Characterization of Nuclear Respiratory Factor 1 using transgenic zebrafish

Abstract

Nuclear respiratory factor 1 (NRF1) is a transcription factor essential for the embryonic development of vertebrates. The zebrafish mutant nrf is characterized by apoptotic photoreceptor death during embryogenesis followed by late larval lethality. In zebrafish nrf1 is initially expressed throughout the CNS at a high level at days 1 and 2 and later fades to levels undetectable by in situ hybridization. This study shows that the initial phase of expression is crucial for photoreceptor maintenance, but not for their initial development, as homozygous mutant cells can form normal photoreceptors in a wild type background. By utilizing reporter expression of a marker insertion near the wild type allele of nrf1 combined with transgenic rescue, I show that a single heat shock induced pulse of nrf1 expression at any time between 24 and 54 hours post fertilization is sufficient to rescue the mutant phenotype and delay photoreceptor degeneration until larval stages. Moreover, no ectopic defects are detected after ubiquitous expression of the gene, suggesting that Nrf1 serves no detectable instructive role during embryogenesis. These results suggest that Nrf1 plays a permissive role in zebrafish photoreceptor maintenance and is crucial for the formation and survival of the outer nuclear layer, but is not strictly necessary for the initial development of individual photoreceptors. Gene expression comparison analysis identifies several up and downregulated genes in the nrf mutant, suggesting that the intraflagellar transport machinery of the photoreceptor connecting cilium might be defect.