Assessing the role of the gut microbiome in methylmercury demethylation and elimination in humans and gnotobiotic mice.

The risk of methylmercury (MeHg) toxicity following ingestion of contaminated foodstuffs (e.g., fish) is directly related to the kinetics of MeHg elimination among individuals.

Organomercurial Lyase (MerB)-Mediated Demethylation Decreases Bacterial Methylmercury Resistance in the Absence of Mercuric Reductase (MerA).

The operon encodes enzymes that transform and detoxify methylmercury (MeHg) and/or inorganic mercury [Hg(II)]. Organomercurial lyase (MerB) and mercuric reductase (MerA) can act sequentially to demethylate MeHg to Hg(II) and reduce Hg(II) to volatile elemental mercury (Hg) that can escape from the cell, conferring resistance to MeHg and Hg(II). Most identified operons encode either MerA and MerB in tandem or MerA alone; however, microbial genomes were recently identified that encode only MerB.

Latent effects of early-life methylmercury exposure on motor function in Drosophila.

The developmental toxicant, methylmercury (MeHg), can elicit motor deficits that last well into adulthood. Recent studies using Drosophila showed that the developing musculature is sensitive to high doses of MeHg, where a larval feeding paradigm resulted in compromised myotendinous junction (MTJ) formation during development, by a mechanism involving the NG2 homologue, kon-tiki (kon). Low-dose exposures to MeHg that do not produce muscle pathology during development, nevertheless result in impaired flight behavior later in adult life.

Neuroligin-1 Is a Mediator of Methylmercury Neuromuscular Toxicity.

Methylmercury (MeHg) is a developmental toxicant capable of eliciting neurocognitive and neuromuscular deficits in children with in utero exposure. Previous research in Drosophila melanogaster uncovered that developmental MeHg exposure simultaneously targets the developing musculature and innervating motor neuron in the embryo, along with identifying Drosophila neuroligin 1 (nlg1) as a gene associated with developmental MeHg sensitivity. Nlg1 and its transsynaptic partner neurexin 1 (Nrx1) are critical for axonal arborization and NMJ maturation.

Tissue-specific Nrf2 signaling protects against methylmercury toxicity in Drosophila neuromuscular development.

Methylmercury (MeHg) can elicit cognitive and motor deficits due to its developmental neuro- and myotoxic properties. While previous work has demonstrated that Nrf2 antioxidant signaling protects from MeHg toxicity, in vivo tissue-specific studies are lacking. In Drosophila, MeHg exposure shows greatest developmental toxicity in the pupal stage resulting in failed eclosion (emergence of adults) and an accompanying 'myosphere' phenotype in indirect flight muscles (IFMs).

Methylmercury myotoxicity targets formation of the myotendinous junction.

Methylmercury (MeHg) is a ubiquitous environmental contaminant and developmental toxicant known to cause a variety of persistent motor and cognitive deficits. While previous research has focused predominantly on neurotoxic MeHg effects, emerging evidence points to a myotoxic role whereby MeHg induces defects in muscle development and maintenance. A genome wide association study for developmental sensitivity to MeHg in Drosophila has revealed several conserved muscle morphogenesis candidate genes that function in an array of processes from myoblast migration and fusion to myotendinous junction (MTJ) formation and myofibrillogenesis.

Drosophotoxicology: Elucidating Kinetic and Dynamic Pathways of Methylmercury Toxicity in a Drosophila Model.

The risks of methylmercury (MeHg) toxicity are greatest during early life where it has long been appreciated that the developing nervous system is an especially sensitive target. Yet, understanding the discrete mechanisms of MeHg toxicity have been obscured by the wide variation in the nature and severity of developmental outcomes that are typically seen across individuals in MeHg exposed populations. Some insight has come from studies aimed at identifying a role for genetic background as a modifier of MeHg toxicity, which have predominantly focused on factors influencing MeHg toxicokinetics, notably, polymorphisms in genes related to glutathione (GSH) metabolism.

Editor's Highlight: Glutathione S-Transferase Activity Moderates Methylmercury Toxicity During Development in Drosophila.

Glutathione (GSH) pathways play a central role in methylmercury (MeHg) metabolism and elimination, largely due to formation of a more readily transported MeHg-GSH conjugate. Glutathione S-transferases (GSTs) have therefore been proposed to facilitate MeHg elimination by catalyzing MeHg-GSH conjugation. A role for human GSTP1 in MeHg disposition is suggested by the association of two common polymorphisms in the coding region (Ile105Val and Ala114Val) with Hg levels in either blood or hair.

Methods for Individualized Determination of Methylmercury Elimination Rate and De-Methylation Status in Humans Following Fish Consumption.

Methylmercury (MeHg) exposure via fish in the diet remains a priority public health concern. Individual variation in response to a given MeHg exposure and the biotransformation of MeHg that follows complicate our understanding of this issue. MeHg elimination from the human body occurs slowly (elimination rate (kel) approximately 0.

Genome-wide association analysis of tolerance to methylmercury toxicity in Drosophila implicates myogenic and neuromuscular developmental pathways.

Methylmercury (MeHg) is a persistent environmental toxin present in seafood that can compromise the developing nervous system in humans. The effects of MeHg toxicity varies among individuals, despite similar levels of exposure, indicating that genetic differences contribute to MeHg susceptibility. To examine how genetic variation impacts MeHg tolerance, we assessed developmental tolerance to MeHg using the sequenced, inbred lines of the Drosophila melanogaster Genetic Reference Panel (DGRP).

Developmental toxicity assays using the Drosophila model.

The fruit fly (Drosophila melanogaster) has long been a premier model for developmental biologists and geneticists. In toxicology studies, Drosophila has only recently gained broader recognition as a tool to elaborate molecular genetic mechanisms of toxic substances. In this article, two practical applications of Drosophila for developmental toxicity assays are described.

Mutational analysis of the mouse aryl hydrocarbon receptor tyrosine residues necessary for recognition of dioxin response elements.

Tyrosine phosphorylation of the aryl hydrocarbon receptor (AhR), a member of the basic helix-loop-helix/PER-ARNT-SIM transcription factor family, has been shown to regulate its dioxin response elements (DRE) binding ability, although no specific residues have been directly demonstrated to be phosphorylated. Of the 23 tyrosines in the mouse AhR, 19 are conserved across all mammalian species sequenced thus far. The studies presented here were conducted to examine tyrosine residue(s) that are both likely candidates of phosphorylation and necessary for DNA binding and/or transcriptional activity of the AhR.

Function of estrogen receptor tyrosine 537 in hormone binding, DNA binding, and transactivation.

The human estrogen receptor (hER) is a ligand-activated transcription factor which, like many other members of the nuclear receptor protein family, exhibits a dimerization-dependent transcriptional activation. Several previous reports have provided evidence of the phosphorylation of the hER at tyrosine 537 (Y537). However, the exact function of a putative phosphorylation at this site remains controversial.

The role of phosphorylation in human estrogen receptor function.

We have studied the role of phosphorylation of the human estrogen receptor (hER) at serine 118, which has been previously identified as a site important for transactivation. We have tested this transactivation in yeast and cell-free transcription assays, and have shown that mutation of serine 118 to alanine results in a 30-40% decrease in hER-dependent transcription. Furthermore, we investigated the functional significance of phosphorylation at this site by hormone binding and DNA binding.

Phosphorylation of serine-167 on the human oestrogen receptor is important for oestrogen response element binding and transcriptional activation.

We have studied the role of phosphorylation of the human oestrogen receptor (hOR; otherwise known as hER) at serine-167, which has been identified previously as the major oestrogen-induced phosphorylation site. We have tested transactivation by the hOR in yeast and cell-free transcription assays, and shown that mutation of serine-167 results in a 70% decrease in hOR-dependent transcription. Furthermore we explored the functional significance of phosphorylation at this site by hormone binding and DNA binding.

Transcriptional activation of the human estrogen receptor by DDT isomers and metabolites in yeast and MCF-7 cells.

In this study, we determined whether the DDT isomers p,p'-DDT [1,1,1,-trichloro-2,2-bis(p-chlorophenyl)ethane], o,p'-DDT [1,1,1-trichloro-2(p-chlorophenyl)-2-(o-chlorophenyl)ethane], and their metabolites p,p'-DDD [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane], o,p'-DDD [1,1-dichloro-2-(p-chlorophenyl)-2-(o-chlorophenyl)ethane], p,p'-DDE [1,1,-dichloro-2,2-bis(p-chlorophenyl)ethylene], o,p'-DDE [1,1-dichloro-2-(p-chlorophenyl)-2-(o-chlorophenyl)ethylene], and p,p'-DDA [2,2-bis(p-chlorophenyl)acetic acid], could bind to and transcriptionally activate the human estrogen receptor (hER). Novel results from competitive binding assays showed that o,p'-DDD, o,p'-DDE, and p,p'-DDT, as well as the established environmental estrogen o,p'-DDT, were able to bind specifically to the hER with approximately 1000-fold weaker affinities for the hER than that of estradiol. In contrast, only o,p'-DDT, but not p,p'-DDT, bound to the rat estrogen receptor.

Estradiol-binding mechanism and binding capacity of the human estrogen receptor is regulated by tyrosine phosphorylation.

We have investigated the effects of tyrosine phosphorylation on the estradiol-binding mechanism and binding capacity of the human estrogen receptor (hER). The wild type hER and a point mutant form of the hER, in which tyrosine 537 was mutated to phenylalanine (Y537F hER), were expressed in Sf9 insect cells. The wild type hER, but not the Y537F hER, reacted with a anti-phosphotyrosine monoclonal antibody, indicating that tyrosine 537 was the only tyrosine phosphorylated on the hER.

Phosphorylation of tyrosine 537 on the human estrogen receptor is required for binding to an estrogen response element.

We report here that the phosphorylation of tyrosine 537 on the human estrogen receptor (hER) controls the receptor's dimerization and DNA binding ability. The DNA-binding form of both the hER from human MCF-7 mammary carcinoma cells and the hER overexpressed in Sf9 insect cells was isolated using estrogen response element (ERE) affinity chromatography. Western blot analyses demonstrated that the DNA-binding form of the hER from MCF-7 or Sf9 cells was (i) phosphorylated at tyrosine 537, (ii) localized in the nucleus of estradiol-treated MCF-7 cells with an apparent molecular mass of 67 kDa, and (iii) hyperphosphorylated at serine residue(s).