Tetrabromobisphenol A acts a neurodevelopmental disruptor in early larval stages of Mytilus galloprovincialis

Abstract

Tetrabromobisphenol A-TBBPA, a widely used brominated flame retardant detected in aquatic environments, is considered a potential endocrine disruptor-ED for its reproductive/developmental effects in vertebrates. In aquatic invertebrates, the modes of action of most EDs are largely unknown, due to partial knowledge of the mechanisms controlling neuroendocrine functions.In the marine bivalve Mytilus galloprovincialis, TBBPA has been previously shown to affect larval development in the 48 hours larval toxicity assay at environmental concentrations. In this work, the effects of TBBPA were further investigated at different times post-fertilization. TBBPA, from 1 µg/L, affected shell biogenesis at 48 hours post fertilization-hpf, as shown by phenotypic and SEM analysis.The mechanisms of action of TBBPA were investigated at concentrations of the same order of magnitude as those found in highly polluted coastal areas (10 µg/L). At 28-32 hpf, TBBPA significantly affected deposition of both the organic matrix and CaCO3 in the shell. TBBPA also altered expression of shell-related genes from 24 to 48 hpf, in particular of tyrosinase, a key enzyme in shell matrix remodeling. At earlier stages (24 hpf), TBBPA affected the development of dopaminergic, serotoninergic and GABAergic systems, as shown by in situ hybridization-ISH and immunocytochemistry. These data contribute draw adverse outcome pathways-AOPs, where TBBPA affects the synthesis of neutrotransmitters involved in key events (neurodevelopment and shell biogenesis), resulting in phenotypic changes on individuals (delayed or arrested development) that might lead to detrimental consequences on populations.

Physiological Roles of Serotonin in Bivalves: Possible Interference by Environmental Chemicals Resulting in Neuroendocrine Disruption

“…Overall, the results demonstrate that in mussel early larvae exposure to known EDCs interfere with neurogenesis of the serotoninergic system. The effects were associated with altered shells and expression of key genes involved in shell biogenesis, tyrosinase and chitinase in particular, resulting in altered larval phenotypes (107)(108)(109). These observations also support the hypothesis that first shell formation, a key step in early bivalve development, is modulated by monoamine neurotransmitters 5- HT and dopamine (DA) through TGF-b Smad mediated pathways that trigger the expression of tyrosinase to form initial shell (110).…”

“…Irrespective of the concentration, the most evident effects of BPA were detected at 48 hpf, when both the number of 5-HT-ir cells and the amount of 5-HTR transcripts were reduced. In contrast, TBBPA did not affect mRNA levels for 5-HTR at any time pf (109). However, TBBPA, although at higher concentrations (10 mg/L) induced a decrease in the number of 5-HT-ir neurons at all times pf, with a significant decrease with respect to controls at 48 hpf (Figure 2B and 109) (Figure 2B).…”

“…g . catecholaminergic, cholinergic, enkephalinergic, dopaminergic, gamma-aminobutyric acid-ergic, and neuropeptide systems (13,111), whose pathways can be strictly interconnected, as suggested by data obtained in cuttlefish embryos with fluoxetine (96) and in mussel larvae with TBBPA (109). In addition, it must be considered that adrenergic signaling seems absent in invertebrates, and analogous functions are performed by the biogenic amines octopamine and its precursor tyramine, that act via related families of G-protein-coupled receptors and are regarded as the invertebrate equivalents of vertebrate norepinephrine (reviewed in 112).…”

“…Therefore, development of serotonergic components were investigated in more detail between 24 and 48 hpf, when the key processes leading to the formation of the first larval shell occur (107), in larvae exposed to BPA and to its brominated derivative, the flame retardant tetrabromobisphenol A (TBBPA) (108,109). Basal expression of 5-HTRs indicated a strong upregulation in early larvae (24 hpf) with respect to eggs, followed by a decrease at 28 and 32 hpf and a further increase at 48 hpf (Figure S1).…”

“…However, TBBPA, although at higher concentrations (10 mg/L) induced a decrease in the number of 5-HT-ir neurons at all times pf, with a significant decrease with respect to controls at 48 hpf (Figure 2B and 109) (Figure 2B). Moreover, TBBPA altered the expression pattern of components of dopamine metabolism and signalling (tyrosine hydroxylase, dopamine-b-hydroxylase and dopamine receptor 1), and affected development of GABAergic neurons (109).…”