Exocytic Mechanisms of Storage and Release of Brevetoxin in the Dinoflagellate Karenia Brevis

Abstract

Karenia brevis is a broadly distributed toxic dinoflagellate responsible for Red Tide outbreaks throughout the world. Deleterious effects of these blooms are caused by brevetoxin, a potent neurotoxin that binds to sodium channels in nerve and muscle cell leading to substantial marine life mortality and human morbidity. Little is known about the mechanisms of storage and release of toxins in algal species responsible for harmful algal blooms. Toxins have been thought to be exported from these unicellulars by an ill-defined “exudation” mechanism. Although secretion is a standard strategy of material export in plants and animal cells, regulated exocytosis has only recently begun to be explored in dinoflagellates (FEBS Letters 2006, 580:2201-2206). Results presented here using fluorescently-labeled antibrevetoxin antibodies show that brevetoxin is present in Karenia's secretory vesicles and is released following blue light-stimulated exocytosis. The matrix of secretory granules functions as a caging polymer network that holds immobilized high payloads of active molecules including hormones, antibacterial peptides, or in this case toxins (Ann. Rev. Physiol 1990.52: 157-176). Upon release, Karenia's vesicle matrix undergoes typical phase transition from condensed to solvated phase, with characteristic first order kinetics swelling, and release of its brevetoxin payload. These observations support the notion that Karenia b functions as a typical secretory cell, opening the way for a better understanding of Red Tide blooms dynamics.