Cytoskeleton Alterations by Tributyltin (TBT) in Tunicate Phagocytes

The effects of tributyltin chloride (TBT) on cytoskeletal components, as possible cell targets of toxicity, were examined on cultured hemocytes of the colonial ascidianBotryllus schlosseriby means of indirect immunofluorescence. The immunotoxic effect of 10 μM TBT (sublethal concentration) consists of (1) inhibition of yeast phagocytosis, Ca2+ATPase activity, and respiratory burst; (2) increase in intracellular Ca2+concentration; and (3) alterations in cell morphology. After 60 min, TBT-exposed amebocytes become spherical, withdrawing their long pseudopodia, and lose motility. Their microfilaments assemble in clusters around the peripheric cytoplasm, indicating massive disassembly, with the exception of unaltered adhesion plaques. Analogously, their microtubules reveal extensive disaggregation, being scattered in the cytoplasm and not recognizable as single filaments, whereas the microtubule organizing center (MTOC) is still visible. Treatment together with 20 μg/ml calmodulin (CaM) can partially restore the cytoskeleton architecture. These results suggest a relationship between TBT and Ca2+homeostasis in ascidian hemocytes. By interfering with Ca2+ATPase activity through CaM inhibition, either directly or indirectly, TBT induces an excess of intracellular Ca2+accumulation, which first causes internal disorganization of cytoskeletal proteins and consequently inhibition of phagocytosis, beginning from chemotaxis and particle adhesion.