Failed inflation of the swim bladder and DIO enzyme inhibition
Exposure to individual chemicals for 96 h caused swim bladder inflation defects in zebrafish larvae, taking TBC as an example (Figure 1). Figure 1b showed failed inflation in the gas bladder of larvae exposed to 1000 µg/L TBC after 96 hpf. Figure 1c showed the decreased area in inflation of gas bladder of larvae exposed to 1000 µg/L TBC after 96 hpf.
Increasing chemical concentrations showed a significantly decreased relative area of the gas bladder(Figure 2). Increasing chemical concentrations showed significantly increased failed inflation of the gas bladder(Figure 3). There were no significant differences in the relative area of the larval gas bladder between the 250, 500 µg/L HBCD exposure group and the control group (Figure 2b). However, compared with the control group, there were significant differences in the relative area of the swim bladder in the 1000 and 2500 µg/L HBCD exposure group (Figure 2b). Also, there were no significant differences in the relative area of the larval gas bladder between the 2, 4 µg/L Cu exposure group, 100, 200 µg/L Zn exposure group, and the control group(Figure 2d and h).
HBCD could cause zebrafish larvae gas bladder defects, which led to the failure of gas bladder inflation, and the pathological phenomenon of losing the capacity of free swimming, which could ultimately lead to the death of zebrafish larvae. Following embryonic exposure to CPs, deformity effects of gas bladders of zebrafish larvae were observed in CPs-treated groups and as the concentration of the CPs increased, deformity of gas bladders of zebrafish larvae increased, with an obvious dose-response relation (Figure 3c).
Increasing concentrations of TBC, HBCD, CPs, Cu, and Zn inhibited the enzyme activity of DIO1 in larvae after 96 h exposure, while increasing concentrations of Cd and Pb elevated the enzyme activity of DIO1(Figure 4). TBC, HBCD, CPs, Cu, Cd, Pb, Ag, and Zn all inhibited the enzyme activity of DIO2 in larvae after 96 h exposure(Figure 5). Most chemicals that were both strong DIO1 and DIO2 inhibitors had an adverse effect on posterior chamber inflation or gas bladder area. For example, PFOS exposure also caused spinal curvatures in zebrafish larvae and affected the gas bladder inflation(Hagenaars et al. 2014).
With an increase in Ag+ concentration in the larval body, the enzyme activity of DIO1 did not vary (Figure 4), whereas the activity of DIO2 was inhibited and showed a credible logistic dose-response relationship (Figure 5). This result indicates that DIO2 enzymes might play a vital role in the aeration of the swim bladder. In support of this phenomenon, most compounds that strongly reflect DIO1 and DIO2 affect posterior chamber ventilation or surface area(Stinckens et al. 2018). Some compounds are only strong DIO2 inhibitors, whereas Ag+, like tetrachlorobisphenol A, is a strong DIO1 inhibitor but not a DIO2 inhibitor(Stinckens et al. 2018). In a previous study, 4 days after fertilization, peaks of DIO1 and DIO2 mRNA expression were observed in normal developing zebrafish(Vergauwen et al. 2018), indicating that either isotype is dominant at this point. However, DIO2 is suggested to be the main contributor to TH activation in developing zebrafish embryos(Darras et al. 2015). Studies have shown that the knockout of DIO1 mRNA affects the embryonic development of zebrafish, whereas the knockout of DIO2 delays the development of ear capsule length, head-trunk angle, and pigmentation index(Houbrechts et al. 2016).