Competing for PH Domains

Abstract

The inositol phospholipid PtdIns(3,4,5)P 3 is an important signaling lipid, the effects of which are predominantly mediated by its interaction with proteins containing pleckstrin homology (PH) domains. In neutrophils and the model organism Dictyostelium discoideum , PtdIns(3,4,5)P 3 plays an important role in chemotaxis. In D. discoideum , a soluble inositol polyphosphate diphosphoinositol pentakisphosphate (InsP 7 ) was found to inhibit chemotaxis by competing for PH domains. Now Jia et al . show that in neutrophils, Ins(1,3,4,5)P 4 appears to serve as the soluble competitor for PH domains and consequently negatively influences neutrophil response to the chemoattractant fMLP. Stimulation of HL60 cells, a human cell line with characteristics of neutrophils, with fMLP rapidly and transiently increased the concentration of Ins(1,3,4,5)P 4 , increased the activity of the enzyme that produces this inositol polyphosphate, InsP3K, and increased the association of Ins(1,3,4,5)P 4 with a PH domain (using a PH domain linked to green fluorescent protein PH Akt -GFP). Application of a membrane-permeant derivative of Ins(1,3,4,5)P 4 was used to artificially increase Ins(1,3,4,5)P 4 concentration and, in HL60 cells, this inhibited chemoattractant-induced membrane translocation of PH Akt -GFP and phosphorylation of endogenous Akt, which is dependent on membrane association. In mouse neutrophils, artificial elevation of Ins(1,3,4,5)P 4 inhibited fMLP-stimulated chemotaxis (both directed movement and speed of migration). InsP3KB was the only isoform of InsP3K detected in mouse or human neutrophils. Analysis of neutrophils from mice deficient in InsP3KB showed that responses to fMLP were augmented (higher proportion of the kinase Akt was phosphorylated, abundance of filamentous actin was increased, and superoxide production was enhanced) compared with those in wild-type cells. Furthermore, the InsP3KB-deficient cells showed enhanced chemotaxis and increased sensitivity to chemoattractant stimulation. InsP3KB-deficient mice (in a mouse peritoneal inflammation assay) or InsP3KB-deficient neutrophils (in an adoptive transfer assay) showed enhanced recruitment to the site of bacterial or inflammatory agent injection compared with that of wild-type mice or cells from the host animal. Thus, Ins(1,3,4,5)P 4 appears to serve as a negative regulator of neutrophil stimulation, which may help keep the inflammatory response in check. Y. Jia, K. K. Subramanian, C. Erneux, V. Pouillon, H. Hattori, H. Jo, J. You, D. Zhu, S. Schurmans, H. R. Luo, Inositol 1,3,4,5-tetrakisphosphate negatively regulates phosphatidylinositol-3,4,5-trisphophate signaling in neutrophils. Immunity 27 , 453-467 (2007). [PubMed]