A new method of high-speed cellular protein separation and insight into subcellular compartmentalization of proteins

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

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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

Mechanistic role of transglutaminase-2 in focal adhesions

Transglutaminase (TG)-2 interacts with matrix proteins and integrins, forming focal adhesions (FA) to initiate cell migration, thus playing a vital role in wound healing. Previously we showed that TG-2 influenced phosphorylation of paxillin and other FA proteins. Here, we aimed to investigate the molecular mechanism of TG-2 regulation of paxillin. Human corneal epithelial cells expressing shRNA against TG-2 (shTG) and scrambled sequence control (shRNA) were cultured. TG-2 was pulled down by anti-paxillin antibody, but not MAP3K12. Cell-free interaction assay with immobilized paxillin shows that TG-2 bind to paxillin directly. JNK was the strongest kinase for paxillin phosphorylation in the in-vitro kinase screen, but TG-2 could not phosphorylate paxillin directly. Increasing TG-2 concentrations did not increase the amount of JNK in the TG-2/paxillin complex. Immunofluoresent staining shows that TG-2 colocalises with vinculin and paxillin in FA of migrating cells. TG-2 binds to paxillin and JNK-containing FA but does not recruit JNK directly. Taken together with previous findings, TG-2 binds paxillin non-covalently, and JNK can phosphorylate paxillin, these processes critically regulate corneal epithelial adhesion and migration.