Junctate boosts phagocytosis by recruiting endoplasmic reticulum Ca2+ stores near phagosomes

Local intracellular Ca2+ elevations increase the efficiency of phagocytosis, a process that is essential for innate and adaptive immunity. These local Ca2+ elevations are generated in part by the store-operated Ca2+ entry (SOCE) sensor STIM1, which recruits endoplasmic reticulum (ER) cisternae to phagosomes and opens phagosomal Ca2+ channels at ER-phagosome junctions. However, residual ER-phagosome contacts and periphagosomal Ca2+ hotspots remain in Stim1(-/-) cells. Here, we tested whether junctate (also called ASPH isoform 8), a molecule that targets STIM1 to ER-plasma-membrane contacts upon Ca2+-store depletion, cooperates with STIM1 at phagosome junctions. Junctate expression in Stim1(-/-) and Stim1(-/-); Stim2(-/-) phagocytic fibroblasts increased phagocytosis and periphagosomal Ca2+ elevations, yet with only a minimal impact on global SOCE. These Ca2+ hotspots were only marginally reduced by the SOCE channel blocker lanthanum chloride (La3+) but were abrogated by inositol trisphosphate receptor inhibitors 2-APB and xestospongin-C, revealing that unlike STIM1-mediated hotspots, junctate-mediated Ca2+ originates predominantly from periphagosomal Ca2+ stores. Accordingly, junctate accumulates near phagosomes and elongates ER-phagosome junctions in Stim1(-/-) cells. Thus, junctate mediates an alternative mechanism for generating localized Ca2+ elevations within cells, promoting Ca2+ release from internal stores recruited to phagosomes, thereby boosting phagocytosis.