Effects of marine toxins on Xenopus laevis early development

Okadaic acid (OA) is a lipophilic compound produced by several marine dinoflagellates, almost exclusively accumulated in mussel digestive gland. The consumption of contaminated animals provokes a syndrome in humans known as diarrheic shellfish poisoning. Palytoxin (PTX) is a large, water soluble polyalcohol found in a variety of marine organisms ranging from dinoflagellates to fish, implicated in seafood poisoning and classified as neurotoxin. Our experiments were performed by using the Frog Embryo Teratogenesis Assay-Xenopus (FETAX) protocol, and X. laevis embryos at early gastrula stage were treated with different toxin concentrations (0.1, 1 and 10 nM for OA and 0.37, 37 and 370 nM for PTX) for 5 days. The bioassay showed that both toxins affected Xenopus development in terms of mortality, delayed growth and embryo malformation. In particular significant mortality rates were observed with PTX higher dose and the initial sample population decreased by about 80% at assay end. The observation of surviving larvae showed a marked tail folding. Further histological and histochemical studies revealed disorders to the nervous system (the most sensitive tissue) and to the tail skeletal musculature, while alterations also involved the main visceral organs. Molecular biology-based experiments assessed the expression of four genes (siamois, engrailed-2, bmp4, and myf5) involved in the early events of Xenopus development (stages 11-47) and showed that PTX induces an increase in expression levels in all genes, while the response to OA is stage-dependent, with the embryonic development stages more sensitive than the larval stages. The de-regulated gene expression patterns may account for FETAX and histological data.