Adverse Outcome Pathways for Chronic Copper Toxicity to Fish and Amphibians

Abstract

In the present review, we synthesize information on the mechanisms of chronic copper (Cu) toxicity using an adverse outcome pathway framework and identify three primary pathways for chronic Cu toxicity: disruption of sodium homeostasis, effects on bioenergetics, and oxidative stress. Unlike acute Cu toxicity, disruption of sodium homeostasis is not a driving mechanism of chronic toxicity, but compensatory responses in this pathway contribute to effects on organism bioenergetics. Effects on bioenergetics clearly contribute to chronic Cu toxicity with impacts at multiple lower levels of biological organization. However, quantitatively translating these impacts into effects on apical endpoints such as growth, amphibian metamorphosis, and reproduction remains elusive and requires further study. Copper-induced oxidative stress occurs in most tissues of aquatic vertebrates and is clearly a significant driver of chronic Cu toxicity. Although antioxidant responses and capacities differ among tissues, there is no clear indication that specific tissues are more sensitive than others to oxidative stress. Oxidative stress leads to increased apoptosis and cellular damage in multiple tissues, including some that contribute to bioenergetic effects. This also includes oxidative damage to tissues involved in neuroendocrine axes and this damage likely alters the normal function of these tissues. Importantly, Cu-induced changes in hormone concentrations and gene expression in endocrine-mediated pathways such as reproductive steroidogenesis and amphibian metamorphosis are likely the result of oxidative stress-induced tissue damage and not endocrine disruption. Overall, we conclude that oxidative stress is likely the primary driver of chronic Cu toxicity in aquatic vertebrates, with bioenergetic effects and compensatory response to disruption of sodium homeostasis contributing to some degree to observed effects on apical endpoints.

Influence of systemic copper toxicity on early development and metamorphosis in Xenopus laevis

“…Overall, results of the current study show that Cu effects on growth and development in X. laevis larvae occurred via a non‐thyroidal AOP consistent with that proposed by Brix et al (2022). This pathway involves multi‐organ cytotoxicity and, at the highest tested Cu concentration, complete growth arrest.…”

“…Likewise, the effects of Cu exposure on advanced development in amphibians have been noted by several investigators (Chai et al, 2017;Fort, Rogers, et al, 2000;Lance et al, 2012;Wang et al, 2016) and is also summarized in Brix et al (2022). Chai et al (2017) found that Cu exposure in B. gargarizan from pre-metamorphic through metamorphic climax decreased metamorphic success, increased HLL, and down-regulated hsp, sod, and phospholipid hydroperoxide glutathione peroxidase (phgpx) implicating the importance of oxidative stress during metamorphosis.…”

“…The effects of Cu exposure on early development in several amphibian species as described in Brix et al (2022) included developmental delay (Chen et al, 2007; Flynn et al, 2015; Peles, 2013; Rumrill et al, 2016), reduced body mass (Peles, 2013), impaired swimming behavior and decreased survival (Chen et al, 2007), and abnormalities involving the eye, craniofacial region, and gut in both X. laevis and S. tropicalis (Fort et al, 2004). However, while excess Cu can induce toxicity, due to its essential role in a variety of biological processes a deficiency in Cu can also cause adverse effects as demonstrated in X. laevis (Fort, Stover, et al, 2000).…”

“…The role of oxidative stress in amphibian metamorphosis at the tissue level, including processes regulating apoptosis, has been well established (Allen, 1991; Buchholz, 2017; Menon & Rozman, 2007). The importance of reactive oxygen species (ROS) in Cu‐mediated toxicity is also well understood (Brix et al, 2022; Huang et al, 2020). Huang et al (2020) further suggested that Cu‐induced increases in ROS could potentially disrupt metamorphic via non‐thyroidal processes including genes involved in fatty acid β‐oxidation, lipid synthesis, and management of oxidative stress.…”

“…Although several studies have hypothesized that copper might directly affect the HPT axis and thus act as an endocrine disruptor in anurans (Chai et al, 2017; Wang et al, 2016), evidentiary support for this hypothesis is not apparent as discussed recently in Brix et al (2022) who proposed an adverse outcome pathway (AOP) for the chronic toxicity of Cu in fish and amphibians. The purpose of the present study was to investigate the effects of early Cu exposure on subsequent somatic growth and metamorphic development in Xenopus laevis with specific focus on examination of several key developmental benchmarks related to thyroid function, and use of histopathology to assess more general systemic toxicity effects.…”