title: An antimicrobial peptide tachyplesin acts as a secondary secretagogue and amplifies lipopolysaccharide-induced hemocyte exocytosis.
abstract: In the horseshoe crab, bacterial lipopolysaccharide (LPS) induces exocytosis by granular hemocytes, resulting in the secretion of various defense molecules, such as lectins and antimicrobial peptides, via a G protein-mediating signaling pathway. This response is a key component of the horseshoe crab innate immune response against infectious microorganisms. Here, we report an endogenous amplification mechanism for LPS-induced hemocytes exocytosis. The concentration of LPS required for maximal secretion decreased in proportion to the density of hemocytes, suggesting the presence of a positive feedback mechanism for secretion via a mediator secreted from hemocytes. The exocytosed fluid of hemocytes was found able to induce hemocyte exocytosis in the absence of LPS. Furthermore, tachyplesin, a major antimicrobial peptide of hemocytes, was able to trigger exocytosis in an LPS-independent manner, which was inhibited by a phospholipase C inhibitor, U-73122, and a G protein inhibitor, pertussis toxin. Surface plasmon resonance analysis showed that tachyplesin directly interacts with bovine G protein. These findings suggest that the tachyplesin-induced hemocyte exocytosis also occurs via a G protein-mediating signaling pathway. We concluded that tachyplesin functions not only as an antimicrobial substance, but also as a secondary secretagogue of LPS-induced hemocyte exocytosis, leading to the amplification of the innate immune reaction at sites of injury.
authors.affiliation: Department of Biology, Faculty of Sciences, Kyushu University, Fukuoka, Japan.
authors.forename: Aya
authors.initials: A
authors.lastname: Ozaki
authors.forename: Shigeru
authors.initials: S
authors.lastname: Ariki
authors.forename: Shun-Ichiro
authors.initials: S
authors.lastname: Kawabata
mesh_terms: D000818:Animals; D023181:Antimicrobial Cationic Peptides / Q000502:physiology*; D002417:Cattle; D004268:DNA-Binding Proteins / Q000502:physiology*; D005089:Exocytosis / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006434:Hemocytes / Q000502:physiology*; D006737:Horseshoe Crabs / Q000502:physiology; D036341:Intercellular Signaling Peptides and Proteins; D008070:Lipopolysaccharides / Q000494:pharmacology*; D010455:Peptides / Q000378:metabolism; D010456:Peptides, Cyclic / Q000502:physiology*; D015398:Signal Transduction; D014862:Wasp Venoms / Q000378:metabolism
chemical_list: D023181:Antimicrobial Cationic Peptides; D004268:DNA-Binding Proteins; D036341:Intercellular Signaling Peptides and Proteins; D008070:Lipopolysaccharides; D010455:Peptides; D010456:Peptides, Cyclic; D014862:Wasp Venoms; C057636:tachyplesin peptide, Tachypleus tridentatus; C037500:mastoparan; D019204:GTP-Binding Proteins