Corneal epithelial development and homeostasis

Abstract

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.

Keywords: Cornea; Development; EMT; Epithelium; Homeostasis; KLF4; OSSN; Plane of division; Polarity.

Figure 1.. Corneal epithelial homeostasis.

Schematic representation of a sagittal section through the limbal epithelial region of an adult mouse ocular surface is shown to illustrate the XYZ hypothesis for CE homeostasis. For CE homeostasis to be normal, the rate at which the LESC and TA cells divide (X) needs to match the rate at which the basal cells differentiate and migrate upwards (Y), which in turn should equal the rate at which the superficial cells are lost to sloughing (Z). The life cycle of a typical CE cell, starting from its inception at the limbus to being sloughed off at the surface lasts between two and three months.

Figure 2.. Corneal epithelial stratification- a balancing act by KLF5 and KLF4.

A. Expression of KLF5 and KLF4 in adult mouse CE, detected by immunofluorescent staining. Note that pro-proliferative KLF5 is enriched in the basal CE cells while the anti-proliferative KLF4 is expressed uniformly through the CE. Our work over the last decade has demonstrated that KLF4 and KLF5 suppress epithelial-mesenchymal transition (EMT) and orchestrate CE stratification and homeostasis by coordinating the CE cell polarity, and plane of division. B. Mechanistic basis for pro- and anti-proliferative activities of KLF5 and KLF4, respectively. KLF5 promotes cyclin-D1 expression and suppresses the cell cycle inhibitor p27 Kip1 to promote G1 to S transition during cell cycle in basal CE cells, while KLF4 does the opposite, to promote suprabasal CE cell differentiation. The overall effect is that epithelial phenotype is promoted, while suppressing the mesenchymal properties. If this balance is disrupted, it results in EMT as observed in ocular surface squamous neoplasia (OSSN).

Figure 3.. KLF4 regulates CE cellular polarity and plane of division facilitating CE homeostasis.

Ablation of Klf4 results in EMT and downregulation of ABP core complex components, disrupting actin cytoskeleton. In the absence of KLF4, Klf4^Δ/ΔCE cells undergo increased number of symmetrical divisions with a vertical plane of division (VPD) unlike the asymmetrical divisions with a horizontal plane of division (HPD) observed in the control. These changes are shown in greater detail in the lower panels. By demonstrating that Klf4 ablation affects CE expression of ABP markers and Rho family GTPase Cdc42, cytoskeletal actin organization and the plane of cell division, and that KLF4 is downregulated in OSSN tissues that display EMT and lack ABP, our work elucidates the key integrative role of KLF4 in coordinating CE cell polarity and plane of division, loss of which results in EMT and OSSN. This figure originally appeared in the publication by Tiwari et al (2020). It is reproduced here with permission from IOVS.