The effects of methylmercury on Notch signaling during embryonic neural development in Drosophila melanogaster

Abstract

Methylmercury (MeHg) is a ubiquitous toxicant that targets the developing fetal nervous system. MeHg interacts with the Notch signaling pathway, a highly-conserved intercellular signaling mechanism required for normal development. Notch signaling is conveyed by activation of the genes in the Enhancer of Split (E(spl)) locus in Drosophila. We have previously shown that acute high doses of MeHg upregulate several E(spl) genes in Drosophila neural-derived C6 cells. Furthermore, MeHg induction of E(spl) can occur independent of the Notch receptor itself. We now show that MeHg, unlike inorganic mercury (HgCl2), preferentially upregulates E(spl)mδ and E(spl)mγ in Drosophila C6 cells. This is distinct from Delta ligand-induced Notch signaling in which no induction of E(spl)mδ is seen. MeHg is also seen to specifically upregulate E(spl)mδ in Drosophila embryos where HgCl2 showed no such effect. Additionally, treatment of embryos with MeHg caused a consistent failure in axonal outgrowth of the intersegmental nerve (ISN). This ISN phenotype was partially replicated by genetic activation of the Notch pathway, but was not replicated by increasing expression of E(spl)mδ. These data suggest a role for Notch signaling and the E(spl)mδ target gene in MeHg toxicity, however, the site of action for E(spl)mδ in this system remains to be elucidated.

Differential protein expression of hepatic cells associated with MeHg exposure: deepening into the molecular mechanisms of toxicity

“…Our study suggests that the hepatic cells are responding to MeHg damage not only by transiently increasing the GSH levels as observed in our previous work [8], but also increasing the expression of other anti-apoptotic proteins.…”

“…The generation of reactive oxygen species (ROS) by MeHg as well as the active involvement of the mitochondria in the process has been shown in previous studies [6,7]. Specific proteins and mechanisms related with methylmercury-induced cell death [7] and neurotoxicity [8,9] have also been reported using different biochemical approaches. However, little has been done to reveal the toxic effects on other organs, such as liver, that can also be considered as an important target for MeHg-induced injury [4,10].…”

A Method of Permeabilization of <em>Drosophila</em> Embryos for Assays of Small Molecule Activity

“…The effects of MeHg are seen in the irregular patterning and clustering of the lateral chordotonal neuron cell bodies (elav-positive, labeled in red and denoted with white arrows in B versus A). In addition, a characteristic branching of the segmental (SN) (solid green arrows in A') is seen to be highly variable with MeHg exposure (solid green arrows in B") consistent with previously reported effects of MeHg on the embryo 15 . Projection of the intersegmental and segmental nerves at their roots are seen to be displaced posteriorly with MeHg exposure (open green arrow in B').…”

The Notch target E(spl)mδ is a muscle-specific gene involved in methylmercury toxicity in motor neuron development

“…MeHg causes transcriptional upregulation of the Enhancer of Split [E(spl)] complex gene m-Delta ( E(spl)mδ ) in the Drosophila embryo model (Bland and Rand 2006; Engel et al 2012; Rand et al 2008). The E(spl) locus consists of a group of 13 canonical Notch response genes; E(spl)mδ is one of the seven basic helix-loop-helix (bHLH) transcriptional repressors in the locus that are well documented effectors of Notch signaling (reviewed in (Portin 2002)).…”

“…Analysis of motor nerves in MeHg-treated embryos previously demonstrated a failure of the ISN to project dorsally (Engel et al 2012). Yet, ectopic E(spl)mδ expression in neurons does not alter ISN axonal outgrowth (Engel et al 2012). We now show that E(spl)mδ is endogenously expressed in developing somatic muscle cells.…”