Genetic factors in methylmercury-induced neurotoxicity: What have we learned from Caenorhabditis elegans models?

1. Alternative and complementary models for mercury toxicity testing

Mercury pollution in its organic form, methylmercury (MeHg) food sources may cause human diseases represented by a cluster of neurological syndromes (Clarkson and Strain, 2020). Investigations in human and other animals such as cat revealed that there is a cross-species sensitivity to MeHg-induced neurotoxicity (Bakir et al., 1973; Charbonneau et al., 1976). This was followed by the establishment of rodent and primate models to characterize the neurotoxic effects of MeHg with various doses and exposure paradigms (Farina et al., 2011; Weiss et al., 2002). The high toxic potential of MeHg compared to its pure metal form—mercury, and the widespread presence of MeHg in aquatic environment sparked a wide interest in alternative and complementary models that can be used for fast and convenient toxicity testing and screening for aquatic and soil contaminants (Best et al., 1981; Donkin and Dusenbery, 1993; McElwee and Freedman, 2011; Williams and Dusenbery, 1990). Girardia dorotocephala is a freshwater planarian that can be conveniently cultured in laboratory settings. The animal can regenerate its head region after decapitation, which is promoted by catecholaminergic neurons (Cebrià, 2007). MeHg exposures dose-dependently inhibits the regeneration process after decapitation (Best et al., 1981), establishing that the integrity of the neuronal regeneration process was targeted by the exposure. The finding coincided with the ex vivo and in vitro models which showed that the catecholaminergic neuronal system is a critical target of MeHg exposures (Farina et al., 2017).

The introduction of C. elegans (C. elegans) to the field of genetics had enabled the use of an extremely useful tool in dissecting the genetic basis of biological phenotypes (Ankeny, 2001). Due to its many advantages, C. elegans was also used as an alternative and complementary model for toxicity testing of metals in aquatic and soil environment, even before the time of complete decoding of its entire genomic sequences and neuronal wirings (Donkin and Dusenbery, 1993; Williams and Dusenbery, 1990). Early studies used C. elegans to test many metals, including mercury in the soluble salt forms and compared the toxicity in the animal to mammalian and aquatic models (Williams and Dusenbery, 1988). To investigate mercury-induced ultrastructural changes of cells under electronic microscope, Popham et al. used the wildtype Bristol N2 strain worms that were fed with the B strain of Escherichia coli on the agar pad that was topped with mercuric chloride solution overnight (Popham and Webster, 1982). The authors showed that the microvilli of the intestinal cells were shortened and the presence of amorphous masses of electron-dense material in the cytoplasm and whorls of electron-dense material, which recapitulated the characteristics of cellular morphological demonstrations after mercury exposure in other models, such as human skin specimen and rats (Popham and Webster, 1982). C. elegans has a very small number of somatic (810) and neuronal cells (302) compared to a mammalian model, yet the animal’s response to toxic metals can be similar to mammalian models, which could be ascribed to the genes of these animals shared a common ancestor and high homology of cellular mechanisms in the handling of metal overexposure between species (Ruszkiewicz et al., 2018b). The unique strength of using C. elegans models in the studies on MeHg toxicity can be ascribed to the rich resources on genetic mutations that can be used to investigate inherent molecular mechanisms underlying the toxicity. Increasing evidence showed that C. elegans models is indispensable for these studies (Caito and Aschner, 2016; Caito et al., 2013; Chen et al., 2019; Helmcke et al., 2009; Ke et al., 2020b; McElwee and Freedman, 2011; Ruszkiewicz et al., 2018a, 2019; Vanduyn and Nass, 2014a; Vanduyn et al., 2010).

2. C. elegans model of MeHg toxicity

The popularity of C. elegans models in biological studies and the convenience for genetic manipulations to the animal propelled the use of the animal to mechanistically investigate the molecular pathways in MeHg-induced toxic endpoints. The number of research papers in PubMed on MeHg-induced toxicity in C. elegans models has drawn increased attention in recent years. Several C. elegans models of chronic or acute MeHg exposures were established to explore the genes and molecular signatures that were evoked by the exposure (Caito and Aschner, 2016; Caito et al., 2013; Helmcke and Aschner, 2010; Helmcke et al., 2009; Hu et al., 2021; Ke et al., 2020a,b,c, 2021a,b; Ruszkiewicz et al., 2019; Vanduyn and Nass, 2014a). C. elegans has four developing larval stages (L1–L4) that are marked by the molting cycles between younger and older larvae. The older L4 stage worms are more resistant to MeHg toxicity than the younger L1 stage worms (1h MeHg exposure, LD50 of L4: 137.9–149.9μM; LD50 of L1: 36.8–44.8μM) (Helmcke et al., 2009), highlighting the developmental stage is the most significant target of MeHg toxicity that observed in other models (Farina et al., 2011). The C. elegans models of MeHg toxicity empowers molecular studies of genetic pathways that may be involved in real-life exposures. For example, an earlier study showed that in the chronic exposure model (24h MeHg exposure), the LD50 of L1 stage worms is 22.9μM (Ke and Aschner, 2019). A recent study employed a sublethal dose of 5μM MeHg for 48h, which caused worm body mercury of 31.02ng Hg/mg protein (Ke et al., 2021a). This level of worm body mercury is near to the reported brain mercury (68.5ng Hg/mg protein) in human after ingesting MeHg contaminated bread in Iraq (Aschner, 2012; Choi et al., 1978). Table 1 provides a comparison of exposure paradigm and internal mercury levels in biomarkers in different species following MeHg exposures.

Table 1.

Mercury levels in different species following acute or chronic MeHg exposure.

	Exposure dose	Exposure route	Exposure time	Sampling time for biomarker	Mercury level in biomarker
C. elegans	5 μM	In liquid culture	48 h from L1 stage	24 h post exposure	31.02 ngHg/mg protein (Ke et al., 2021a)
	40 μM	In M9 buffer	1 h at L4 stage	24 h post exposure	70–80 ng Hg/mg protein (Ruszkiewicz et al., 2019)
	1 μM	On agar plate	48 h from L1 stage	After exposure	15–20 ng/mg wet worms (Vanduyn and Nass, 2014)
Rat	500 μg/kg	Gavage	Acute exposure	5 days after exposure	150–200 ppb in blood (Rodrigues et al., 2010)
Mouse	250 μg/kg	In diet	57 days	after exposure	210 ppb in blood (Bourdineaud et al., 2012)
Primate	50 μg/kg	In diet	4 months	After exposure	1100 ppb in blood (Vahter et al., 1994)
Human	60 μg/kga	In bread	55 days	~65 days after exposure	2000–3000 ppb in blood (Bakir et al., 1973)

^aThe value was an estimation from a total amount of 200 mg ingested mercury in 55 days for a person with a body weight of 60 kg.

The C. elegans models of MeHg toxicity are very useful in characterizing the molecular pathways of toxic endpoints consequent to the exposure (Arantes et al., 2016; Helmcke and Aschner, 2010; Helmcke et al., 2009; Hu et al., 2021; Ke et al., 2020a,b,c, 2021a, 2022; Ruszkiewicz et al., 2019; VanDuyn and Nass, 2014b; Vanduyn et al., 2010), particularly given the existing powerful genetic manipulation resources. Meanwhile, the epigenetic mechanisms that sustain the population and individual variations in MeHg toxicity must be fully investigated, which likely contribute to variations in the reported acute toxicity of MeHg in the isogenic worms synchronized at the young larvae stages (Helmcke et al., 2009; Martinez-Finley et al., 2013; Ruszkiewicz et al., 2018a, 2019). As the early development stage of worms as well as mammals is the most vulnerable period to the toxic effects of various environmental exposures (Grandjean et al., 2015; Helmcke et al., 2009; Ke et al., 2021a; Ruszkiewicz et al., 2018a), the current protocol used to isolate C. elegans eggs with the bleach treatment could have a long-lasting effect on worms’ sensitivity to MeHg (Porta-de-la-Riva et al., 2012). Further, the parental stress level should also be considered, as studies have shown that maternal stress can be inherited to next generations with epigenetic reprogramming (Braun et al., 2022; Das et al., 2020). These variations shall be carefully controlled for the aim to compare worms’ sensitivity to MeHg, especially in the experiments with developmental defect worms with various mutations that are important for dissecting molecular pathways underlying MeHg toxicity.

2.1. Mitochondria toxicity and antioxidant response

Oxidative stress is a hallmark of metal-induced neurotoxicity, particularly for MeHg that is highly electrophilic (Antunes Dos Santos et al., 2018). MeHg exposure compromises the molecular integrity of mitochondria respiratory apparatus, through a combined effect on the respiratory molecular complexes and calcium homeostasis that are essential to mitochondrial functions (Usuki et al., 2011). The toxic consequences, such as damaged mitochondria and cell death are closely related to the elevated reactive oxygen species (ROS) through direct (depletion of glutathione (GSH) and inhibition of enzymes that maintaining cellular redox balance) or indirect effects of MeHg exposure, namely glutamate-induced excitotoxicity (Farina and Aschner, 2019). The glutathione S-transferase family plays an important role in the detoxification of toxic substances through conjugation of reduced GSH with toxicants (Pizzorno, 2014). MeHg exposure induces overexpression of glutathione S-transferase (GST) proteins and increases the conjugation MeHg to GSH to from MeHg-SG complex and promotes the exportation of the complex through multi-drug resistance transporters (Granitzer et al., 2020). These studies in the worm have translational significance, since human studies have shown that body mercury levels are associated with the polymorphism of the homologues of GST gene (Chan et al., 2020; Schläwicke Engström et al., 2008).

The C. elegans GST-4 is a homolog of human prostaglandin-D synthase (Pohl et al., 2019), which belongs to the sigma class of GST family member (Jowsey et al., 2001). The in vivo expression dynamics of gst genes can be visualized by creating a transcriptional reporter C. elegans strain that carries a construct bearing GFP under a gst-4 promoter (Helmcke and Aschner, 2010). With the reporter stain, the increased expression levels of gst-4 in C. elegans following various MeHg exposure scenarios were characterized (Helmcke and Aschner, 2010; Ke et al., 2020a), further supporting a critical role of the gene in the detoxification of the metal. Furthermore, the importance of skn-1 (the C. elegans homolog of nuclear factor erythroid 2–related factor 2 (Nrf2)) in MeHg toxicity was demonstrated in the studies with RNA interference (RNAi) or skn-1 knockout animals (Martinez-Finley et al., 2013; Vanduyn et al., 2010).

The importance of anti-oxidative molecules rich in sulfhydryl group in MeHg toxicity is also supported by studies on the metal-binding protein-metallothionein (Arantes et al., 2016; Helmcke and Aschner, 2010; McElwee et al., 2013), which is a cysteine-rich metal-binding protein that plays important roles in zinc and copper homeostasis as well as protection against metal toxicity (Aschner et al., 2006; Koh and Lee, 2020). C. elegans has two genes (mtl-1 and mtl-2) that encode isoforms of metallothionein (Helmcke and Aschner, 2010). Studies with the transcriptional reporter strains showed that the expression of mtl-1 was induced following acute MeHg exposure; however, neither mtl-1 nor mtl-2 was changed in worms after chronic MeHg exposure (Helmcke and Aschner, 2010). In worms with either single knockout of mtl-1 or mtl-2 or double knockout of mtl-1/mtl-2, the animals were more sensitive to MeHg-induced toxicity (Helmcke and Aschner, 2010), corroborating that the metallothionein family affords protection against metal toxicity (Leiva-Presa et al., 2004; Lima et al., 2018; Yasutake and Nakamura, 2011).

2.2. Sex-specific response

Experimental studies in the rodent model showed that MeHg-induced neurotoxicity exhibits sex-specific patterns (Ruszkiewicz et al., 2016; Weston et al., 2014). The sex-specific responses to MeHg toxicity likely involves the sex hormones that can modulate the animals’ antioxidant capacity (Sumien et al., 2021). C. elegans can be used as an alternative and complementary model to investigate sex-specific toxicity of MeHg (Ruszkiewicz et al., 2019). The natural C. elegans isolates have two sexes: hermaphrodite and male. The occurrence of male is rather low (1 in 500) in a population under normal conditions (Hodgkin et al., 1979), but stress such as heat and starvation can increase the number of male offspring (Frézal and Félix, 2015). The hermaphrodites have two X chromosomes (XX), while the males have only one X chromosome (XO). The males are generated via X-chromosome nondisjunction during oogenesis (Hodgkin et al., 1979). With genetic mutations that increase X-chromosome nondisjunction, a large proportion of male offspring can be created to investigate the sex-specific response in MeHg toxicity (Ruszkiewicz et al., 2019). The studies on sex-specific responses to MeHg may be used to infer analogous pathways in mammals given that the principle of sex-determination is similar among different animal species (McElreavey, 1996); however, it has to be mentioned that the divergency of sex-determining genes among different species should be considered for extrapolation of C. elegans studies (Parkhurst and Meneely, 1994).

The thioredoxin system is a critical target of MeHg (Wagner et al., 2010). The function of selenol groups in thioredoxin reductase can be inhibited by MeHg (Pickering et al., 2020). In the him-8 mutant worms which produce 37% male offspring, it was shown that MeHg toxicity was more pronounced in hermaphrodites than males at the mature stages including L4 and adults (Ruszkiewicz et al., 2019). With translational GFP reporter C. elegans strains, additional investigations revealed that the male worms had a higher basal expression of thioredoxin (TRX-1) compared to hermaphrodites; however, the expression of thioredoxin reductase (TRXR-1) was reduced only in hermaphrodites with MeHg but not in males (Ruszkiewicz et al., 2019). The sex-specific modulation of TRX-1 and TRXR-1 likely attributes the effect of sex-specific hormones. For example, the steroid signaling in C. elegans acts on the nuclear receptor DAF-12 to assist the nuclear accumulation of DAF-16 for the upregulation of antioxidant ability and lifespan extension (McCulloch and Gems, 2007). Likewise, the male-specific steroid signaling may mobilize the transcriptional activity of trx-1 or trxr-1 through targeting the nuclear hormones. On the other hand, the rodent model has shown that expression of TrxR, Trx and glutathione peroxidase (GPx) in the brain tissue was increased in females, but repressed in males (Ruszkiewicz et al., 2016); however, in the C. elegans model, the expression level of TRX-1 and TRXR-1 were the in vivo measurement of global expression levels (Ruszkiewicz et al., 2019). The discrepancy between these models can be resolved with the neuronal-specific C. elegans reporter strains in which the expression levels can be compared in specific neuronal cells (Fung et al., 2020; Lorenzo et al., 2020). In addition, the molecular events that trigger the increased basal expression of TRX-1 in males and repressed expression of TRXR-1 in females are currently unknown (Ruszkiewicz et al., 2019). C. elegans has approximately 284 nuclear receptors, among which 25 are responsive to steroids (Novillo et al., 2005). Further studies with mutant worms that are defect in these nuclear receptors can help to characterize the upstream pathways that augment males’ antioxidant effector mechanisms against MeHg-induced toxicity.

2.3. Transgenerational effects

In experimental conditions, the effects of environmental toxicants could be passed down to the future generations that were not directly exposed to the toxicants, which is termed transgenerational effect (Skinner, 2014). The transgenerational effect involves non-genetic mechanisms that mainly work on epigenetic programs including post-translational modifications of histones and small RNAs to establish “inherited phenotypes” ( Jirtle and Skinner, 2007).

C. elegans has a unique advantage to investigate the mechanisms underlying transgenerational effects of MeHg (Hu et al., 2021; Weinhouse et al., 2018), particularly its short life span of ~20days and large brood size (Muschiol et al., 2009). Early studies showed the hormetic effect of MeHg in C. elegans, highlighting an invoked protective mechanism by initial MeHg exposures in the animals against the toxicity of re-exposure to this metal (Helmcke and Aschner, 2010). The hermetic effect of MeHg likely involves the mobilization of the surveillance system for protein translation and mitochondria health (Dalton and Curran, 2018; Liu et al., 2014), which are both targeted by MeHg exposure (Dreiem and Seegal, 2007; Dreiem et al., 2005; Kuznetsov et al., 1986; Lee et al., 2016; Omata et al., 1978; Syversen, 1981). In addition, the toxicity of MeHg can be transferred to future worm generations without direct exposures (Hu et al., 2021). In C. elegans model, parental exposure to MeHg caused multigenerational toxicity which is associated with upregulation of genes involved in apoptosis and DNA damage (Hu et al., 2021); yet, the mechanisms of MeHg-induced transgenerational toxicity are not clear. It may be argued that the effects observed in the offspring with parental MeHg exposure were attributed to the remaining MeHg that was transferred from maternal worms to the next generation, as it was shown in other models of prenatal MeHg exposure (Albores-Garcia et al., 2016; Stringari et al., 2008). However, a recent study showed that the effect associated with parental MeHg exposures persisted even in the second-generation of offspring which were not directly exposed to the metal (Carvan 3rd et al., 2017). In fish models, MeHg-induced transgenerational toxicity was associated with changes in sperm epimutation, a heritable change in the epigenetic landmarks of DNA (Carvan 3rd et al., 2017). The post-translational modifications of DNA histones are the major components in the epigenetic mechanism to modulate gene expression, which is also regulated by nuclear small RNAs that can be propagated in the germline cells (Ni et al., 2016). It has been shown that the parental exposures to various noxious stimulus could trigger transgenerational effects via the passage of small RNAs in the germline cells to modify gene expressions in the offspring (Devanapally et al., 2015; Houri-Zeevi et al., 2021; Rechavi, 2020). These advancements may serve an important base for the further investigations on the transgenerational effects of MeHg and epigenetic mechanisms of heritable traits following parental MeHg exposures.

2.4. Neurodegeneration

Oxidopamine, or 6-hydroxydopamine (6-OHDA), is a neurotoxicant that preferentially destroys dopaminergic neurons in various models (Cordero-Llana et al., 2015; Nass et al., 2002; Vijayanathan et al., 2017). C. elegans has only eight dopaminergic neurons, which include two pairs of cephalic sensilla (CEP) and one pair of anterior deirids (ADE) in the head region and one pair of posterior deirids (PDE) located at the posterior body between vulva and tail. The C. elegans dopaminergic neurons are also sensitive to the toxicity of 6-OHDA (Nass et al., 2002). The 6-OHDA-induced dopaminergic neurodegeneration in C. elegans is widely used as an alternative and complementary model to investigate mechanisms of neuronal degenerative diseases (Nass et al., 2002).

MeHg-induced neuronal toxicity and pathogenesis of neurodegeneration share common pathways including mitochondrial damage and oxidative stress (Ferrer et al., 2022; Mori et al., 2011). Yet, a caustic link between MeHg exposure and age-related neurodegeneration has not been demonstrated (Weiss et al., 2002). In the accidental high-level dimethylmercury (diMeHg) exposure, the relatively long symptom-free period was followed by progressive deterioration of neurological function, resembling the clinical prognosis of Parkinson’s disease (PD) (Nierenberg et al., 1998). Although the dopaminergic system appears a critical target of MeHg exposure (Bridges et al., 2017; Ke et al., 2020a; Tiernan et al., 2015), the pathogenic role of MeHg in PD has yet to be fully confirmed. It appears that C. elegans’ dopaminergic neurons are more resistant to high-level MeHg exposure compared to other models (Daher et al., 2014; Renaud et al., 2018), at least for the morphological endpoints such as the integrity of the dopaminergic dendrites (Götz et al., 2002; Ke et al., 2020b). Surviving worms exposed to high-level MeHg maintain morphologically intact dopaminergic neurons (Ke et al., 2020a). Although dopaminergic neurons are not essential for the survival of C. elegans, they are involved in many regulated behaviors including mechanical sensation, swimming and egg-laying, to name a few (McDonald et al., 2006). Interestingly, exposure to low doses of MeHg causes long-lasting behavior effects in C. elegans as well as other models (Glazer and Brennan, 2021; Ke et al., 2021a), which involve the functional endpoints of dopaminergic neurons, such as dopamine-mediated modulation of motor programs in worms (Ke et al., 2021a; McDonald et al., 2007) and animals’ response to psychostimulants (Rasmussen and Newland, 2001; Wagner et al., 2007).

Unlike humans (Davis et al., 1994), the survival C. elegans preserves a relatively normal lifespan and integrity of neuronal cells after high-level MeHg exposures (Helmcke et al., 2009; Ke et al., 2020a). The young larvae worms have already developed three pairs of dopaminergic neurons in the head region (Flames and Hobert, 2009), which can be used to track the integrity of neuronal morphology following larval stage exposure to MeHg (Ke et al., 2020a,b,c; Martinez-Finley et al., 2013). Thus, the animals must have a unique detoxification and/or protective mechanism to deal with high-level MeHg exposure. This is reasonable, as stresses from xenobiotic exposures and pathogenic microorganisms are common to worms in the wild environment—the humid soil (Schulenburg and Félix, 2017). Recent studies with C. elegans showed that the toxic insults by MeHg could be coped with the removal of toxic substances in the bulk through budding of vesicular structures in dopaminergic neurons (Ke et al., 2020b,c). The dopaminergic neurons of wildtype worms generate vesicular structures at a slowly basal rate. In the worms with STI-1 (stress-inducible protein-1) knockout, low levels of MeHg substantially increase the genesis of vesicular structures (Ke et al., 2020b). STI-1 is a protein cochaperone that has important functions in protein quality control (Flom et al., 2007), which suggests that the protein quality surveillance is tightly coupled to the detoxification of xenobiotics. The fate of dopaminergic versicular structures remains elusive. One possible route is that these exported toxic wastes can be eventually transported and degraded in remote tissues which was shown in another independent study (Melentijevic et al., 2017), highlighting a coordinated mechanism among anatomically remote tissues to deal with neurotoxicity. Furthermore, C. elegans has four larvae stages that are marked by the four cycles of molting. The C. elegans cuticle has important functions in immunity as well as in the initiation of protective response to various stresses (Dalton and Curran, 2018; Dodd et al., 2018). Future studies on how the molting process affect the fate of MeHg metabolism and its significance in the protection of worms from neurotoxicity will shed new light on these cross-tissue mechanisms in the challenge of toxicant exposures.

3. Unique strength of C. elegans models in deciphering long-term effects of developmental methylmercury exposures

Exposure to environmental toxicants at developmental stages has a profound impact on the wellness and disease susceptibility in the later adult stages (Jirtle and Skinner, 2007). It has long been appreciated that MeHg-induced toxicity manifests after a long silent period following developmental exposures; yet the mechanism remains elusive (Weiss et al., 2002).

MeHg, at low doses that mimics environmental exposures, could alter the DNA histone markers in in vitro models (Bose et al., 2012). A recent study showed C. elegans exposed to MeHg at the developmental stage had an altered swimming behavior at the adult stage through the modulation of dopaminergic signaling (Ke et al., 2021a). C. elegans has a life span of ~20days. It has already shown a strong potential in the study of age-related biological questions (Johnson, 2013; Pu et al., 2015). For example, the gene expression pattern from young to old worms shifted in worms subjected to proteotoxic stress (Golden and Melov, 2007; Shai et al., 2014; Walther et al., 2017), a common pathogenic factor in a variety of age-related diseases. A recent study in the C. elegans model of MeHg toxicity showed that the gene expression levels of C. elegans in the dopaminergic system undergo a significant change between young and old adult stages (Ke et al., 2022). The age-dependent changes in gene expression in the animals is fulfilled by the spatial and temporal regulation of DNA histone markers that repress or activate genes for the purposes of physiological fitness (Costello and Petrella, 2019). MeHg exposure can alter DNA histone modifications (Go et al., 2021), either directly or indirectly, causing a shift in the histone methylation levels at specific gene locus to affect gene expression levels (Rudgalvyte et al., 2017). These effects could be masked at the stage when there is no active modulation of the gene expression, but turned on when the locus is constantly activated or repressed in the later adult stage. It has been shown that a time-dependent regulation of gene expression determines the chorological arrangement of physiological and morphological traits (Ruvkun and Giusto, 1989): for example, molting (Kostrouchova et al., 2001) and maturation of reproduction (Ririe et al., 2008). The disrupted gene expression may serve an important basis for the latent effects of early MeHg exposures.

The effects of developmental MeHg exposure on the methylation levels of DNA histone markers can be investigated with the chromatin immunoprecipitation (ChIP) in C. elegans (Rudgalvyte et al., 2017). For the investigations on the genomic methylation level of DNA histones, the isolation chromatin can be used for ChIP-sequencing (ChIP-seq) (Jänes et al., 2018). These techniques may speed up the investigations on the vulnerable gene locus that can be epigenetically modified by MeHg exposures and its impacts on disease outcomes in the adult life-stage.

4. Conclusion

C. elegans is increasingly used as an alternative and complementary model for the investigation of MeHg-induced toxicity. The high homology of the C. elegans genome with mammals’ is an advantage of the model for the understanding the genetic basis for the vulnerability and pathways towards MeHg-induced toxic endpoints. Although MeHg remains highly toxic to the animals, the worms’ unique strategy to the handling of the MeHg exposure deserve a full investigation for the comparison of novel mechanisms to what we have learned in the models of mammals. The advancement on the understanding of C. elegans’ antioxidant response to MeHg exposure, and MeHg-induced neurotoxic effects as well as long-term and transgenerational effects has provided molecular insights into an organism’s response to MeHg toxicity. C. elegans is a species that has homologous and non-homologous genes in different biological domains compared to the human. The translational value of discoveries in C. elegans models to humans can be complemented with the human relevant models such as the platform of induced pluripotent stem cells (iPS). The unique strength of C. elegans models in the investigations on age-related biology and transgenerational effects will greatly aid our efforts in the understanding of MeHg exposures and its significance in the health of elderlies and future generations.