Inositol Pyrophosphates Mediate Chemotaxis in Dictyostelium via Pleckstrin Homology Domain-PtdIns(3,4,5)P3 Interactions

Abstract

Inositol phosphates are well-known signaling molecules, whereas the inositol pyrophosphates, such as diphosphoinositol pentakisphosphate (InsP7/IP7) and bis-diphosphoinositol tetrakisphosphate (InsP8/IP8), are less well characterized. We demonstrate physiologic regulation of Dictyostelium chemotaxis by InsP7 mediated by its competition with PtdIns(3,4,5)P3 for binding pleckstrin homology (PH) domain-containing proteins. Chemoattractant stimulation triggers rapid and sustained elevations in InsP7/InsP8 levels. Depletion of InsP7 and InsP8 by deleting the gene for InsP6 kinase (InsP6K/IP6K), which converts inositol hexakisphosphate (InsP6/IP6) to InsP7, causes rapid aggregation of mutant cells and increased sensitivity to cAMP. Chemotaxis is mediated by membrane translocation of certain PH domain-containing proteins via specific binding to PtdIns(3,4,5)P3. InsP7 competes for PH domain binding with PtdIns(3,4,5)P3 both in vitro and in vivo. InsP7 depletion enhances PH domain membrane translocation and augments downstream chemotactic signaling activity.

Moving Forward: Mechanisms of Chemoattractant Gradient Sensing

“…Also supporting this model is the recent observation that diphosphoinositol pentakisphosphate (IP 7 ) acts as a negative regulator of chemotaxis in vivo by competing with PIP 3 for binding to PH domains (29). Production of this second messenger is stimulated by cAMP in Dictyostelium, and, interestingly, cells unable to produce IP7 aggregate faster in response to starvation than wild-type cells.…”

Regulation of innate immunity by inositol 1,3,4,5-tetrakisphosphate

“…14,15 Recently, we demonstrated that two inositol phosphates, InsP7 and Ins(1,3,4,5)P4, compete for Akt-PH domain binding with PtdIns(3,4,5)P3 both in vitro and in vivo, providing another level of regulation for Akt membrane translocation and activation. 16,17 In hematopoietic progenitor cells, depletion of Ins(1,3,4,5)P4 by disrupting InsP3KB gene enhances membrane translocation of PtdIns(3,4,5)P3 specific PH domain, thus augments the PtdIns(3,4,5)P3 downstream signals such as Akt. As a result, the phosphorylation of cell cycle inhibitory protein p21 cip1 , a target of Akt, is enhanced, leading to diminished cell cycle inhibitory effect of p21 cip1 .…”

Novel Mechanism of PTEN Regulation by Its Phosphatidylinositol 4,5-Bisphosphate Binding Motif Is Critical for Chemotaxis

“…Recently, both PI3K and PTEN have been shown to be required for PI(3,4,5)P 3 production and chemotaxis toward cAMP (11,12). In wild type Dictyostelium cells, the level of PI(3,4,5)P 3 transiently increases within 5 s upon cAMP stimulation and returns to basal levels within 60 s. However, in mutants lacking two of three PI3K genes, the production of PI(3,4,5)P 3 induced by cAMP is dramatically impaired, and the mutant cells are defective in chemotaxis and development (13,14).…”