Effect of Arsenic Exposure on Early Eye Development in Zebrafish (Danio rerio)

Arsenic is a metalloid that contaminates drinking water supplies worldwide. Due to concerns for human health, the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) have established a safe level in drinking water of ≤ 10 ppb. Arsenic has been shown to have carcinogenic effects in humans at high and low doses. Chronic exposure may result in dermal conditions such as hyperkeratosis and hyperpigmentation. Recently, arsenic exposure has also been linked to lower IQ values in children. The effect of arsenic on neurogenesis, specifically eye development, has not been widely explored. This study aimed to examine the effect of environmentally relevant concentrations of arsenic on early eye development by morphological and molecular analysis. The zebrafish (Danio rerio) was chosen to model the impacts of arsenic on retinogenesis because of its similarities to human eye development both structurally and genetically. The effect of arsenic exposure on the gross morphology of the eye and tissue development via histology were examined at three larval stages. It was found that exposure to arsenic has an effect on eye morphology resulting in a significant increase in eye diameter at 14 dpf (days post fertilization) relative to control under all treatment conditions. This was coupled with a trend in thinning of the retinal pigmented epithelium (RPE) layer in fish exposed to 500 ppb arsenic. The impacts of arsenic exposure on gene expression were analyzed following treatments of 10, 50, and 500 ppb from 1 – 72 hpf (hours post fertilization). Molecular analysis of genes associated with eye development was investigated by RT-qPCR at 32 and 48 hpf. RT-qPCR results revealed differential expression of Pax6a, Pax2a, Ngn1, Sox2, and Shha relative to control. Specifically Pax6a, Pax2a, and Sox2 are found to be important in the formation of the RPE. Proper formation of the RPE is necessary for growth of the sclera which, in turn, is responsible for maintaining the shape of the eye. Although we observed a thinning of the RPE we also noted an overall increase in eye size of 14 dpf zebrafish. This could potentially be explained by the disruption of gene expression under arsenic exposure during critical time points in early eye development