The toxic effect of gadolinium ions on sea urchin embryos: comparison among phylogenetically distant species and focus on mechanisms regulating stress response and skeletogenesis.

Gadolinium (Gd) is a metal of the lanthanide series of the elements whose chelates are commonly employed as contrast agents for magnetic resonance imaging, and subsequently released into the aquatic environment. We investigated the consequences of sea urchin embryo exposure to sublethal Gd concentrations, comparing the effects on the development of four phylogenetically and geographically distant species: two Mediterranean species, Paracentrotus lividus and Arbacia lixula, and two species living in the East coast of Australia, Heliocidaris tuberculata and Centrostephanus rodgersii. Measures of the Gd content inside embryos by ICP-MS showed a time- and dose-dependent increase. In all these species, we observed a general delay of embryo development at 24 hours post-fertilization, and a strong inhibition of skeleton growth at 48 hours, frequently displayed by an asymmetrical pattern, with species-specific threshold levels of sensitivity. Further experiments were carried out on P. lividus embryos. Removal of Gd after 24 hours caused partial recovery of embryo development 48 hours post fertilization, demonstrating a partially reversible effect. We detected an increase of the LC3 autophagic marker at 24 and 48h, while confocal microscopy studies confirmed the increased number of autophagosomes and autophagolysosomes, suggesting that the autophagic process is acting as a cell survival strategy. RT-PCR gene expression analysis showed the misregulation of several genes implicated both in the skeletogenic and the left-right axis specification networks, including: transcription factors (Alx-1, Nodal), signaling molecules (univin, VEGF, VEGF-R, FGF) and skeletal matrix proteins (p16, p19 and msp130). Taken together, the results pose serious questions on the hazard of Gd in the marine environment and indicate that Gd is able to affect three different levels of the stress response in sea urchin embryos: morphogenesis, survival strategies such as autophagy, and gene expression