cytolytic secretory pathway

Cytotoxic lymphocytes clear infected and transformed cells by releasing the con-tent of lytic granules at cytolytic syn-apses, and the ability of cytolytic effec-tors to kill in an iterative manner has been documented previously. Although bidirec-tional trafficking of cytolytic machinery components along the endosomal path-way has begun to be elucidated, the mo-lecular mechanisms coordinating granule retrieval remain completely unexplored. In the present study, we focus on the lytic granule priming factor Munc13-4, the muta-tion of which in familial hemophagocytic lymphohistiocytosis type 3 results in a pro-found defect of cytotoxic function. We ad-dressed the role of phosphatidylinositol (4,5)-bisphosphate (PIP2) in the regulation of Munc13-4 compartmentalization. We ob-served that in human natural killer cells, PIP2 is highly enriched in membrane rafts. Granule secretion triggering induces a tran-sient Munc13-4 raft recruitment, followed by AP-2/clathrin–dependent internalization. Phosphatidylinositol 4-phosphate 5-kinase (PIP5K)gene silencing leads to the impair-ment of granule secretion associated with increased levels of raft-associated Munc13-4, which is attributable to a defect in AP-2 membrane recruitment. In such condi-tions, the ability to subsequently kill mul-tiple targets was significantly impaired. These observations indicate that Munc13-4 reinternalization is required for the mainte-nance of an intracellular pool that is func-tional to guarantee the serial killing poten-tial. (Blood. 2012;119(10):2252-2262