Exposure effects of synthetic glucocorticoid drugs on skeletal developmental and immune cell function in zebrafish

Synthetic glucocorticoids (GCs) are used to treat a wide range of human health conditions and as such are frequently detected in the aquatic environment. This, together with the highly conserved nature of the glucocorticoid system across vertebrates means that the potential for biological effects of GCs in fish is relatively high. Here, we found that exposure of zebrafish (Danio rerio) to environmentally relevant concentrations of 4 of the most widely used synthetic GCs (beclomethasone dipropionate, budesonide, fluticasone propionate, and prednisolone), from 0 to 4 days post fertilisation (dpf), resulted in no effects on embryo-larval development or bone and cartilage formation. However, after exposure to equivalents of human therapeutic plasma levels, developmental abnormalities were observed that included pericardial oedema, blood pooling and alterations in jaw cartilage. Furthermore, using a double transgenic zebrafish osteoblast and chondrocyte reporter line, exposure up to 10 dpf resulted in alterations to lower jaw cartilage and bone development for all compounds at, and above, human therapeutic plasma concentrations. In the case of beclomethasone dipropionate, a reduction in lower jaw intercranial distance was observed at the environmentally relevant concentration of 0.1 μg/L. Using further transgenic reporter lines with fluorescently tagged neutrophils and macrophages, we also show exposure of embryo-larvae (0-4 dpf) to the GCs tested resulted in altered immune cell migration, but only at relatively high exposure concentrations. Collectively, our findings show GC exposure impacts embryo-larval zebrafish development, immune function, and skeletal formation, but predominantly at concentrations greater than those currently reported for the aquatic environment. Despite this, however, it is suggested that studies with longer exposure times, and to mixtures of multiple GCs (many GCs act via the same mechanism of action) are warranted before we can confidently assert that these commonly detected contaminants do not pose a risk to fish in the wild.