title: Munc13-4 restricts motility of Rab27a-expressing vesicles to facilitate lipopolysaccharide-induced priming of exocytosis in neutrophils.
abstract: LPS is an efficient sensitizer of the neutrophil exocytic response to a second stimulus. Although neutrophil exocytosis in response to pathogen-derived molecules plays an important role in the innate immune response to infections, the molecular mechanism underlying LPS-dependent regulation of neutrophil exocytosis is currently unknown. The small GTPase Rab27a and its effector Munc13-4 regulate exocytosis in hematopoietic cells. Whether Rab27a and Munc13-4 modulate discrete steps or the same steps during exocytosis also remains unknown. Here, using Munc13-4- and Rab27a-deficient neutrophils, we analyzed the mechanism of lipopolysaccharide-dependent vesicular priming to amplify exocytosis of azurophilic granules. We found that both Munc13-4 and Rab27a are necessary to mediate LPS-dependent priming of exocytosis. However, we show that LPS-induced mobilization of a small population of readily releasable vesicles is a Munc13-4-dependent but Rab27a-independent process. LPS-induced priming regulation could not be fully explained by secretory organelle maturation as the redistribution of the secretory proteins Rab27a or Munc13-4 in response to LPS treatment was minimal. Using total internal reflection fluorescence microscopy and a novel mouse model expressing EGFP-Rab27a under the endogenous Rab27a promoter but lacking Munc13-4, we demonstrate that Munc13-4 is essential for the mechanism of LPS-dependent exocytosis in neutrophils and unraveled a novel mechanism of vesicular dynamics in which Munc13-4 restricts motility of Rab27a-expressing vesicles to facilitate lipopolysaccharide-induced priming of exocytosis.
authors.affiliation: Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, California 92037, USA.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Johnson
authors.forename: Hong
authors.initials: H
authors.lastname: Hong
authors.forename: Jlenia
authors.initials: J
authors.lastname: Monfregola
authors.forename: William B
authors.initials: WB
authors.lastname: Kiosses
authors.forename: Sergio D
authors.initials: SD
authors.lastname: Catz
mesh_terms: D000818:Animals; D005089:Exocytosis / Q000187:drug effects* / Q000502:physiology; D005786:Gene Expression Regulation / Q000187:drug effects* / Q000502:physiology; D008070:Lipopolysaccharides / Q000494:pharmacology*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008817:Mice, Mutant Strains; D009504:Neutrophils / Q000378:metabolism* / Q000648:ultrastructure; D022142:Secretory Vesicles / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D020691:rab GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D000075745:rab27 GTP-Binding Proteins
chemical_list: D008070:Lipopolysaccharides; D008565:Membrane Proteins; C520225:Unc13d protein, mouse; D000075745:rab27 GTP-Binding Proteins; C512962:Rab27a protein, mouse; D020691:rab GTP-Binding Proteins