Differential expression of keratins in goldfish optic nerve during regeneration

The goldfish visual pathway, unlike the visual pathway of higher vertebrates, retains continuous growth and development throughout life and is capable of functional regeneration. The structure and expression of proteins that support the physiological attributes of this system are of interest. Glial cells in this pathway express keratins as the predominant intermediate filament proteins rather than the expected glial fibrillary acidic protein. Previously we identified and characterized cDNA clones representing two type I keratins from the goldfish optic nerve, GK48 and GK49. The GK48 protein is the type I keratin partner to the type II keratin ON3, while the GK49 protein is expressed in a different cell type. Here, we extend our studies on the expression of mRNA for the GK48, GK49, and ON3 proteins at the early stages of optic nerve regeneration. RNase protection assays show that at 10 days post-crush, there is no overall change in levels of mRNA for these proteins as compared to uncrushed control nerves and nerves from unoperated fish. In addition, we show by in situ hybridization that the GK49 protein shows no changes in its distribution of mRNA in the optic nerve after crush. In contrast, the levels of GK48 and ON3 mRNA are greatly reduced within the crush zone. However, these two mRNAs are differentially expressed at different time points during regeneration, with GK48 mRNA appearing in the crush zone before ON3. These results indicate that the mRNA for the GK48 and ON3 proteins are differentially regulated during regeneration and that these two proteins are expressed in a different cell type from the GK49 protein.