Developmental and metabolic responses of zebrafish (Danio rerio) embryos and larvae to short-chain chlorinated paraffins (SCCPs) exposure
Graphical abstract
Introduction
Short-chain chlorinated paraffins (SCCPs) are a large and complex family of chlorinated n-alkanes with carbon chain lengths of 10–13 and chlorine content of 30%–70% (mass weight) (Barber et al., 2005). They are widely applied in metal-working fluids, paints, sealants, adhesives, leather softener, and as flame retardants and plasticizers for plastics and textiles (Fiedler, 2010). In view of their properties of environmental persistence, long-range transport potential, bioaccumulation potential and high toxicity to aquatic organisms (UNEP, 2009), SCCPs have been listed as a new group of POPs by the Convention's POP Review Committee (POPRC) of Stockholm Convention (UNEP, 2017).
SCCPs were ubiquitously found in the aquatic ecosystem, and they could be accumulated in aquatic organisms (United Nations Environment Programme (UNEP), 2009, van Mourik et al., 2016). The concentrations of total SCCPs generally ranged from 4 to 1700 ng/L in natural surface water (Nicholls et al., 2001, Castells et al., 2004, Zeng et al., 2011, Ma et al., 2014a), and 10 to 4000 ng/g wet weight in fishes and aquatic invertebrates (Houde et al., 2008, Yuan et al., 2012, Ma et al., 2014a, Ma et al., 2014b, Wei et al., 2016). Previous studies have indicated that SCCPs are highly toxic to aquatic invertebrates (UNEP, 2009). The no observed effect concentration (NOEC) of SCCPs for daphnids (Daphnia magna) were determined to be 5 μg/L based on a 21-day chronic exposure (Thompson and Madeley, 1983a), and 28-day chronic NOEC of SCCPs for the mysid shrimp (Mysidopsis bahia) were determined to be 7.3 μg/L (Thompson and Madeley, 1983b). Fish embryos and larvae were also found to be sensitive to SCCPs exposure. The acute toxicity of SCCPs to Japanese Medaka (Oryzias latipes) is narcosis, and the lowest observed effect concentration (LOEC) in embryo-larval assays varied from 55 μg/L to 460 μg/L (Fisk et al., 1999).
SCCPs could cause sub-chronic toxicity in the kidney (Warnasuriya et al., 2010), liver (Cooley et al., 2001) and thyroid (Wyatt et al., 1993) on laboratory animals, and induce developmental malformation of Xenopus laevis frog embryos (Burýškova et al., 2006). In addition, several previous studies indicated that SCCPs could act as endocrine disruptor (Wyatt et al., 1993, Cooley et al., 2001, Liu et al., 2016, Zhang et al., 2016). SCCPs exposure could affect gene expression in the hypothalamic-pituitary-thyroid (HPT) axis and level of thyroid hormone in zebrafish larvae (Liu et al., 2016). A study with H295R cells indicated that SCCPs not only exerted potential endocrine-disrupting effects through nuclear receptors but also disrupted the production of steroid hormones via non-receptor-mediated mechanisms (Zhang et al., 2016). Moreover, the aliphatic structure could also make SCCPs act as PPAR-α activator to induce the peroxisome proliferation (Warnasuriya et al., 2010). These endocrine-disrupting effects together with PPAR-α activation imply the possible adverse effects of SCCPs at the environmentally-relevant concentrations on aquatic organisms.
Metabolomics is a powerful tool of understanding metabolic regulation by systematically detecting low-molecule metabolites present in biological samples (Chen et al., 2013). It can provide new sights for how mechanistic biochemistry relates to the phenotypic state of an organism, because metabolites act as direct signatures of biochemical activity (Patti et al., 2012). As a crucial “omics” science in system biology, metabolomics has been extensively used for drug safety evaluation, disease diagnosis and toxicity assessment (Xu et al., 2015). In this study, the metabolomics strategy was adopted to investigate the aquatic toxicity of SCCPs. The embryos and larvae of zebrafish (Danio rerio) were adopted as the animal model. Zebrafish is an attractive aquatic model for chemical toxicity assessment, and its embryos and larvae is more sensitive to environmental stress compared with adult. The developmental toxicity was first evaluated, and then a pseudotargeted metabolomics approach was adopted to explore the impact of SCCPs exposure on the metabolism in zebrafish embryos. The acquired results are looked forward to offering a better knowledge about the aquatic toxicity of SCCPs for fish, and to providing new evidence and clues concerning the toxicological mechanisms of SCCPs from a metabolomics perspective.
Section snippets
Chemicals and reagents
SCCP mixtures with different carbon chain lengths (mass ratio, C10-CPs:C11-CPs:C12-CPs:C13-CPs = 1:1:1:1) were respectively synthesized by chlorination of the n-alkanes according to the method described by Tomy et al. (2000). The chlorine content of test SCCP mixtures was determined to be 56.5%. The corresponding chromatogram and congener group abundance profile are shown in Fig. S1 (Supplementary data). Acetonitrile and Methanol (LC-MS grade) were obtained from Merck (Germany). Formic acid
Developmental toxicity
Hatching is a crucial point in the embryonic development of zebrafish life cycle and also an important toxicological endpoint to evaluate the influence of chemicals on fish (Cao et al., 2016). In this study, there were no observable effects on the hatching rate and malformation of zebrafish embryos/larvae in any of SCCPs exposure groups relative to the control group. At 72 hpf, only exposure to SCCPs at 10 μg/L caused a slight reduction in survival rate of zebrafish embryos relative to the
Conclusion
Exposure to SCCPs at environmentally relevant concentrations (1–5 μg/L) slightly disturbed the overall metabolism in zebrafish embryos, accelerated the β-oxidation of USFAs and VLCFAs, and inhibited the transformation of guanine to xanthine in the pathway of purine metabolism. When the exposure concentrations of SCCPs were in the range of 50–200 μg/L, the metabolism in zebrafish embryos was significantly disturbed. The most relevant pathways affected by SCCPs expose were glycerophospholipid
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
Financial support from the National Natural Science Foundation of China (Grant Nos: 21277141 and 21337002).
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