The Phosphoinositide‐Gated Lysosomal Ca2+ Channel, TRPML1, Is Required for Phagosome Maturation

TRPML1 is a lysosomal Ca2+ channel gated by phosphatidylinositol‐3,5‐bisphosphate, a signaling lipid that controls late endosome and lysosome function. Here, we show that TRPML1 is necessary to trigger fusion of lysosomes docked to phagosomes, for efficient killing of phagocytosed bacteria and the release of lysosomal Ca2+ into the cytosol after phagocytosis. Macrophages internalize and sequester pathogens into a phagosome. Phagosomes then sequentially fuse with endosomes and lysosomes, converting into degradative phagolysosomes. Phagosome maturation is a complex process that requires regulators of the endosomal pathway including the phosphoinositide lipids. Phosphatidylinositol‐3‐phosphate and phosphatidylinositol‐3,5‐bisphosphate (PtdIns(3,5)P2), which respectively control early endosomes and late endolysosomes, are both required for phagosome maturation. Inhibition of PIKfyve, which synthesizes PtdIns(3,5)P2, blocked phagosome–lysosome fusion and abated the degradative capacity of phagosomes. However, it is not known how PIKfyve and PtdIns(3,5)P2 participate in phagosome maturation. TRPML1 is a PtdIns(3,5)P2‐gated lysosomal Ca2+ channel. Because Ca2+ triggers membrane fusion, we postulated that TRPML1 helps mediate phagosome–lysosome fusion. Using Fcγ receptor‐mediated phagocytosis as a model, we describe our research showing that silencing of TRPML1 hindered phagosome acquisition of lysosomal markers and reduced the bactericidal properties of phagosomes. Specifically, phagosomes isolated from TRPML1‐silenced cells were decorated with lysosomes that docked but did not fuse. We could rescue phagosome maturation in TRPML1‐silenced and PIKfyve‐inhibited cells by forcible Ca2+ release with ionomycin. We also provide evidence that cytosolic Ca2+ concentration increases upon phagocytosis in a manner dependent on TRPML1 and PIKfyve. Overall, we propose a model where PIKfyve and PtdIns(3,5)P2 activate TRPML1 to induce phagosome–lysosome fusion.