Small molecules that inhibit the late stage of Munc13-4–dependent secretory granule exocytosis in mast cells

Ca2+-dependent secretory granule fusion with the plasma membrane is the final step for the exocytic release of inflammatory mediators, neuropeptides, and peptide hormones. Secretory cells use a similar protein machinery at late steps in the regulated secretory pathway, employing protein isoforms from the Rab, Sec1/Munc18, Munc13/CAPS, SNARE, and synaptotagmin protein families. However, no small-molecule inhibitors of secretory granule exocytosis that target these proteins are currently available but could have clinical utility. Here we utilized a high-throughput screen of a 25,000-compound library that identified 129 small-molecule inhibitors of Ca2+-triggered secretory granule exocytosis in RBL-2H3 mast cells. These inhibitors broadly fell into six different chemical classes, and follow-up permeable cell and liposome fusion assays identified the target for one class of these inhibitors. A family of 2-aminobenzothiazoles (termed benzothiazole exocytosis inhibitors or bexins) was found to inhibit mast cell secretory granule fusion by acting on a Ca2+-dependent, C2 domain-containing priming factor, Munc13-4. Our findings further indicated that bexins interfere with Munc13-4-membrane interactions and thereby inhibit Munc13-4-dependent membrane fusion. We conclude that bexins represent a class of specific secretory pathway inhibitors with potential as therapeutic agents.