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Exp Eye Res 2019 Jul 01;184:107-125. doi: 10.1016/j.exer.2019.04.010.
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Molecular markers for corneal epithelial cells in larval vs. adult Xenopus frogs.

Sonam S , Srnak JA , Perry KJ , Henry JJ .

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.

PubMed ID: 30981716
PMC ID: PMC6697113
Article link: Exp Eye Res
Grant support: [+]

Species referenced: Xenopus laevis
Genes referenced: abcb5 abcg2 bmi1 cad cdh1 cdh2 cebpd ctnnb1 fzd7 itgb1 klf4 krt19 ngfr pax6 plin1 sox2 tcf4 tcf7l2 tp63 vim
GO keywords: eye development [+]
Antibodies: Abcg2 Ab1 Cdh1 Ab1 Ctnnb1 Ab9 Itgb1 Ab1 Tp63 Ab2 Vim Ab1 abcb5 Ab1 bmi1 Ab1 cdh2 Ab5 cebpd Ab1 fzd7 Ab1 klf4 Ab1 krt19 Ab2 ngfr Ab1 pax6 Ab4 sox2 Ab8 tcf7l2 Ab1

Article Images: [+] show captions
References [+] :
Akiyama, Growth factor and growth factor receptor localization in the hair follicle bulge and associated tissue in human fetus. 1996, Pubmed