Abstract

Maitotoxin (MTX) is a potent shellfish toxin widely used as an in vitro tool for increasing intracellular Ca²⁺ and studying Ca²⁺-dependent processes. MTX also induces membrane blebbing and nonselective pores similar to those elicited by the P2X7 receptor (P2X7R), an ATP-gated cation channel expressed in inflammatory leukocytes. We therefore tested whether MTX treatment of lipopolysaccharide-primed murine macrophages would mimic the ability of activated P2X7R to induce secretion of the proinflammatory cytokine interleukin-1β (IL-1β). MTX at ≤0.6 nM predominantly induced processing and nonlytic release of mature IL-1β (mIL-1β), whereas >0.6 nM of MTX induced cytolytic release of unprocessed proIL-1β. MTX-dependent release of mIL-1β (but not cytolysis) was inhibited by the elimination of the trans-plasma membrane K⁺ gradient. MTX-induced cytokine release and cytolysis were both abrogated in the absence of extracellular Ca²⁺. On the other hand, extracellular glycine (5 mM) blocked MTX-induced cytolytic release of proIL-1β without affecting regulated secretion of mIL-1β. Because MTX has profound effects on plasma membrane permeability, we used time-lapse videography to examine the morphologic response of individual macrophages to MTX. MTX treatment led to biphasic propidium dye uptake and dilated blebbing coincident with cytolysis. Glycine completely blocked the second, lytic phase of dye uptake and prevented MTX-induced bleb dilation. These results indicate that the inflammatory macrophage can assemble the necessary signaling components to initiate both regulated and lytic release of IL-1β in response to MTX. This suggests that the hyperactivation of proinflammatory cytokine secretion may be a significant component of the in vivo response to MTX during shellfish seafood poisoning.