TMEM55a localizes to macrophage phagosomes to down-regulate phagocytosis

Abstract

TMEM55a is an enzyme that dephosphorylates phosphatidylinositol (PtdIns) (4,5)P2 to form PtdIns(5)P in vitro. However, the in vivo conversion of the polyphosphoinositide to PtdIns(5)P by the phosphatase has not yet been demonstrated, and its role remains poorly understood. 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, IpgD to 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 substantially localized to the phagosome. The accumulation of PtdIns(4,5)P2, PtdIns(3,4,5)P3 and F-actin on the phagocytic cup is increased in TMEM55a deficient cells, as monitored by live-cell imaging. Phagosomal PtdIns(5)P was decreased in the knock down cells, but the augmentation of phagocytosis in these cells was unaffected by the exogenous addition of PtdIns(5)P. Together, these results suggest that TMEM55a negatively regulates the phagocytosis of large particles by reducing phagosomal PtdIns(4,5)P2 accumulation during cup formation.