STIM1 Juxtaposes ER to Phagosomes, Generating Ca2+ Hotspots that Boost Phagocytosis

Background: Endoplasmic reticulum (ER) membranes are recruited to phagosomes, but the mechanism and functional significance of this ER recruitment is not known. Here, we show that the ER Ca2+ sensor stromal interaction molecule 1 (STIM1) sustains high-efficiency phagocytosis by recruiting thin ER cisternae that interact productively but do not fuse with phagosomes. Results: Endogenous STIM1 was recruited to phagosomes upon ER Ca2+ depletion in mouse neutrophils, and exogenous YFP-STIM1 puncta coincided with localized Ca2+ elevations around phagosomes in fibroblasts expressing phagocytic receptors. STIM1 ablation decreased phagocytosis, ER-phagosome contacts, and periphagosomal Ca2+ elevations in both neutrophils and fibroblasts, whereas STIM1 re-expression in Stim-1(-/-) fibroblasts rescued these defects, promoted the formation and elongation of tight ER-phagosome contacts upon ER Ca2+ depletion and increased the shedding of periphagosomal actin rings. Re-expression of a signaling-deficient STIM1 mutant unable to open Ca2+ channels recruited ER cisternae to the vicinity of phagosomes but failed to rescue phagocytosis., actin shedding, and periphagosomal Ca2+ elevations. The periphagosomal Ca2+ hotspots were decreased by extracellular Ca2+ chelation and by Ca2+ channels inhibitors, revealing that the Ca2+ ions originate at least in part from phagosomes. Conclusions: Our findings indicate that STIM1 recruits ER cisternae near phagosomes for signaling purposes and that the opening of phagosomal Ca2+ channels generates localized Ca2+ elevations that promote high-efficiency phagocytosis.