Transglutaminase-2 is critical for corneal epithelial barrier function via positive regulation of Claudin-1

Transglutaminase (TG)-2 is a ubiquitous multi-functional protein expressed in all living cells. The purpose of the current study was to investigate the role of TG-2 in corneal barrier function and its potential regulation of epithelial junctional proteins and transcription factors.

Corneal barrier function to ions in TG-2^-/- and TG-2^+/+ mice was assessed by Ussing chamber assay. Hypo-osmolar water or FITC-dextran was applied on top of mouse eyes to evaluate the corneal barrier function to water and macromolecules. Western blots, qPCR and immunofluorescent staining were used to investigate the expression of tight junction proteins in TG-2^-/- and TG-2^+/+ mouse corneas, and also in TG-2 knockdown human corneal epithelial cells.

Corneal explants from TG-2^-/- mice had a lower trans-epithelial electrical resistance compared to TG-2^+/+ mice. When challenged by hypo-osmolar water, the central corneal thickness of TG-2^-/- mice increased faster, and these mice had a faster rise of fluorescence in the anterior chamber after ocular exposure to FITC-dextran, compared to TG-2^+/+. Claudin-1 protein and transcript levels were reduced in the cornea of TG-2^-/- mice and in TG-2 knockdown human corneal epithelial cells. Slug which previously reported suppressing Claudin-1 transcription, was increased at both protein and transcript level in TG-2 knockdown cells. TG-2 and Claudin-1 protein levels were unchanged in shRNA and shTG cells after MG132 treatment, while Slug accumulated in treated cells.

TG-2 may positively regulate Claudin-1 through repressing Slug at transcript level, and thus it is critical for normal corneal barrier function.