The inositol 1,4,5-triphosphate signal transduction pathway and magnesium ion mediate lipopolysaccharide-induced exocytosis in the hemocyte of the horseshoe crab, Limulus polyphemus (L.)

The Limulus polyphemus (L.) granulocyte (GR) responds to bacterial contact by degranulating to release coagulation, agglutination and antibacterial factors that mediate an immune response. Degranulation in different invertebrate and vertebrate cells occurs through the process of exocytosis, which can be mediated by the inositol 1,4,5-triphosphate (IP{dollar}sb3{dollar}) signal transduction pathway. In this pathway, extracellular ligands, such as neurohormone analogs and bacterial lipopolysaccharide, bind to surface receptors that are linked to guanine nucleotide-binding protein (G-protein), which activate phospholipase C (PLC) to produce inositol 1,4,5-triphosphate (IP{dollar}sb3{dollar}) and diacyglycerol (DAG). IP{dollar}sb3{dollar} diffuses through the cytoplasm and binds to receptors (IP{dollar}sb3{dollar}R), which are located in the membranes of endoplasmic reticulum and specialized intracellular organelles, and form Ca{dollar}sp{lcub}2+{rcub}{dollar} channels to release stored Ca{dollar}sp{lcub}2+{rcub}{dollar} into the cytoplasm. This rise in cytoplasmic Ca{dollar}sp{lcub}2+{rcub}{dollar} ( (Ca{dollar}sp{lcub}2+{rcub}{dollar}) i) is involved in granule and plasma membrane fusion that occur during exocytosis. In this study I demonstrate, for the first time, that the neurohormone analogs octopamine, serotonin and carbachol induce degranulation and that LPS-induced exocytosis is mediated through IP{dollar}sb3{dollar} pathway, but that a large intracellular Mg{dollar}sp{lcub}2+{rcub}{dollar} ( (Mg{dollar}sp{lcub}2+{rcub}{dollar}) {dollar}sb{lcub}rm i{dollar}) flux occurs in addition to the characteristic (Ca{dollar}sp{lcub}2+{rcub}{dollar}) {dollar}sb{lcub}rm i{rcub}{dollar} flux. I also provide evidence to support the hypothesis that a (Mg{dollar}sp{lcub}2+{rcub}{dollar}) {dollar}sb{lcub}rm i{rcub}{dollar} flux is the key mediator in exocytosis in the GR.