Twenty-five percent Gy is a safe dose and a low dose for humans, but the value of a low irradiation dose for LRCC cannot be completely determined. Previous studies have shown that 1.94 Gy caesium-137 irradiation can cause changes in the levels of SOD, GSH-PX and HSP70, molecules related to oxidative stress, in the liver of LRCC. J. Lemos used 0.1~1.0 Gy X-ray irradiation to stress zebrafish, and the results showed that the offspring’s peripheral blood DNA was damaged to different degrees within 24 hours after irradiation (Lemos et al., 2017). We collectively considered the results of research on radiation dose effects related to humans, zebrafish and LRCC and established 0.3 Gy as the low dose of caesium-137 irradiation in this study. This study showed that exposure to 0.3-Gy irradiation stress did not cause visible damage or death in the experimental LRCC. Analysis of liver sections showed that some cells had nuclear pyknosis, and differential expression of miRNAs could still be detected.
The biological effects of low-dose radiation are categorized as excitatory effects and adaptive effects, among which excitatory effects are mainly manifested in promoting growth, prolonging the lifespan, and enhancing immune function (Jargin, 2020). The excitatory effect of radiation, as proposed by Luckey T.D., indicates that the energy of low-dose rate irradiation can be absorbed by the organism and converted into chemical energy that can be used by itself, accelerate metabolism, and even produce certain benefits to the organism(Luckey, 2006; Sagan, 1989). This study also showed that the target genes of differentially expressed miRNAs are involved in substance metabolism, cell damage repair, immune responses, etc. In particular, the metabolism of fats, carbohydrates, amino acids, and pyruvate is enhanced, and the assembly of the nucleus, cytoplasm, and cell membrane is active.
In this study, the irradiation group and nonirradiation group of LRCC were used as the research objects, and 34 differentially expressed miRNAs were identified through RNA-seq and bioinformatics analysis. Among these miRNAs, 10—abu-miR-99b, dre-miR-126a-3p, dre-miR-202-5p, dre-miR-24b-3p, abu-miR-19b, ccr-miR-222, gmo-miR-140-5p, gmo-miR-21-3p, gmo-miR-27d-3p, ola-miR-27d-3p—exhibited extremely significant differential expression, with P<0.01. Studies have shown that miR-99 family miRNAs are related to the repair of radiation-induced DNA damage. By targeting the SWI/SNF chromatin remodelling factor SNF2H/SMARCA5, the speed and overall efficiency of DNA damage repair after irradiation can be reduced, which is helpful for fractional radiotherapy(Mueller et al., 2013). The miR-24 family negatively regulates FERMT1, which can enhance the sensitivity of cells to ionizing radiation(Yan et al., 2019). The miR-27 family is related to fat metabolism and lipid cell differentiation and plays an important role in regulating dynamic homeostasis of energy metabolism(Chen et al., 2012; Vickers et al., 2013). Wen-Jun Wei used high-energy iron ions to irradiate the whole body of mice and found that the expression levels of miR-21a and miR-200b in the circulating blood were increased with a strong dose-effect relationship(Malkani et al., 2020). The differentially expressed miRNAs identified in this experiment may be used as preselected biomarkers for the biological effects of irradiation in aquatic animals.
LRCC has many advantages as a model fish and has shown certain applications in environmental monitoring of chemicals, heavy metals and pesticides in aquatic environments and aquatic ecotoxicology research(Mennigen et al., 2017; Yang et al., 2020; Zhou et al., 2019).
The effects of radiation observed in previous research and this study show that LRCC also have a high sensitivity to nuclear radiation. A radiation dose of 0.3 Gy can cause nuclear constriction in hepatocytes and changes in liver miRNA expression; 1.94 Gy can cause changes in blood physiology and biochemistry, enzymology, protein expression and other parameters; and 31 Gy may cause death in 50% of LRCC. Therefore, LRCC may have great research and application importance in evaluating the biological effects of radiation in aquatic environments and in evaluating the safety of radiation, and this fish can be used as a model experimental animal for radiation biology research.