A TRP Channel in the Lysosome Regulates Large Particle Phagocytosis via Focal Exocytosis

Phagocytosis of large extracellular particles such as apoptotic bodies requires delivery of the intracellular endosomal and lysosomal membranes to form plasmalemmal pseudopods. Here, we identified mucolipin TRP channel 1 (TRPML1) as the key lysosomal Ca2+ channel regulating focal exocytosis and phagosome biogenesis. Both particle ingestion and lysosomal exocytosis are inhibited by synthetic TRPML1 blockers and are defective in macrophages isolated from TRPML1 knockout mice. Furthermore, TRPML1 overexpression and TRPML1 agonists facilitate both lysosomal exocytosis and particle uptake. Using time-lapse confocal imaging and direct patch clamping of phagosomal membranes, we found that particle binding induces lysosomal PI(3,5)P2 elevation to trigger TRPML1-mediated lysosomal Ca2+ release specifically at the site of uptake, rapidly delivering TRPML1-resident lysosomal membranes to nascent phagosomes via lysosomal exocytosis. Thus phagocytic ingestion of large particles activates a phosphoinositide- and Ca2+-dependent exocytosis pathway to provide membranes necessary for pseudopod extension, leading to clearance of senescent and apoptotic cells in vivo. [Display omitted] •Phagocytic uptake of apoptotic bodies is defective in TRPML1 knockout macrophages•Large particle binding induces lysosomal PI(3,5)P2 elevation in macrophages•Particle binding activates TRPML1, lysosomal Ca2+ release, and focal exocytosis•Focal exocytosis provides intracellular membranes for phagosome biogenesis Phagocytosis of large extracellular particles such as apoptotic bodies requires delivery of the intracellular endosomal and lysosomal membranes to form plasmalemmal pseudopods. In this study, Samie et al. identified TRPML1 as the key lysosomal Ca2+ channel regulating focal lysosomal exocytosis, providing the membranes necessary for pseudopod extension and phagosome biogenesis.