Exocytic machineries differentially control mediator release from allergen-triggered RBL-2H3 cells

Mast cells utilize SNAREs (soluble-N-ethyl-maleimide sensitive factor attachment protein receptors) and SM (Sec1/Munc18) proteins to secrete/exocytose a variety of proinflammatory mediators. However, whether a common SNARE-SM machinery is responsible remains unclear.

Four vesicle/granule-anchored SNAREs (VAMP2, VAMP3, VAMP7, and VAMP8) and two Munc18 homologs (Munc18a and Munc18b) were systematically knocked down or knocked out in RBL-2H3 mast cells and antigen-induced release of β-hexosaminidase, histamine, serotonin, and TNF was examined. Phenotypes were validated by rescue experiments. Immunofluorescence studies were performed to determine the subcellular distribution of key players.

The reduction of VAMP8 expression inhibited the exocytosis of β-hexosaminidase, histamine, and serotonin but not TNF. Unexpectedly, however, confocal microscopy revealed substantial co-localization between VAMP8 and TNF, and between TNF and serotonin. Meanwhile, the depletion of other VAMPs, including knockout of VAMP3, had no impact on the release of any of the mediators examined. On the other hand, TNF exocytosis was diminished specifically in stable Munc18b^knockdown cells, in a fashion that was rescued by exogenous, RNAi-resistant Munc18b. In line with this, TNF was co-localized with Munc18b (47%) to a much greater extent than with Munc18a (13%).

Distinct exocytic pathways exist in mast cells for the release of different mediators.