From stress proteins to apoptosis and autophagy in sea urchin embryos

Marine invertebrates inhabit a key position as intermediate consumers in the pelagic as well as in the benthonic food chains, making them suitable model systems for ecotoxicological studies. Among benthonic organisms, echinoderms represent a simple, though significant, model system to test how specific stress can simultaneously provoke dangerous effects on growth and vitality of organisms. Sea urchins provide an attractive and exceptional model for investigating environmental pollution. Most studies investigating the effects of Cd stress were conducted on Paracentrotus lividus, one of the most important marine invertebrates used as bioindicator of metal/heavy metal pollution and an important model organism in developmental biology. It was shown that exposure to different toxics causes the synthesis of heat-shock proteins (HSPs), providing a protective role during stress conditions. The synthesis of a specific set of HSPs was demonstrated in P. lividus embryos continually exposed to 1mM of CdCl2 at the blastula (15 h) and gastrula (24 h) stages. Cd insult induces an alteration of normal development, probably inducing alternative pathways of growth, as demonstrated from the presence of several typologies of embryo morphology. Contextually, embryos activate other molecular defense mechanisms such as apoptosis. Studies on the apoptotic processes activated in P. lividus were conducted after a long-lasting exposure to low Cd concentrations, similar to those found in moderately or highly polluted seawaters. These exposures caused severe developmental delays and abnormalities in the larvae, suggesting that even very small amounts of Cd, if accumulated in cells, can produce significant cytotoxic effects and apoptosis. Finally, we can assume that in sea urchin embryos/larvae, apoptosis can be considered part of a defense strategy that, by sacrificing a few cells, can safeguard the whole organism and the developmental program, provided that the exposure to Cd is not excessively prolonged or too intense. Another molecular process studied in sea urchin embryos is autophagy, a mechanism of self-eating described as an important intracellular pathway responsible for degradation and recycling of long-term proteins and cytoplasmic organelles. Autophagy has been recently observed in eggs and embryos, in response to stress induced by Cd and other stressors. Results of these studies revealed a higher level of autophagosomes in embryos exposed to Cd for 18 h. Several experiments have been conducted on sea urchin embryos exposed to Cd to study the relationship between autophagy and apoptosis. Results suggest that autophagy may have a key role providing the energy supply necessary for apoptosis, delivering ATP molecules by recycling damaged cellular components. Recent studies suggest that autophagy is important for the clearance of protein aggregates that are formed in cells following stress, and in this process, the p62/SQSTM1 protein (sequestosome 1) appears to play a key role. p62/SQSTM1 is a multifunctional, multidomain adaptator protein which resides at the autophagosome membranes. It is an autophagosome cargo protein that targets other proteins that bind to it for selective autophagy. To highlight any protein aggregates concomitantly with the peak of autophagic vacuolation, control P. lividus embryos and embryos exposed to 1 mM CdCl2 for 18 and 24 h were submitted to the immunofluorescence/confocal laser scan microscopy protocol, using the anti-p62/SQSTM1 heterologous antibody. Qualitative analysis showed a diffuse globular signal that is often referred to as Ibs (inclusion bodies), suggesting that the presence of polyubiquitinated protein aggregates was intended for autophagic degradation. Taken together, these data indicate that in conjunction with the peak of autophagic vacuolation, there is an intense formation of protein aggregates that would be sent to degradation mediated by the autophagic process. Reported data about the defense mechanisms induced by Cd in P. lividus suggest a hierarchical choice of defense strategies; strikingly, the temporal choice of activation of different mechanisms depends on the fact that the embryo tries to face the stress conditions using, initially, defense strategies that are less deleterious to preserve the developmental program. If these processes are not sufficient to offset the damage, the autophagic and apoptotic mechanisms are activated