Munc13-4 Regulates Granule Secretion in Human Neutrophils

Abstract

The neutrophil plays a central role in the innate host immune defense. Regulated exocytosis of its granules and release of antimicrobial and cytotoxic substances are key events to limit the spread of pathogens. However, the molecular mechanisms that control exocytosis of neutrophil granules are ill-defined. Recently, it was shown that Munc13-4 is essential for the priming of granules in several hematopoietic cells. In this study, we show that Munc13-4 is expressed in human neutrophils, and that its expression is increased during granulocytic differentiation of HL-60 and PLB-985 cells. Cell fractionation analysis reveals that Munc13-4 is mainly cytosolic and is recruited rapidly to membranes following stimulation with fMLF (N-formyl-methionyl-leucyl-phenylalanine). Moreover, a pool of Munc13-4 associated with mobilizable secondary and tertiary granules is relocalized to the plasma membrane after stimulation with fMLF. The fMLF-induced translocation of Munc13-4 is strictly dependent on calcium in neutrophils. C2 domains of Munc13-4 are essential for binding to phospholipid vesicles in a Ca2+-independent manner. Finally, down-regulation of Munc13-4 using small interfering RNA decreases exocytosis of tertiary granules in PLB-985 cells, whereas overexpression of Munc13-4 enhances secretion of MMP-9 (matrix metalloproteinase-9) from tertiary granules. Our findings suggest a role for Munc13-4 as a component of the secretory machinery in neutrophils.

Macrophage activation syndrome as part of systemic juvenile idiopathic arthritis: diagnosis, genetics, pathophysiology and treatment

“…42,43 In addition, sJIA/MAS patients with bi-allelic mutations in the MUNC13-4 gene reported in FHLH have been described, 43 suggesting that there is likely a genetic component that overlaps between MAS and FHLH. Interestingly, mutations in MUNC13-4 also affect the degranulation of neutrophils 44 and platelets. 45 Not surprisingly, FHLH patients with bi-allelic mutations in MUNC13-4 often have atypical clinical features with some overlap with sJIA.…”

MUNC13-4 Protein Regulates the Oxidative Response and Is Essential for Phagosomal Maturation and Bacterial Killing in Neutrophils

“…Because exocytosis is considered an important factor for NADPH oxidase activation (18) and MUNC13-4 regulates exocytosis in neutrophils (29,30), we sought to analyze a possible function for MUNC13-4 in ROS production regulation. First, we determined the subcellular distribution of MUNC13-4 in relationship to that of flavocytochrome b 558 by cell fractionation and confocal microscopy.…”

“…In neutrophils, MUNC13-4 regulates exocytosis of azurophilic (30) and tertiary (29) granules, but the increased susceptibility of patients with FHL3 to bacterial infections (34) suggests further defects in neutrophil function. Possible roles for MUNC13-4 in the control of the secretion of other neutrophil granules, ROS production, phagocytosis maturation, or NET formation have not been explored.…”

“…In neutrophils, RAB27A controls azurophilic granule exocytosis and regulates the activation of the NADPH oxidase at the plasma membrane (26 -28). The RAB27A effector MUNC13-4 is also involved in the regulation of granule exocytosis (27,29,30) and controls the priming of myeloperoxidase release in response to bacterial lipopolysaccharide (30), but its role in other neutrophil functions remains unknown. Importantly, defects in the rab27a or the unc13-4 genes are associated with immunodeficiencies in humans (31).…”

Munc13-4 Restricts Motility of Rab27a-expressing Vesicles to Facilitate Lipopolysaccharide-induced Priming of Exocytosis in Neutrophils

“…Other studies suggest a possible combined role for Munc13-4 and Rab27a during lytic granule maturation (16). In neutrophils, both Rab27a and Munc13-4 regulate the exocytosis of secretory organelles (3,7,17), although whether they jointly or independently regulate transport toward the plasma membrane (PM), tethering, docking, or fusion is currently unknown and needs further characterization.…”