Functions of Phosphatidylinositol 4-Phosphate 5 Kinases in Actin Cytoskeletal Regulation During Phagocytosis
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Phosphatidylinositol (4,5)-bisphosphate (PIP2) is a crucial signaling phosphoinositide at the plasma membrane (PM) which mediates a variety of biochemical activities and cellular functions. It is primarily synthesized by type I phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) through the phosphorylation on the D-5 position of the inositol ring of phatidylinositol 4-phosphate [PI(4)P]. Mammals have three PIP5K isoforms named a, b, and g (human isoform designation) which have a highly conserved central kinase homology domain and divergent amino and carboxyl terminal extensions. There is now extensive evidence suggesting that PIP5Ks have unique functions and regulations in many cellular processes which provide the key to understand how functionally, and possibly physically, segregated PIP2 pools are generated. The actin cytoskeleton is dynamically remodeled during Fcg receptor (FcgR)-mediated phagocytosis in a PIP2-dependent manner. I investigated the role of PIP5Kg and a isoforms, which synthesize PIP2, during phagocytosis. PIP5Kg-/- bone marrow-derived macrophages (BMM) have a highly polymerized actin cytoskeleton and are defective in attachment to IgG-opsonized particles and FcgR clustering. Delivery of exogenous PIP2 rescued these defects. PIP5Kg knockout BMM also have more RhoA and less Rac1 activation and pharmacological manipulations establish that they contribute to the abnormal phenotype. Likewise, depletion of PIP5Kg by RNA interference (RNAi) inhibits particle attachment. In contrast, PIP5Ka knockout or silencing has no effect on attachment but inhibits ingestion by decreasing Wiskott-Aldrich syndrome protein (WASP) activation and hence actin polymerization, in the nascent phagocytic cup. In addition, PIP5Kg but not a is transiently activated by spleen tyrosine kinase (Syk)-mediated phosphorylation. I propose that PIP5Kg acts upstream of Rac/Rho and that the differential regulation of PIP5Kg and a allows them to work in tandem to modulate the actin cytoskeleton during the attachment and ingestion phases of phagocytosis.