Real-time in vitro monitoring of the subcellular toxicity of inorganic Hg and methylmercury in zebrafish cells

Mercury (Hg) is a prominent environmental contaminant and can cause various subcellular effects. Elucidating the different subcellular toxicities of inorganic Hg (Hg²⁺) and methylmercury (MeHg) is critical for understanding their overall cytotoxicity. In this study, we employed aggregation-induced emission (AIE) probes to investigate the toxicity of Hg at the subcellular level using an aquatic embryonic zebrafish fibroblast cell line ZF4 as a model. The dynamic monitoring of lysosomal pH and the mapping of pH distribution during Hg²⁺ or MeHg exposure were successfully realized for the first time. We found that both Hg²⁺ and MeHg decreased the mean lysosomal pH, but with contrasting effects and mechanisms. Hg²⁺ had a greater impact on lysosomal pH than MeHg at a similar intracellular concentration. In addition, Hg²⁺ in comparison to MeHg exposure led to an increased number of lysosomes, probably because of their different effects on autophagy. We further showed that MeHg (200 nM) exposure had an inverse effect on mitochondrial respiratory function. A high dose (1000 nM) of Hg²⁺ increased the amount of intracellular lipid droplets by 13%, indicating that lipid droplets may potentially play a role in Hg²⁺detoxification. Our study suggested that, compared with other parameters, lysosome pH was most sensitive to Hg²⁺ and MeHg. Therefore, lysosomal pH can be used as a potential biomarker to assess the cellular toxicity of Hg in vitro.