Bioimaging tools reveal copper processing in fish cells by mitophagy.

As an essential trace metal, copper (Cu) regulation, distribution and detoxification among different cellular organelles remain much unknown. In the current study, bioimaging tool was used in visualizing the locations of Cu among different organelles in fish fin cells isolated from rabbitfish Siganus fuscescens. Exposure concentration of Cu directly affected the Cu bioaccumulation and toxicity. When the exposure dosage of Cu reached 100 µM, it began to damage the cells and affect the cell viability after 10 min of exposure. Remarkably, while various Cu concentrations (50∼150 µM) initially reduced the cell viability, they did not lead to a further loss in viability over extended exposure period. Upon entry to the cells, Cu was mainly targeted to the mitochondria whose number, size and network responded immediately to the incoming Cu. However, Cu toxicity did not increase time-dependently, strongly indicating that these mitochondria damaged by Cu could be removed and its cytotoxicity could be relieved. Bioimaging results showed that lysosomes interacted with the mitochondria, which were subsequently digested within a few minutes. Meanwhile the lysosomal number increased, and the size and pH of lysosomes decreased. These reactions were in line with the observed mitophagy, suggesting that mitochondrial Cu could be detoxified, and the damaged mitochondria were removed by lysosome via mitophagy. By further purifying the cellular organelles, the mitochondrial and lysosomal Cu amounts were quantified and found to be in line with the imaging results. The present study suggested that excessive mitochondrial Cu could be removed via mitophagy to relieve the Cu toxicity.