of CK3/12.

RESULTS

The culture media collected every 12h, from 20µg/mL mitomycin pretreatment S-ihCECs could significantly promote the proliferation of hAECs. In the period of differentiation, the morphology of differentiated hAECs was obviously different compared with the control group, and the distinctive CK3/12 for corneal epithelial cells was detected.

CONCLUSION

This study showed that hAECs can differentiate into corneal epithelial-like cells by in vitro replication of the corneal epithelial microenvironment, using the culture media collected from S-ihCECs, and it is possible that S-ihCECs culture media could be used in corneal tissue engineering.

Expression of claudins -2 and -4 and cingulin is coordinated with the start of stratification and differentiation in corneal epithelial cells: retinoic acid reversibly disrupts epithelial barrier

Although tight junctions (TJ) have been extensively studied in simple epithelial cells, it is still unknown whether their organization is coupled to cell differentiation in stratified epithelia. We studied the expression of TJ in RCE1(5T5) cells, an in vitro model which mimics the sequential steps of rabbit corneal epithelial differentiation. RCE1(5T5) cells expressed TJ components which were assembled once cells constituted differentiated epithelia, as suggested by the increase of transepithelial electrical resistance (TER) which followed a similar kinetic to the expression of the early differentiation marker Pax-6. TJ were functional as indicated by the establishment of an epithelial barrier nonpermeable to ruthenium red or a biotin tracer. In immunostaining experiments, TJ were located at the superficial cells from the suprabasal layers; Western blot and RT-PCR suggested that TJ were composed of claudins (cldn) -1, -2, -4, cingulin (cgn), occludin (ocln) and ZO-1. Semi-quantitative RT-PCR and TER measurements showed that TJ became organized when cells began to form a 3–5 layers stratified epithelium; TER increased once cells reached confluence, with a time course comparable to the raise in the expression of cgn, cldn-2 and -4. Nevertheless, cldn-1, -2, ZO-1 and ocln were present in the cells from the beginning of cultivation, suggesting that TER increases mainly depend on TJ assembly. While EGF increased epithelial barrier strength, retinoic acid disrupted it, increasing paracellular flux about 2-fold; this effect was concentration dependent and completely reversible. Our results suggest that TJ assembly is tightly linked to the expression of corneal epithelial terminal phenotype.

Asymmetrical cell division and differentiation are not dependent upon stratification in a corneal epithelial cell line

To determine whether asymmetrical cell division takes place during growth and differentiation of corneal epithelial cells, we analyzed the expression of some proteins required for the correct execution of the asymmetric division in cultured RCE1-(5T5) cells, which mimic the differentiation of corneal epithelial cells. RT-PCR and immunostaining showed that Par-3, LGN (GPSM2), NuMA, and the mammalian homolog of inscuteable (Insc) are expressed by the cultured cells. Semi-quantitative RT-PCR demonstrated that Insc mRNA levels were stable throughout the experiment. Conversely, LGN and NuMA mRNAs increased slightly and steadily in proliferative cells, reaching a peak of about 20% above basal levels when cells were confluent. At later times, LGN and NuMA mRNAs decreased to become barely detectable when cells organized into a four-layered epithelium and expressed terminal phenotype as indicated by the highest expression of LDH-H mRNA. Cultivation under low Ca2+ conditions (0.09 mM) reduced about 50% Insc mRNA expression both in proliferating and confluent cultures, but did not affect the levels of LGN and NuMA mRNAs. Hence, asymmetric cell division seems to take place with a lower frequency in cells grown with low Ca2+ concentrations, in spite of the absence of stratification. Immunostaining experiments raise the possibility of an interaction between k3/K12 keratin cytoskeleton and Par-3. The results show for the first time the coordination between the expression of corneal epithelial cell differentiation and the expression of cell polarity machinery. They also suggest that asymmetric division does not depend on stratification; instead, it seems to be part of the differentiation program.

Corneal stem cells and their origins: significance in developmental biology

Adult corneal stem cells (SCs) have been the subject of substantial research over the past 2 decades, with promising clinical applications being devised, refined, and tried. However, there have been few studies on the early development of these cells in humans, perhaps due to ethical and practical constraints. This review highlights work that has yielded significant insights from developmental studies in the cornea and other SC repositories. This field merits further research to improve our current knowledge of the origin of SCs, their location, phenotype, function, and niche structure, as well as providing fresh insight into the pathogenesis of congenital diseases and new therapeutic avenues for treating a range of blinding corneal diseases.