TMEM55a localizes to macrophage phagosomes to downregulate phagocytosis

TMEM55a (also known as PIP4P2) is an enzyme that dephosphorylates the phosphatidylinositol (PtdIns) PtdIns(4,5)P₂ to form PtdIns(5)P in vitro However, the in vivo conversion of the polyphosphoinositide into PtdIns(5)P by the phosphatase has not yet been demonstrated, and the role of TMEM55a remains poorly understood. Here, we found that mouse macrophages (Raw264.7) deficient in TMEM55a showed an increased engulfment of large particles without affecting the phagocytosis of Escherichia coli Transfection of a bacterial phosphatase with similar substrate specificity to TMEM55a, namely IpgD, into Raw264.7 cells inhibited the engulfment of IgG-erythrocytes in a manner dependent on its phosphatase activity. In contrast, cells transfected with PIP4K2a, which catalyzes PtdIns(4,5)P₂ production from PtdIns(5)P, increased phagocytosis. Fluorescent TMEM55a transfected into Raw264.7 cells was found to mostly localize to the phagosome. The accumulation of PtdIns(4,5)P₂, PtdIns(3,4,5)P₃ and F-actin on the phagocytic cup was increased in TMEM55a-deficient cells, as monitored by live-cell imaging. Phagosomal PtdIns(5)P was decreased in the knockdown cells, but the augmentation of phagocytosis in these cells was unaffected by the exogenous addition of PtdIns(5)P. Taken together, these results suggest that TMEM55a negatively regulates the phagocytosis of large particles by reducing phagosomal PtdIns(4,5)P₂ accumulation during cup formation.