Plasticin and gefiltin are intermediate filament (IF) proteins which are expressed in the goldfish visual pathway. These two proteins are structurally and physiologically homologous to IF proteins which are expressed during growth and development in mammalian nerves. Thus, these developmentally regulated IF proteins may support the sequential processes of axonogenesis. Our histological and biochemical results indicate that plasticin and gefiltin have structural attributes that support the ongoing developmental program of axonal outgrowth. More specifically, plasticin and gefiltin are synthesized in retinal ganglion cells and transported into the optic nerve. In normal goldfish, plasticin is preferentially expressed in young ganglion cells, whereas gefiltin is expressed in all ganglion cells. After optic nerve injury, there is a dramatic increase in plasticin mRNA expression. Gefiltin mRNA expression occurs later when the ganglion cell axons reach their tectal targets. This leads to the following hypotheses: plasticin expression supports initial axonogenesis whereas gefiltin is more essential to the formation of normal retinal projection terminals. The organism that is best suited to test these hypotheses is the zebrafish (Danio rerio). It shares with the goldfish the capacity for optic nerve regeneration and has an identical distribution of intermediate filament proteins. However, the zebrafish is amenable to genetic and transgenic manipulations that allow easy visualization of phenotypes resulting from alterations in genetic expression. The long term goal of these studies is to establish the molecular basis of optic nerve development and regeneration.
The specific aims of this proposal are: to determine the spatial and temporal expression patterns of plasticin and gefiltin and to determine the functions of these two proteins in axonogenesis by manipulating their expression in zebrafish embryos.