Details of Research Outputs

The unicellular eukaryotic dinoflagellates shed their flagella and form a new pellicle cyst wall in response to environmental stress. This encystment process can also be induced by indoleamines such as melatonin and 5-methoxytryptamine. To decipher the complex signaling events which lead to encystment, we have investigated the functional roles of Ca²⁺ and inositol phosphates in indoleamine-induced encystment of the dinoflagellates Alexandrium catenella and Crypthecodinium cohnii. Pretreatment with EGTA, but not with EDTA, effectively blocked the indoleamine-induced encystment of A. catenella in a dose-dependent manner. Conversely, agents that facilitate the influx of Ca²⁺ (Bay K 8644, A23187 and ionomycin) dose-dependently induced encystment of A. catenella. Endoplasmic Ca²⁺-ATPase inhibitors such as thapsigargin and the peptide toxin melittin also induced encystment of A. catenella. These results suggest that an elevation of intracellular [Ca²⁺] may be involved in the encystment response. In terms of the regulation of phospholipase C, melatonin doseand time-dependently stimulated the formation of inositol phosphates in C. cohnii. The rank order of potency for several indoleamines to stimulate inositol phosphates formation was 2-iodomelatonin > 5-methoxytryptamine ≥ melatonin >> N-acetylserotonin > 5-hydroxytryptamine. This rank order was the same as for the indoleamine-induced encystment of C. cohnii as previously reported. Our results indicate that indoleamineinduced activation of phospholipase C and elevation of intracellular [Ca²⁺] may be proximal steps in the signal transduction pathway leading to encystment in dinoflagellates. Moreover, this is the first demonstration of the possible involvement of Ca²⁺ and inositol phosphates as second messengers in dinoflagellates.