Estrogenic compounds as exogenous modulators of physiological functions in molluscs: Signaling pathways and biological responses

Ecological significance of G protein-coupled receptors in the Pacific oyster (Crassostrea gigas): Pervasive gene duplication and distinct transcriptional response to marine environmental stresses

Marine ecosystems with ocean warming and industry pollution threaten the survival and adaptation of organisms. G protein-coupled receptors (GPCRs) play critical roles in various physiological and toxicological processes in vertebrates and invertebrates. The Pacific oyster (Crassostrea gigas) was widely used to study the adaptation of marine molluscs to coastal environments. In this work, we identified a total of 586 GPCRs in C. gigas genome. The C. gigas GPCRs were divided into five classes (including class A, B, C, E and F) with different degrees of expansion. Meta-analysis of multiple RNA-seq datasets revealed that transcriptional expression patterns of GPCRs in C. gigas were distinct in response to high temperature, salinity, air exposure, heavy metal, ostreid herpes virus 1 (OsHV-1) and Vibrio challenge. This work for the first time characterized the GPCR gene family and provided insights into the potential roles of GPCRs in adaptation of marine molluscs to stressful coastal environment.

The strategy for estrogen receptor mediated-risk assessment in environmental water: A combination of species sensitivity distributions and in silico approaches

Risk assessment for molecular toxicity endpoints of environmental matrices may be a pressing issue. Here, we combined chemical analysis with species sensitivity distributions (SSD) and in silico docking for multi-species estrogen receptor mediated-risk assessment in water from Dongjiang River, China. The water contains high levels of phenolic endocrine-disrupting chemicals (PEDCs) and phthalic acid esters (PAEs). The concentration of ∑₄PEDCs and ∑₆PAEs ranged from 2202 to 3404 ng/L and 834-4368 ng/L, with an average of 3241 and 2215 ng/L, respectively. The SSD approach showed that 4-NP, BPA, E2 of PEDCs, and DBP, DOP, and DEHP could severely threaten the aquatic ecosystems, while most other target compounds posed low-to-medium risks. Moreover, binding affinities from molecular docking among PEDCs, PAEs, and estrogen receptors (ERα, Erβ, and GPER) were applied as toxic equivalency factors. Estrogen receptor-mediated risk suggested that PEDCs were the main contributors, containing 53.37-69.79% of total risk. They potentially pose more severe estrogen-receptor toxicity to zebrafish, turtles, and frogs. ERβ was the major contributor, followed by ERα and GPER. This study is the first attempt to assess the estrogen receptor-mediated risk of river water in multiple aquatic organisms. The in silico simulation approach could complement toxic effect evaluations in molecular endpoints.

Studies on a widely-recognized snail model species (Lymnaea stagnalis) provide further evidence that vertebrate steroids do not have a hormonal role in the reproduction of mollusks

Experiments were carried out to determine whether, as with other mollusks that have been studied, the snail, Lymnaea stagnalis, can absorb, esterify and store vertebrate steroids that are present in the water. We also carried out experiments to determine whether neural tissues of the snail could be immunohistochemically stained with an antibody to human aromatase (a key enzyme that catalyzes the conversion of testosterone [T] to 17β-estradiol [E₂]); and, if so, to determine the significance of such staining. Previous studies on other mollusks have reported such staining and have proposed this as decisive evidence that mollusks have the same steroid synthesis pathway as vertebrates. We found that snails absorb, esterify and retain esterified T, E₂, progesterone and ethinyl-estradiol (albeit with an absorption rate about four times slower, on a weight basis, than the mussel, Mytilus edulis). We also found that not only anti-human aromatase, but also anti-human nuclear progesterone receptor (nPR) and anti-human gonadotropin-releasing hormone antibodies immunohistochemically stained snail neural cells. However, further experiments, involving gel electrophoretic separation, followed by immunostaining, of proteins extracted from the neural tissue, found at least two positively-stained bands for each antibody, none of which had masses matching the human proteins to which the antibodies had been raised. The anti-aromatase antibody even stained the 140 kDA ladder protein used as a molecular weight marker on the gels. Mass spectrometric analysis of the bands did not find any peptide sequences that corresponded to the human proteins. Our findings confirm that the presence of vertebrate-like sex steroids in molluscan tissues is not necessarily evidence of endogenous origin. The results also show that immunohistochemical studies using antibodies against human proteins are grossly non-specific and likely to have little or no value in studying steroid synthesis or activity in mollusks. Our conclusions are consistent with the fact that genes for aromatase and nPR have not been found in the genome of the snail or of any other mollusk. Our overarching conclusion, from this and our previous studies, is that the endocrinology of mollusks is not the same as that of humans or any other vertebrates and that continuing to carry out physiological and ecotoxicological studies on mollusks on the basis of this false assumption, is an unconscionable waste of resources.

Identification of essential β-oxidation genes and corresponding metabolites for oestrogen degradation by actinobacteria

Steroidal oestrogens (C₁₈) are contaminants receiving increasing attention due to their endocrine‐disrupting activities at sub‐nanomolar concentrations. Although oestrogens can be eliminated through photodegradation, microbial function is critical for removing oestrogens from ecosystems devoid of sunlight exposure including activated sludge, soils and aquatic sediments. Actinobacteria were found to be key oestrogen degraders in manure‐contaminated soils and estuarine sediments. Previously, we used the actinobacterium Rhodococcus sp. strain B50 as a model microorganism to identify two oxygenase genes, aedA and aedB, involved in the activation and subsequent cleavage of the estrogenic A‐ring respectively. However, genes responsible for the downstream degradation of oestrogen A/B‐rings remained completely unknown. In this study, we employed tiered comparative transcriptomics, gene disruption experiments and mass spectrometry‐based metabolite profile analysis to identify oestrogen catabolic genes. We observed the up‐regulation of thiolase‐encoding aedF and aedK in the transcriptome of strain B50 grown with oestrone. Consistently, two downstream oestrogenic metabolites, 5‐oxo‐4‐norestrogenic acid (C₁₇) and 2,3,4‐trinorestrogenic acid (C₁₅), were accumulated in aedF‐ and aedK‐disrupted strain B50 cultures. Disruption of fadD3 [3aα‐H‐4α(3'‐propanoate)‐7aβ‐methylhexahydro‐1,5‐indanedione (HIP)‐coenzyme A‐ligase gene] in strain B50 resulted in apparent HIP accumulation in oestrone‐fed cultures, indicating the essential role of fadD3 in actinobacterial oestrogen degradation. In addition, we detected a unique meta‐cleavage product, 4,5‐seco‐estrogenic acid (C₁₈), during actinobacterial oestrogen degradation. Differentiating the oestrogenic metabolite profile and degradation genes of actinobacteria and proteobacteria enables the cost‐effective and time‐saving identification of potential oestrogen degraders in various ecosystems through liquid chromatography–mass spectrometry analysis and polymerase chain reaction‐based functional assays.

Nuclear Receptors and Development of Marine Invertebrates

Nuclear Receptors (NRs) are a superfamily of transcription factors specific to metazoans that have the unique ability to directly translate the message of a signaling molecule into a transcriptional response. In vertebrates, NRs are pivotal players in countless processes of both embryonic and adult physiology, with embryonic development being one of the most dynamic periods of NR activity. Accumulating evidence suggests that NR signaling is also a major regulator of development in marine invertebrates, although ligands and transactivation dynamics are not necessarily conserved with respect to vertebrates. The explosion of genome sequencing projects and the interpretation of the resulting data in a phylogenetic context allowed significant progress toward an understanding of NR superfamily evolution, both in terms of molecular activities and developmental functions. In this context, marine invertebrates have been crucial for characterizing the ancestral states of NR-ligand interactions, further strengthening the importance of these organisms in the field of evolutionary developmental biology.

A critical evaluation of some of the recent so-called 'evidence' for the involvement of vertebrate-type sex steroids in the reproduction of mollusks

Many studies on the control of reproduction in mollusks have focused on hormones (and proteins associated with the production and signaling of those hormones) which were originally discovered in humans, in the belief that if they are also present in mollusks, they must have the same role. However, although human sex steroids can be found in mollusks, they are so readily absorbed that their presence is not necessarily evidence of endogenous synthesis. A homolog of the vertebrate nuclear estrogen receptor has been found in mollusks, but it does not bind to estrogens or indeed to any steroid at all. Antibodies against human aromatase show positive immunostaining in mollusks, yet the aromatase gene has not been found in the genome of any invertebrates (let alone mollusks). This review will deal with these and other examples of contradictory evidence for a role of human hormones in invertebrate reproduction.