Science Inventory

OECD Series on Adverse Outcome Pathways No. 25: Deiodinase 1 inhibition leading to increased mortality via reduced anterior swim bladder inflation

Citation:

Vergauwen, L., E. Stinckens, D. Villeneuve, AND D. Knapen. OECD Series on Adverse Outcome Pathways No. 25: Deiodinase 1 inhibition leading to increased mortality via reduced anterior swim bladder inflation. OECD Press, Paris, France, 2022. https://doi.org/10.1787/9f651c16-en

Impact/Purpose:

This Adverse Outcome Pathway (AOP) describes the linkage between Deiodinase 1 inhibition and increased mortality via reduced anterior swim bladder inflation. The swim bladder is a gas-filled organ found in many bony fish species and typically consists of two gas-filled chambers. The posterior chamber inflates during early development (embryo), while the anterior chamber inflates during late development (larva). Both chambers are important for fish to control buoyancy and the anterior chamber has an additional role in hearing. This AOP is part of a network of 5 AOPs describing how disruption of the thyroid hormone system can affect developmental processes involved in swim bladder inflation. The network includes three molecular initiating events representing the inhibition of enzymes that are important for thyroid hormone synthesis and activation. It describes how inhibition of thyroperoxidase and/or deiodinase, leads to reduced swim bladder inflation, resulting in reduced swimming performance, increased mortality and ultimately, decreased population trajectory in fish. This AOP network is currently mainly based on experimental evidence from studies on fish species with a two-chambered swim bladder. This AOP is referred to as AOP 158 in the Collaborative Adverse Outcome Pathway Wiki (AOP-Wiki).

Description:

This AOP describes the sequence of events leading from deiodinase inhibition to increased mortality via reduced anterior swim bladder inflation. Thyroid hormones (THs) are critical during development and disruption of the TH system can interfere with normal development. Three types of iodothyronine deiodinases (DIO1-3) have been described in vertebrates that activate or inactivate THs and are therefore important mediators of TH action. While type II deiodinase (DIO2) has thyroxine (T4) as a preferred substrate and is mostly important for converting T4 to the more biologically active triiodothyronine (T3), type I deiodinase is capable of both converting T4 into T3 and converting rT3 to the inactive thyroid hormone 3,3’ T2. Inhibition of DIO1 thus reduces T3 levels. However, partly because rT3, rather than T4, is the preferred substrate for DIO1, DIO1 inhibition is probably less important in causing reduced T3 levels when compared to DIO2 inhibition. Swim bladder inflation is known to be under TH control (Brown et al.,1988; Liu and Chan, 2002). Many fish species have a swim bladder which is a gas-filled organ that typically consists of two chambers (Robertson et al., 2007). The posterior chamber inflates during early development in the embryonic phase, while the anterior chamber inflates during late development in the larval phase. Both the posterior and the anterior chamber have an important role in regulating buoyancy, and the anterior chamber has an additional role in hearing (Robertson et al., 2017). This AOP describes how inhibition of DIO1 reduces levels of T3, thereby prohibiting proper inflation of the anterior chamber. Due to its role in regulating buoyancy, this results in reduced swimming performance. Since reduced swimming performance resuls in a decreased ability to forage and avoid predators, this reduces chances of survival. The final adverse outcome is a decrease of the population trajectory. Since many AOPs eventually lead to this more general adverse outcome at the population level, the more specific and informative adverse outcome at the organismal level, increased mortality, is used in the AOP title. Support for this AOP is mainly based on chemical exposures in zebrafish and fathead minnows (Cavallin et al., 2017; Godfrey et al., 2017; Stinckens et al., 2020). This AOP is part of a larger AOP network describing how decreased synthesis and/or decreased biological activation of THs leads to incomplete or improper inflation of the swim bladder, leading to reduced swimming performance, increased mortality and decreased population trajectory (Knapen et al., 2018; Knapen et al., 2020; Villeneuve et al., 2018). Other than the difference in deiodinase (DIO) isoform, the current AOP isidentical to the corresponding AOP leading from DIO2 inhibition to increased mortality via anterior swim bladder inflation (https://aopwiki.org/aops/156). The overall importance of DIO1 versus DIO2 in fish is notexactly clear. DIO1 inhibitors are often also inhibitors of DIO2 (Olker et al., 2019; Stinckens et al. 2018). In the ToxCast DIO1 inhibition single concentration assay, 219 out of 1820 chemicals were positive and 177 of these were also positive for DIO2 inhibition (viewed on 5/7/2022). This complicates the distinction between the relative contribution of DIO1 and DIO2 inhibition to reduced swim bladder inflation. The current state of the art suggests that DIO2 is more important than DIO1 in regulating swim bladder inflation. Six out of seven DIO1 inhibitors impaired posterior chamber inflation, but almost all of these compounds also inhibit DIO2 (Stinckens et al., 2018)). Tetrachlorobisphenol A (TCBPA), the only compound that inhibits DIO1 and not DIO2, had no effect on the posterior swim bladder. Exposure to strong DIO2 inhibitors on the other hand affected posterior chamber inflation and/or surface area in all cases. Therefore...