Investigating the Role of Transcription Factor AP-2β in Modulating Corneal Epithelial Stratification Through the Wnt/β-catenin/BMP4 Signaling Axis

Abstract

The cornea, which is the outermost part of the anterior segment of the eye, is composed of an outer stratified epithelium, an inner endothelial monolayer, and a collagen-rich, avascular stroma. The corneal endothelium and stroma are derived from neural crest cells (NCCs), while the corneal epithelium develops from surface ectoderm (SE). Our lab has developed a mutant mouse model lacking expression of activating protein 2β (AP-2β), a transcription factor critical in corneal development, in NCC-derived tissues utilizing the Wnt1Cre transgene. In this mutant mouse model, we have observed an absent endothelium, a vascularized stroma, and a non-stratified epithelium. The lack of proper corneal epithelial stratification is interesting to note as AP-2β expression is unaltered in the SE-derived corneal epithelium. A possible mechanism for the observed lack of stratification may be due to altered Wnt/β-catenin/bone morphological protein 4 (BMP4) signalling between the corneal stroma and the epithelium. Thus, the purpose of this thesis was to investigate changes in Wnt/β-catenin/BMP4 signaling in the absence of stromal AP-2β expression and determine whether the lack of corneal epithelial stratification can be rescued. Immunohistochemical staining of corneal sections revealed that compared to controls, AP-2β NCC KO mutant mice experienced elevated Wnt/β-catenin signaling and reduced BMP4 signaling at later timepoints. Due to the decrease in BMP4 signaling, we hypothesized that administration of exogenous BMP4 delivered via subconjunctival injections would increase BMP4 downstream signaling and trigger corneal epithelial stratification in mutant mice. Haematoxylin and eosin staining of BMP4 treated mice revealed a change in corneal epithelial stratification in control mice compared to untreated control mice. However, no change in corneal epithelial stratification was observed between untreated and BMP4 treated mutant eyes. Ultimately, these results indicate that AP-2β expression in NCC-derived tissues are critical for corneal epithelial stratification.