Comparative toxicology of mercurials in Caenorhabditis elegans

Abstract

Mercury (Hg) is a toxic metal that can exist in multiple chemical species. Humans are commonly exposed to methylmercury and Hg vapor, which are converted to mercuric species in the body. Despite years of research, little information exists on the similarities and differences in the mechanisms of Hg toxicity. The relative toxicity of mercuric chloride (HgCl(2)) and methylmercury chloride (MeHgCl) in Caenorhabditis elegans was determined in assays that measured growth, feeding, reproduction, and locomotion. The effect of HgCl(2) and MeHgCl on the expression of several archetypal stress-response genes was also determined. There was no significant difference between the EC50s of the two mercurials in terms of C. elegans growth. However, MeHgCl was more toxic to C. elegans than HgCl(2) when assessing feeding, movement, and reproduction, all of which require proper neuromuscular activity. Methylmercury chloride exposure resulted in increased steady-state levels of the stress response genes at lower concentrations than HgCl(2). In general, MeHgCl was more toxic to C. elegans than HgCl(2), particularly when assaying behaviors that require neuromuscular function.

Figure. 1

Effect of mercurials on C. elegans growth. Nematode size is expressed as the log of the absorbance (EXT) of individual nematodes at the end of 48 h exposure to HgCl₂ or MeHgCl minus the mean absorbance of all nematodes at the beginning of exposure. Lines in box plots indicate mean, boxes include 25^th–75^th percentile, whiskers include 10^th–90^th percentile. Data represent combined results of three independent experiments. Each box plot represents 500 to 900 nematodes.

Figure. 2

Effect of mercurials on C. elegans feeding. The level of feeding (LogRFP) was measured in individual nematodes following a 5 h exposure to HgCl₂ or MeHgCl followed by a 15 min incubation with fluorescent beads. LogRFP values are the measured fluorescence values following exposure minus mean background fluorescence of all nematodes at the beginning of mercurial exposure [28]. Lines in box plots indicate mean, boxes include 25^th to 75^th percentile, whiskers include 10^th to 90^th percentile. Data represent combined results of 3 independent experiments. Each box plot represents 200 to 400 nematodes.

Figure. 3

Effect of mercurials on C. elegans reproduction. Nematodes in final larval stage were exposed to HgCl₂ (●) or MeHgCl (∎) for 48 h. The number of offspring from treated nematodes was then measured [27]. Data presented are mean ± standard mean error of 3 to 8 independent experiments.

Figure. 4

Effect of mercurials on C. elegans locomotion. Movement of L4 larvae was tracked after a 4 h exposure to various concentrations of HgCl₂ or MeHgCl. Three motion parameters were then calculated: curvilinear distance (upper panel), curvilinear velocity (middle panel), and mean amplitude of the head (ALH mean) (lower panel). Data presented are mean ± standard mean error of three independent experiments. Significant differences relative to untreated are indicated with asterisks.

Figure. 5

Effect of mercurials on stress-response gene expression. Relative mRNA levels of four C. elegans stress-responsive genes, gcs-1, gst-38, hsp-16.2 and hsp-70, were measured from mixed-stage C. elegans populations following a 24 h exposure to HgCl₂ or MeHgCl. Data represent mean ± standard mean error of 3 to 4 independent experiments. Significant differences relative to untreated are indicated with asterisks. Values for the fold change in stress-response gene expression can be found in Supplemental Table 1