We have shown previously that a 1.696 kb upstream fragment of the goldfish ␣1-tubulin promoter was capable of driving green fluorescent protein (GFP) expression in the developing and regenerating zebrafish CNS in a pattern closely mimicking the endogenous ␣1-tubulin gene. Comparison of fish and rat ␣1-tubulin promoters identified a 64 bp region with a conserved repetitive homeodomain (HD) consensus sequence core (TAAT) and a nearby basic helix-loop-helix binding E-box sequence (CANNTG), which led us to speculate that it could be of importance for regulating ␣1-tubulin gene transcription. To address this issue, we examined the ability of deletion mutants of the 1.696 kb promoter to drive expression of GFP in zebrafish retinal cells under normal conditions and after injury. Interestingly, although wild-type 1.696 kb and mutant promoters, lacking the E-box and/or HD sequences, exhibited rather similar patterns of GFP expression in the developing retina, significant differences were noticed in the mature retina. First, although the 1.696 kb promoter directed transgene expression to retinal neurons and progenitor cells, the activity of mutant promoters was drastically reduced. Second, we found that the E-box and HD sequences were necessary for transgene reinduction during optic nerve regeneration, but were not as important for transgene expression in regenerating retinal neurons after eye injury. In this latter lesion model, remarkably, both 1.696 kb and mutant promoters targeted GFP expression to Müller glia-like cells, some of which re-entered the cell cycle. These new findings will be useful for identifying the molecular signals necessary for successful CNS regeneration.
Alpha1-tubulin expression occurs in a neural-specific, temporally regulated, and regeneration-inducible fashion in zebrafish. A GFP reporter driven by the alpha1-tubulin promoter in transgenic zebrafish acts as a stable, in vivo molecular tag that follows neuronal development from birth/specification through postmitotic differentiation to axonal outgrowth and synaptogenesis. We exploited this transgenic system in a reporter expression-dependent (morphology-independent) mutagenesis screen to identify disruptions in genetic loci essential for neuronogenesis and axon elaboration, which would manifest as visually appreciable perturbations in GFP fluorescence. Thirty-two such recessive mutations were obtained, a subset of which was screened through a secondary RNA quantification-based assay to eliminate housekeeping gene defects. Three representative loci, when characterized in detail, were found to exhibit missteps in discrete, sequential stages of embryonic neuronal development. Mutation in sookshma panneurally diminishes the neural precursor pool by affecting cell proliferation in the developing embryo while patterning along the neuraxis remains unperturbed. Disruption of drishti on the other hand ameliorates the mitotic neural population by affecting cell cycle exit of progenitors and stalling their progression to the postmitotic neuronal stage, without impairing subsequent cell fate determination or differentiation. Finally, dhruva is required during neuronal differentiation for axonal branching and terminal innervation in spinal motoaxons and the retinotectal projection. Molecular identification of these loci and analysis of the remaining mutational repertoire will offer unique insights into the genetic inputs that go on to make a mature, differentiated neuron.
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.