Essential and unique roles of PIP5K-γ and -α in Fcγ receptor-mediated phagocytosis

Abstract

The actin cytoskeleton is dynamically remodeled during Fcγ receptor (FcγR)-mediated phagocytosis in a phosphatidylinositol (4,5)-bisphosphate (PIP2)-dependent manner. We investigated the role of type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) γ and α isoforms, which synthesize PIP2, during phagocytosis. PIP5K-γ−/− bone marrow–derived macrophages (BMM) have a highly polymerized actin cytoskeleton and are defective in attachment to IgG-opsonized particles and FcγR clustering. Delivery of exogenous PIP2 rescued these defects. PIP5K-γ knockout BMM also have more RhoA and less Rac1 activation, and pharmacological manipulations establish that they contribute to the abnormal phenotype. Likewise, depletion of PIP5K-γ by RNA interference inhibits particle attachment. In contrast, PIP5K-α knockout or silencing has no effect on attachment but inhibits ingestion by decreasing Wiskott-Aldrich syndrome protein activation, and hence actin polymerization, in the nascent phagocytic cup. In addition, PIP5K-γ but not PIP5K-α is transiently activated by spleen tyrosine kinase–mediated phosphorylation. We propose that PIP5K-γ acts upstream of Rac/Rho and that the differential regulation of PIP5K-γ and -α allows them to work in tandem to modulate the actin cytoskeleton during the attachment and ingestion phases of phagocytosis.

Type I Phosphotidylinosotol 4-Phosphate 5-Kinase γ Regulates Osteoclasts in a Bifunctional Manner*

“…In both conditions, there is dampened activation and expression of osteoclast differentiation markers, including NFAT, which in other cells is regulated by diacylglycerol and PIP 3 (11). The blunted osteoclastogenesis of PIP5KI␥-and phospholipase C␥2-deficient osteoclasts is consistent with the expected reduction of PI(4,5)P 2 metabolites in both circumstances.…”

“…Because osteoclast function is attended by dramatic cytoskeletal reorganization, which in other cells is regulated by PIP5KI␥, we reasoned that deletion of this isoform is most likely to impact the resorptive cell (9,11,17,18). Our finding that PIP5KI␥2 increases in abundance as macrophages commit to the osteoclast phenotype fortifies this hypothesis.…”

“…Talin/integrin binding is a necessary event whereby these heterodimers organize the cytoskeleton, including that of osteoclasts (13,14,28). As most cells possess more than one isoform of PIP5KI, which may in some circumstances be redundant, a reasonable, albeit speculative, hypothesis holds that PIP5K␣ and/or PIP5K␤ prevent osteoclast dysfunction in face of PIP5K␥ deficiency (11,18). This posture is challenged, however, by the absence of MS-detectable PIP 2 …”

“…PIP5KI␥ represents at least three splice variants, with the two most studied being PIP5KI␥1 (PIP5KI␥-87), consisting of 635 amino acids, and PIP5KI␥2 (PIP5KI␥-90) with an additional 26 residue C-terminal extension (4,10). Differential immunostaining of the two species establish PIP5KI␥1 resides diffusely within the cytoplasm, whereas PIP5KI␥2 targets focal adhesions, suggesting it participates in cytoskeletal organization (11)(12)(13)(14). In addition to its enzymatic activity, PIP5KI␥2 binds talin and delivers it to ␤ integrin subunits.…”

Surfactant Protein A (SP-A)-mediated Clearance of Staphylococcus aureus Involves Binding of SP-A to the Staphylococcal Adhesin Eap and the Macrophage Receptors SP-A Receptor 210 and Scavenger Receptor Class A

“…This interaction modulates formation of vesicles and structure of the trans-Golgi network. Besides Golgi, phosphatidylinositol 4-phosphate that is also enriched at the plasma membrane modulates macrophage phagocytosis via IgG receptors (98,99) and may have a role in cytoskeleton reorganization during SP-R210-mediated phagocytosis. It remains to be determined whether physical association between SP-R210 isoforms, SR-A, and MyoIIA mediates phagocytosis of SP-A-opsonized bacteria.…”