Mechanisms of Hg species induced toxicity in cultured human astrocytes: genotoxicity and DNA-damage response

Abstract

The toxicologically most relevant mercury (Hg) species for human exposure is methylmercury (MeHg). Thiomersal is a common preservative used in some vaccine formulations. The aim of this study is to get further mechanistic insight into the yet not fully understood neurotoxic modes of action of organic Hg species. Mercury species investigated include MeHgCl and thiomersal. Additionally HgCl2 was studied, since in the brain mercuric Hg can be formed by dealkylation of the organic species. As a cellular system astrocytes were used. In vivo astrocytes provide the environment necessary for neuronal function. In the present study, cytotoxic effects of the respective mercuricals increased with rising alkylation level and correlated with their cellular bioavailability. Further experiments revealed for all species at subcytotoxic concentrations no induction of DNA strand breaks, whereas all species massively increased H2O2-induced DNA strand breaks. This co-genotoxic effect is likely due to a disturbance of the cellular DNA damage response. Thus, at nanomolar, sub-cytotoxic concentrations, all three mercury species strongly disturbed poly(ADP-ribosyl)ation, a signalling reaction induced by DNA strand breaks. Interestingly, the molecular mechanism behind this inhibition seems to be different for the species. Since chronic PARP-1 inhibition is also discussed to sacrifice neurogenesis and learning abilities, further experiments on neurons and in vivo studies could be helpful to clarify whether the inhibition of poly(ADP-ribosyl)ation contributes to organic Hg induced neurotoxicity.

Toxicity of organic and inorganic mercury species in differentiated human neurons and human astrocytes

“…In accordance with the cytotoxic order of the Hg species in astrocytes [13), organic MeHgCl and thiomersal exerted stronger cytotoxic effects as compared to inorganic HgC1 2 in neurons with thiomersal being the most cytotoxic compound. These results are in line with the low intracellular Hg contents quantified in neurons following 24 h HgC1 2 exposure, whereas an incubation with the organic species resulted in high cellular mercury levels.…”

“…With accumulation factors of about 1000 in case of the organic species, Hg accumulation is about 140 fold higher as compared to CCF-S1TG1 cells [ 13). This might also be one explanation for the enhanced sensitivity of the neurons.…”

“…Since evidence exists that MeHg accumulates preferentially in astrocytes and neuronal dysfunction is secondary to disturbance of astrocytes, a central role for astrocytes in mediating Hg induced neurotoxicity has been proposed [12]. Pieper et al compared cytotoxic effects of MeHg, thiomersal and mercuric mercury on human astrocytes (CCF-STTG1) concluding an increased cytotoxic potential with rising alkylation level [13]. Previous studies have demonstrated similar toxicities of EtHg and MeHg in brain-associated cells, whereas inorganic Hg induced effects were restricted to substantially higher concentrations [14,15].…”

Genetic polymorphisms affecting susceptibility to mercury neurotoxicity in children: Summary findings from the Casa Pia Children's Amalgam Clinical Trial

“…Since adverse neurobehavioral effects were found only among children having gene modifications, genexHg interactions are assumed to underlie these effects. In this regard, Hg directly affects dopaminergic (Eddins et al, 2008), serotonergic (Tsai et al, 1995) and other neurologic processes (Castoldi et al, 2008; Pieper et al, 2014), and these effects are likely to be exaggerated against enzyme or protein products of genetic variants that mediate or facilitate developing neurologic pathways. Of note, Hg 2+ directly impairs the enzymatic activity of CPOX4 (Li and Woods, 2009) and also likely inhibits preferentially the S-containing met forms of the COMT val158met enzyme and the BDNF val66met protein product, both of which play critical roles in neurologic development (Barnett et al, 2009; Burkhalter et al, 2003).…”

Toxicological effects of thiomersal and ethylmercury: Inhibition of the thioredoxin system and NADP+-dependent dehydrogenases of the pentose phosphate pathway

“…However, the mechanism of toxicity has not been well understood and studies with EtHg and TM are scarce in vivo and in vitro (Geir et al, 2009;Ida-Eto et al, 2013;Migdal et al, 2011;Mutkus et al, 2005;Pieper et al, 2014;Sharpe et al, 2012;Westphal et al, 2003). In this context, the better understanding of the molecular mechanism of action of TM and EtHg besides being a priority in toxicology research would help to clarify the epidemiological controversy around TCV and mercury intoxication.…”