Earlier experiments in this laboratory identified a highly expressed 65-68-kDa protein in both mouse and human corneas (Cuthbertson, R. A., Tomarev, S. I., and Piatigorsky J. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 4004 -4008). Here, we demonstrate that this protein is transketolase (TKT; EC 2.2.1.1), an enzyme in the nonoxidative branch of the pentose-phosphate pathway, based on peptide and cDNA isolation and sequence analysis of mouse cornea protein and RNA samples, respectively. While expressed at low levels in a number of tissues, the 2.1-kilobase TKT mRNA was expressed at a 50-fold higher level in the adult mouse cornea. The area of most abundant expression was localized to the cornea epithelial cell layer by in situ hybridization. Western blot analysis confirmed TKT protein abundance in the cornea and indicated that TKT may comprise as much as 10% of the total soluble protein of the adult mouse cornea. Soluble cornea extracts exhibited a correspondingly high level of TKT enzymatic activity. TKT expression increased progressively through cornea maturation, as shown by Northern blot, in situ hybridization, Western blot, and enzymatic analyses. TKT mRNA and protein were expressed at low levels in the cornea prior to eye opening, while markedly increased levels were observed after eye opening. Taken together, these observations suggest that TKT may be a cornea enzymecrystallin, and suggest that the crystallin paradigm and concept of gene sharing, once thought to be restricted to the lens, apply to other transparent ocular tissues.
Our studies suggest that both exposure to incident light and events surrounding the process of eyelid opening play a role in the up-regulation of TKT gene expression observed during corneal maturation in mice. Light appears to play a less important role in the mature cornea in maintaining high levels of TKT gene expression. The low levels of TKT in the cornea of chicken and zebrafish support the notion that TKT acts as a taxon-specific enzyme-crystallin in mammals. The involvement of environmental signals for this putative, mammalian cornea crystallin contrasts with the purely developmental signals involved in the up-regulation of the crystallin genes of the lens.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.