Stinckens Evelyn, Vergauwen Lucia, Schroeder Anthony L, Maho Walid, Blackwell Brett R, Witters Hilda, Blust Ronny, Ankley Gerald T, Covaci Adrian, Villeneuve Daniel L, Knapen Dries
Zebrafishlab, Veterinary Physiology and Biochemistry, Department of Veterinary Sciences, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium.
U.S. Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Blvd, Duluth, MN 55804, USA; University of Minnesota-Twin Cities, Water Resources Center, 1985 Lower Buford Circle, St. Paul, MN 55108, USA.
Aquat Toxicol. 2016 Apr;173:204-217. doi: 10.1016/j.aquatox.2015.12.023. Epub 2016 Jan 18.
Disruption of the thyroid hormone (TH) system, an important mode of action, can lead to ecologically relevant adverse outcomes, especially during embryonic development. The present study characterizes the effects of disruption of TH synthesis on swim bladder inflation during zebrafish early-life stages using 2-mercaptobenzothiazole (MBT), a thyroid peroxidase (TPO) inhibitor. Zebrafish were exposed to different MBT concentrations until 120/168h post fertilization (hpf) and 32days post fertilization (dpf), in two sets of experiments, to investigate the effects of TPO inhibition on posterior and anterior swim bladder inflation respectively, as well as whole body thyroid hormone concentrations (triiodothyronine (T3) and its prohormone, thyroxine (T4)). At 120hpf, MBT did not directly impair posterior chamber inflation or size, while anterior chamber inflation and size was impaired at 32dpf. As previously shown in amphibians and mammals, we confirmed that MBT inhibits TPO in fish. Whole-body T4 decreased after MBT exposure at both time points, while T3 levels were unaltered. There was a significant relationship between T4 levels and the anterior chamber surface at 32dpf. The absence of effects on posterior chamber inflation can possibly be explained by maternal transfer of T4 into the eggs. These maternally derived THs are depleted at 32dpf and cannot offset TPO inhibition, resulting in impaired anterior chamber inflation. Therefore, we hypothesize that TPO inhibition only inhibits swim bladder inflation during late development, after depletion of maternally derived T4. In a previous study, we showed that iodothyronine deiodinase (ID) knockdown impaired posterior chamber inflation during early development. Our findings, in parallel with similar effects observed in fathead minnow (see part I, this issue) suggest that thyroid disruption impacts swim bladder inflation, and imply an important distinction among specific subtypes of TH disrupting chemicals. However, the existence of another - yet unknown - mode of action of MBT impacting swim bladder inflation cannot be excluded. These results can be helpful for delineating adverse outcome pathways (AOPs) linking TPO inhibition, ID inhibition and other TH related molecular initiating events, to impaired swim bladder inflation in fish during early life stages. Such AOPs can support the use of in vitro enzyme inhibition assays for predicting reduced survival due to impaired posterior and anterior chamber inflation.
甲状腺激素（TH）系统的破坏是一种重要的作用模式，可导致具有生态相关性的不良后果，尤其是在胚胎发育期间。本研究使用甲状腺过氧化物酶（TPO）抑制剂2-巯基苯并噻唑（MBT），来表征TH合成中断对斑马鱼早期生命阶段鳔充气的影响。在两组实验中，将斑马鱼暴露于不同浓度的MBT中，直至受精后120/168小时（hpf）和受精后32天（dpf），以分别研究TPO抑制对鳔后室和前室充气以及全身甲状腺激素浓度（三碘甲状腺原氨酸（T3）及其前体激素甲状腺素（T4））的影响。在120hpf时，MBT并未直接损害后室充气或大小，而在32dpf时前室充气和大小受到损害。如先前在两栖动物和哺乳动物中所示，我们证实MBT在鱼类中抑制TPO。在两个时间点暴露于MBT后，全身T4均下降，而T3水平未改变。在32dpf时，T4水平与前室表面之间存在显著关系。对后室充气没有影响可能是由于T4通过母体转移到卵中。这些母体来源的TH在32dpf时耗尽，无法抵消TPO抑制作用，导致前室充气受损。因此，我们假设TPO抑制仅在母体来源的T4耗尽后的发育后期抑制鳔充气。在先前的一项研究中，我们表明碘甲状腺原氨酸脱碘酶（ID）敲低会在早期发育期间损害后室充气。我们的发现与黑头呆鱼中观察到的类似效应（见本期第一部分）一致，表明甲状腺破坏会影响鳔充气，并暗示TH破坏化学物质的特定亚型之间存在重要区别。然而，不能排除MBT影响鳔充气的另一种未知作用模式的存在。这些结果有助于描绘将TPO抑制、ID抑制和其他与TH相关的分子起始事件与鱼类早期生命阶段鳔充气受损联系起来的不良结局途径（AOP）。这样的AOP可以支持使用体外酶抑制试验来预测由于后室和前室充气受损而导致的存活率降低。
