Dual Lipid Pathways for Migration

Abstract

Motile cells respond to extracellular gradients of chemoattractants by moving in a directed manner up the gradient in a process called chemotaxis. Studies of chemotaxis in the slime mold Dictyostelium discoideum revealed a key role for the polarized production of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ], which is produced by phosphatidylinositol 3-kinase (PI3K) and degraded by the lipid phosphatases PTEN (a PI-3 phosphatase) and SHIP (PI-5 phosphatase). Chen et al . now show that, in addition to this PI3K-dependent pathway, there is a parallel pathway dependent on phospholipase A 2 (PLA 2 ) and arachidonic acid derivatives. The chemotaxis of wild-type Dictyostelium was not diminished by low concentrations of inhibitors of PI3K (such as LY294002) but was blocked by high concentrations of such inhibitors. Chen et al . screened for Dictyostelium mutants that showed chemotaxis defects in the presence of low concentrations of LY294002 and were resistant to chemotaxis inhibition by high concentrations of LY294002. Characterization of one of the mutants revealed the disrupted gene to be homologous to genes in plants encoding patatin PLA 2 and in animals encoding calcium-independent PLA 2 , so they named the gene plaA and the encoded protein PLA 2 A. The purified protein from Dictyostelium produced arachidonic acid from phosphatidylcholine in vitro. Targeted disruption of the plaA gene phenocopied the chemotaxis defect of the mutant, and these cells were used for further analysis, along with cells in which two of the PI3K genes were disrupted ( pi3k 1 − /2 − ) and cells in which all three were disrupted ( plaA − , pi3k 1 − /2 − ). The triple-mutant cells failed to aggregate under starvation conditions and exhibited defective migration toward a cAMP point source. The cells in which PLA 2 A or PI3K signaling was individually disrupted did not show such migration defects unless the parallel pathway was inhibited pharmacologically--the plaA − cells were defective in chemotaxis in the presence of low concentrations of LY294002, and the pi3k 1 − /2 − cells were defective in chemotaxis in the presence of the PLA 2 inhibitor bromoenol lactone. PLA 2 A activity was not required for proper PI(3,4,5)P 3 dynamics based on monitoring PI(3,4,5)P 3 accumulation with a pleckstrin homology domain green fluorescent protein fusion protein. Lipid analysis of wild-type and plaA − cells revealed that the chemoattractant-stimulated plaA − cells had a greatly reduced accumulation of an arachidonic acid derivative that was abundant in the chemoattractant-stimulated wild-type cells. Thus, there appear to be two parallel paths contributing to chemotaxis of Dictyostelium , one reliant on PI3K signaling and one reliant on PLA 2 signaling. L. Chen, M. Iijima, M. Tang, M. A. Landree, Y. E. Huang, Y. Xiong, P. A. Iglesias, P. N. Devreotes, PLA 2 and PI3K/PTEN pathways act in parallel to mediate chemotaxis. Dev. Cell 12 , 603-614 (2007). [PubMed]