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Decoding Transcriptional Regulation in Response to Sunlight in Vertebrates
Sunlight is a powerful environmental stimulus for most organisms, known to entrain their circadian clocks, activate their DNA repair systems, and impact various other physiological processes. Cells and tissues of zebrafish (Danio rerio) respond directly to light and the D-box enhancer element has previously been implicated in the subsequent regulation of various circadian and DNA repair genes. Additionally, the PAR-bZip and Nfil3 transcription factors have been identified as D-box regulators. However, the full extent of the transcriptional response to sunlight and its evolutionary nuances remains unclear. In the present study, the cellular light-mediated gene expression was explored in zebrafish and compared to that of the blind Somalian cavefish (Phreatichthys andruzzii), which evolved in perpetual darkness, and whose circadian clock and DNA repair mechanisms are not light-regulated. Two mRNA-sequencing experiments of zebrafish and cavefish cells exposed to blue light (468 nm) and to UV-C (20J/m2) were performed. Gene ontology analyses of zebrafish cells exposed to blue light for 1 to 6 hours revealed the enrichment of genes related to mitochondrial structure and function, as well as heme biosynthesis, transport, and catabolism. The upregulation of these genes was also observed in 5 dpf zebrafish embryos exposed to blue light and in zebrafish cells 18 and 36 hours after exposure to UV-C. Their temporal profile of expression follows that of known D-box-regulated genes and is the result of de novo transcription. Bioinformatic, in vitro, and in vivo analyses supported the notion that in zebrafish the expression of these genes is regulated via D-box enhancer elements in their promoters. However, the upregulation of the mitochondrial and heme-related genes was absent in the cavefish cell line, highlighting the evolutionary adaptability of light-sensing mechanisms. To gain insights into D-box-mediated gene expression and its evolution, the cavefish PAR-bZip and Nfil3 transcription factors were identified, and their function was compared with the zebrafish counterparts. The analyses revealed all PAR-bZip factors of both zebrafish and cavefish can activate transcription via the D-box enhancer element in both cell systems. The lower levels of activated transcription mediated by the cavefish factors suggests they might have been the target of evolution, either through mutation or alterations affecting their phosphorylation, binding efficiency, and/or cellular localization. The present findings expand the known landscape of light-mediated gene expression and identify the D-box as part of a broader mechanism extending beyond circadian clock entrainment and DNA repair. Despite the transcriptional changes in zebrafish cells, preliminary analyses reveal no significant changes in general mitochondrial function or heme levels. Nonetheless, the present study lays the groundwork for further investigation into the functional impact of the observed transcriptomic response, offering insights into its broader physiological implications.
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570 Biowissenschaften, Biologie
570 Life sciences