Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid

Abstract

Inflation of the posterior and/or anterior swim bladder is a process previously demonstrated to be regulated by thyroid hormones. We investigated whether inhibition of deiodinases, which convert thyroxine (T4) to the more biologically active form, 3,5,3'-triiodothyronine (T3), would impact swim bladder inflation. Two experiments were conducted using a model deiodinase inhibitor, iopanoic acid (IOP). First, fathead minnow embryos were exposed to 0.6, 1.9, or 6.0 mg/L or control water until 6 d postfertilization (dpf), at which time posterior swim bladder inflation was assessed. To examine anterior swim bladder inflation, a second study was conducted with 6-dpf larvae exposed to the same IOP concentrations until 21 dpf. Fish from both studies were sampled for T4/T3 measurements and gene transcription analyses. Incidence and length of inflated posterior swim bladders were significantly reduced in the 6.0 mg/L treatment at 6 dpf. Incidence of inflation and length of anterior swim bladder were significantly reduced in all IOP treatments at 14 dpf, but inflation recovered by 18 dpf. Throughout the larval study, whole-body T4 concentrations increased and T3 concentrations decreased in all IOP treatments. Consistent with hypothesized compensatory responses, deiodinase-2 messenger ribonucleic acid (mRNA) was up-regulated in the larval study, and thyroperoxidase mRNA was down-regulated in all IOP treatments in both studies. These results support the hypothesized adverse outcome pathways linking inhibition of deiodinase activity to impaired swim bladder inflation. Environ Toxicol Chem 2017;36:2942-2952. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Gene transcription ontogeny of hypothalamic-pituitary-thyroid axis development in early-life stage fathead minnow and zebrafish

“…2-7). We previously showed that disruption of TH levels during embryonic development interferes with normal inflation of the posterior swim bladder chamber after combined knockdown of dio1 and dio2 or knockdown of dio3b in zebrafish (Bagci et al, 2015), and exposure to a model deiodinase inhibitor in fathead minnow (Cavallin et al, 2017). In both fathead minnow and zebrafish, we observed a peak in the transcript levels of dio1 and in zebrafish we additionally observed a peak of dio2 and dio3b mRNA levels at the time of posterior swim bladder chamber inflation (Psb in Fig.…”

An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish

“…Dio1 could therefore also be important during embryogenesis and metamorphosis of fish, when the demand for THs is high (Orozco et al, 2012). In fathead minnows exposed to IOP, a strong DIO1 and DIO2 inhibitor according to our data, during the period of anterior chamber inflation, dio2 mRNA levels increased, possibly as a compensatory response, while dio1 mRNA levels did not (Cavallin et al, 2017). Therefore, it seems plausible that Dio2 inhibition is more important for causing impaired posterior and anterior chamber inflation.…”

“…In a follow-up study, we also plan to assess whether these in chemico assays can be used to predict endpoints in a 30 days fish early-life stage (FELS) chronic toxicity test, as the FELS test (OECD TG 210; OECD, 2013b) is one of the primary assays used to assess the chronic aquatic toxicity of chemicals in fish. Disruption of thyroid function has already been reported to affect anterior swim bladder chamber inflation (Cavallin et al, 2017;Godfrey et al, 2017;Nelson et al, 2016;Stinckens et al, 2016), a chronic adverse outcome which cannot be observed in acute ZFET experiments.…”

“…Building on our previous work focused on AOP development related to TH disruption in fish (Cavallin et al, 2017;Nelson et al, 2016;Stinckens et al, 2016), we aligned the present study with AOPs that are publicly available in the AOP-Wiki (aopwiki.org), an online database organizing the available knowledge and published research into individual AOP descriptions using a user friendly Wiki interface. More specifically, we focused on two AOPs linking the molecular initiating events (MIEs) Dio1 and Dio2 inhibition to impaired inflation of the posterior swim bladder chamber in fish during early life-stages ( Figure 1; Bagci et al, 2015;Cavallin et al, 2017;Knapen et al, 2018;Nelson et al, 2016;Stinckens et al, 2016;Villeneuve et al, 2018). These AOPs were used to optimize in chemico assays to measure the potential of compounds to inhibit DIO1 and DIO2 enzyme activity, using porcine tissue.…”

ERGO: Breaking Down the Wall between Human Health and Environmental Testing of Endocrine Disrupters

“…For instance, the molecular mechanisms of thyroid organogenesis and the role of TR signaling during embryogenesis are well documented for zebrafish and seem to be highly conserved between zebrafish and mammalian models [26,31]. (Neuro)developmental processes that are, at least partially, regulated by THs include eye development, swim bladder inflation and fin formation during early embryonic development and the metamorphosis from the larval to juvenile stage [33][34][35][36][37]. Exposure to THDs has been shown to affect these important developmental processes, which makes them promising candidates for thyroid-related endpoints in THD fish testing assays.…”