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 Stim1^-/- 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 Ca ²⁺ 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 Ca ²⁺ channels generates localized Ca²⁺ elevations that promote high-efficiency phagocytosis.