Lipid signaling: LPA's ways of actin

Lipidomics Gateway (28 October 2009) [doi:10.1038/lipidmaps.2009.29]

Lysophosphatidic acid (LPA) controls actin dynamics indirectly, through cell surface receptor-mediated signaling, and directly, by binding and inhibiting villin.

Model of F-actin. For full figure and legend, see Oda et al. Nature 457, 441-445 (2009) doi:10.1038/nature07685

Rearrangement of the actin cytoskeleton is crucial for the control of cell morphology and migration. Actin-binding proteins regulate the polymerization of actin into filaments, and the cross-linking of these into bundles. LPA is known to induce cytoskeletal rearrangements, but the mechanisms involved are not well understood. Now, two studies in the Journal of Biological Chemistry show how LPA can influence actin dynamics in different ways. First, Masayuki Masu and colleagues report that in mouse embryos, extracellular LPA controls phosphorylation of an actin binding protein through a cell surface receptor and Rho GTPase signaling cascade, leading to the formation of specialized lysosomes ¹ . Meanwhile, Seema Khurana and colleagues used in vitro methods to show that LPA and phosphatidylinositol 4,5-bisphosphate (PIP₂) compete for binding sites in villin, an actin binding protein, with opposing effects on actin reorganization ² .

Extracellular LPA (See 'Lipid of the month') is produced by the secreted enzyme autotaxin, which is encoded by the Enpp2 gene. Defects, including failures in yolk sac angiogenesis, kill Enpp2^−/− mouse embryos before birth, but the cellular mechanisms involved are not well understood. Masu and colleagues used a whole embryo culture system to probe the role of autotaxin–LPA in specialized yolk sac cells. These visceral endoderm (VE) cells usually have distinctively large lysosomes, but these were fragmented in VE cells from Enpp2^−/− embryos.

Using pharmacological inhibitors and electroporation of dominant-negative constructs, the authors showed that the formation of large lysosomes in VE cells requires LPA receptor signaling, Rho GTPase and its kinase ROCK, and the downstream LIM kinase (LIMK). Cofilin, an actin binding protein that stimulates filament disassembly, is phosphorylated and inactivated by LIMK. The steady-state levels of cofilin phosphorylation were significantly decreased in Enpp2^−/− VE cells, and this correlated with a decrease in polymerized actin. Culturing the embryos with compounds to either disrupt or stabilize actin filaments confirmed that actin polymerization is involved in VE lysosome formation, and that dynamic regulation of actin turnover is required. Without autotaxin–LPA signaling to regulate cofilin, the balance of actin polymerization is lost and the lysosomal defects ensue.

Another actin-binding protein, villin, is specific to endothelial cells, where it regulates actin dynamics, cell morphology and migration. PIP₂ is known to bind villin and modify its actin regulatory functions. Khurana and colleagues found that LPA also binds villin in vitro, and kinetic analysis revealed that villin has a slightly higher affinity for LPA than for PIP₂. On binding, neither lipid induced global changes in the conformation of full-length villin, but each had a different effect on the secondary structure of villin peptides. This difference had functional consequences for full-length villin; c-Src kinase was unable to phosphorylate the recombinant protein whether PIP₂ was present or absent, but it was able to do so in the presence of LPA. In vitro, actin filaments are cross-linked into bundles by villin, but this did not occur in the presence of LPA. LPA also inhibited the actin capping function of villin, and its depolymerizing activity. PIP₂, by contrast, only inhibited the last function.

Whereas the study by Masu and colleagues identifies a pathway by which extracellular LPA modifies the actin cytoskeleton, in vivo binding of villin would involve intracellular LPA. The different cellular membranes have distinct phospholipid compositions, suggesting that LPA or PIP₂ binding might regulate villin localization as well as phosphorylation. Intracellular LPA production involves phospholipase enzymes, not autotaxin, and is more tightly regulated than extracellular production, although it is not certain that the two LPA pools are entirely separate. Nevertheless, control of actin dynamics from inside or outside the cell could be critical for many of the pathological functions of LPA.

Emma Leah