Characterization of the effects of methylmercury on Caenorhabditis elegans

Abstract

The rising prevalence of methylmercury (MeHg) in seafood and in the global environment provides an impetus for delineating the mechanism of the toxicity of MeHg. Deleterious effects of MeHg have been widely observed in humans and in other mammals, the most striking of which occur in the nervous system. Here we test the model organism, Caenorhabditis elegans (C. elegans), for MeHg toxicity. The simple, well-defined anatomy of the C. elegans nervous system and its ready visualization with green fluorescent protein (GFP) markers facilitated our study of the effects of methylmercuric chloride (MeHgCl) on neural development. Although MeHgCl was lethal to C. elegans, induced a developmental delay, and decreased pharyngeal pumping, other traits including lifespan, brood size, swimming rate, and nervous system morphology were not obviously perturbed in animals that survived MeHgCl exposure. Despite the limited effects of MeHgCl on C. elegans development and behavior, intracellular mercury (Hg) concentrations (<or=3 ng Hg/mg protein) in MeHgCl-treated nematodes approached levels that are highly toxic to mammals. If MeHgCl reaches these concentrations throughout the animal, this finding indicates that C. elegans cells, particularly neurons, may be less sensitive to MeHgCl toxicity than mammalian cells. We propose, therefore, that C. elegans should be a useful model for discovering intrinsic mechanisms that confer resistance to MeHgCl exposure.

Figure 1

Dose-response curve of lethality of MeHgCl to C. elegans. Worms treated at L1 (LC₅₀=1.08, n=10 p<0.001) were more sensitive to the toxicant than worms treated at the L4 stage (LC₅₀=4.51, n=6) (A). Toxicity increased as exposure duration increased, L4 worms were treated for 15 hours (LC₅₀=0.33, n=9), for 6 hours (LC₅₀=0.57, n=6), and for 30 minutes (LC₅₀=4.51, n=6) (B).

Figure 2

Hg content in C. elegans following MeHgCl exposure. Hg content was measured as a function of sample protein content (n=3). Hg content significantly increased as the duration of exposure to MeHgCl increased and as the MeHgCl treatment concentration increased.

Figure 3

Body length of C. elegans was shorter following treatment with MeHgCl. After growth for 24 or 48 hours, animals treated at either L1 or L4 stages with the toxicant were significantly (***, p<0.001, n=4) shorter than control animals, as measured using the Nikon Element software to measure their length in pixels (arbitrary units) according to their body contour from the posterior bulb of the pharynx to the anus.

Figure 4

C. elegans larvae were developmentally delayed following exposure to MeHgCl. Animals treated at higher concentrations of MeHgCl took longer to develop through the larval stages and into adults following a 30-minute exposure at L1 stage (A–C) or a 15-hour exposure at L4 stage.

Figure 5

Pharyngeal pumping rates of C. elegans decrease following MeHgCl exposure. Number of pharyngeal pumps per minute significantly decreased in a dose-dependent manner following 30 minute MeHgCl exposure of L1 worms 48 hours following treatment at 0.75 and1 mM MeHgCl (**, p<0.01, n=12) and 72 hours following treatment at 1 mM MeHgCl (*, p<0.05, n=12). Exposure of L4 worms for 15 hours induced a decrease in pharyngeal pumping rate 24 hours following exposure at 0.4, 0.6, and 0.75 mM MeHg (*, p<0.05, n=11). No alteration in pharyngeal pumping rate was noted in progeny of L4-treated animals (n=8).

Figure 6

Representative dopaminergic and GABAergic C. elegans neurons following MeHgCl insult. Dopaminergic (A–B) and GABAergic (C–D) cells and projections are identical under control (A and C) and MeHgCl-treatment (B and D) conditions (at all concentrations observed, 0.1, 0.4, and 1 mM MeHg) following treatment at L1 for 30 minutes or L4 for 15 hours (n=4). Progeny of worms treated at L4 for 15 hours were also unaffected (n=4).