Gefiltin in zebrafish embryos: sequential gene expression of two neurofilament proteins in retinal ganglion cells.

Neurogenesis is correlated with the progressive synthesis of diverse neuronal intermediate filaments (IF) proteins. This apparent developmental regulation of IF protein gene expression suggests that specific neurofilament proteins impart unique structural attributes that support the staged growth of the neuron. In the teleost visual pathway, the sequential expression of two IF genes, plasticin and gefiltin, is linked to the age of retinal ganglion cells (RGCs) and to the regeneration of optic axons after nerve injury. Given this pattern of plasticin and gefiltin expression, we hypothesized that the two proteins would be sequentially expressed in zebrafish retina during development. We analyzed the pattern of gefiltin expression during zebrafish development and compared it to our previous determination of plasticin expression (Canger et al. 1998). Gefiltin is expressed after plasticin, during the later stages of retinal development when axons grow past the optic chiasm and innervate their targets. Thus, during RGC development, expression of plasticin and gefiltin resembles that with optic nerve regeneration. Outside of the visual pathway, gefiltin is predominantly expressed in the central nervous system whereas plasticin is primarily expressed in the peripheral nervous system. These results suggest that the expression of these genes is regulated in a neuron-specific manner. In addition, since plasticin and gefiltin are co-expressed during RGC development, these findings suggest a more complex mechanism of transcriptional regulation which orchestrates the sequential expression of these genes.