Histological, antioxidant, apoptotic and transcriptomic responses under cold stress and the mitigation of blue wavelength light of zebrafish eyes

As global climate changes, severely temperature variations have significant impacts on survival and development of fish. While the potential effects of light wavelength on cold shock and associated mechanisms remain largely unknown in fish. Here, zebrafish (Danio rerio) were pre-exposed to white LEDs (Light emitting diodes) at an irradiance of 0.9 W/m² and blue LEDs (LDB, peak at 450 nm, 0.9 W/m²) for 2 weeks, and then exposed to 26 ℃ or 11 ℃ for 48 h, respectively. Cold shock led to low survival rate. Cold shock altered retinal structure, increased the number of apoptotic cells and Caspase-3 activity, inhibited superoxide dismutase (SOD) and catalase (CAT) activities, up-regulated mRNA expression of nrf2 (NF-E2-related factor 2), p53 (Tumor suppressor), casp3 (Caspase-3) and casp9 (Caspase-9), and down-regulated cat (Catalase) expression in fish eyes. These results demonstrated that acute cold exposure induced oxidative stress and apoptosis in zebrafish eyes, which may lead to mortality. However, cold shock in combination with LDB apparently mitigated these negative effects, which might be involved in the up-regulation of antioxidant response and down-regulation of apoptotic responses at transcriptional and translational levels. Furthermore, cold shock also caused dysregulation of genome-wide gene expression involved in circadian rhythm, phototransduction and il-17 signaling pathway, indicating that cold shock disturbed phototransduction cascade and circadian rhythm signals and caused inflammatory responses. Ten key genes involved in circadian rhythm, phototransduction, cell cycle arrest, RNA processing or inflammatory responses were identified, including muc5d (Mucin 5d), rnps1 (RNA-binding protein with serine-rich domain 1), si:dkey-243i1.1, opn1mw1 (Medium-wave-sensitive opsin 3), gadd45ba (Growth arrest and DNA damage-inducible protein), cebpd (CCAAT/enhancer binding protein delta), btg2 (Anti-proliferative cofactor), si:dkey-242g16.2, nr1d1 (Nuclear receptor subfamily 1 group D member 1) and zgc:122979, which may play an important role in the protection of LDB against cold shock. Finally, our study suggested the relationship between spectrum and cold stress and demonstrated LDB could protect fish against the negative effect of cold stress in the eyes.