Transketolase gene expression in the cornea is influenced by environmentalfactors and developmentally controlled events

Purpose. Transketolase (TKT) has been proposed to be a corneal crystallin, and its gene and protein are abundantly expressed in the corneal epithelium of several mammals. A marked up-regulation of TKT gene expression coincide s with the time of eyelid opening in the mouse. Here, we examined whether e xposure to incident light contributes to the up-regulation of TKT gene expr ession during cornea maturation. Methods. Mice were raised in either standa rd light/dark cycling conditions or total darkness. In some cases, subcutan eous injections of epidermal growth factor (EGF) were given beginning on th e day of birth to induce early eyelid opening. RNA was prepared from the co rneas of mothers and pups and subjected to Northern blot analyses. In addit ion, the relative levels of TKT mRNA and/or enzyme activity were examined i n the corneas of human, bovine, rat, chicken, and zebrafish. Results. TKT m RNA levels were 2.1-fold higher in the corneas of 25-day-old mouse pups (12 days after eyelid opening) that had been born and raised in light/dark con ditions compared to pups born and raised in total darkness. By contrast, th e level of TKT mRNA in the mature corneas of adult mice maintained in the d ark for 2-8 weeks did not vary greatly from those of mice maintained in lig ht/dark conditions. Interestingly, TKT mRNA levels in the corneas of dark-r aised mice, although reduced, did exhibit the increase characteristically o bserved before and after eyelid opening. In addition, TKT mRNA levels were elevated fivefold in the corneas of 28-day-old mice raised in darkness and injected with EGF compared to uninjected mice also deprived of light. The E GF-injected mice opened their eyes 3 days early, and their corneal epitheli um did not grossly differ from that of control mice. TKT mRNA and/or enzyme activity was found to be much higher in the corneas than in other tissues of humans, bovines, and rats but was extremely low in the corneas of chicke n and zebrafish. Conclusion, Our studies suggest that both exposure to inci dent light and events surrounding the process of eyelid opening play a role in the up-regulation of TKT gene expression observed during corneal matura tion in mice. Light appears to play a less important role in the mature cor nea in maintaining high levels of TKT gene expression. The low levels of TK T in the cornea of chicken and zebrafish support the notion that TKT acts a s a taxon-specific enzyme-crystallin in mammals. The involvement of environ mental signals for this putative, mammalian cornea crystallin contrasts wit h the purely developmental signals involved in the up-regulation of the cry stallin genes of the lens.