Lipids: Cofilin set free

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EGF signaling reduces PtdIns(4,5)P₂ levels, which relieves cofilin from PtdIns(4,5)P₂-mediated sequestration at the plasma membrane and permits its relocalization to the leading edge of newly formed lamellipodia.

Epidermal growth factor (EGF) stimulates both chemotaxis and the metastatic activity of mammary carcinoma cells. It regulates actin polymerization and promotes cellular protrusion by activating cofilin — an effect known to require phospholipase C (PLC). In the Journal of Cell Biology, Jacco van Rheenen et al. now report that in rat MTLn3 carcinoma cells, cofilin is activated by its release from the plasma membrane as a result of reduced levels of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂).


PtdIns(4,5)P₂ (green) and cofilin (red) are associated in vivo in a compartment at the plasma membrane that releases active cofilin in response to stimulation of EGF receptors.

Using a fluorescence resonance energy transfer (FRET)-based PtdIns(4,5)P₂ assay, the authors first showed that EGF stimulation led to an increase in cell protrusion, which was accompanied by a substantial drop in PtdIns(4,5)P₂ levels. Both cell protrusion and PtdIns(4,5)P₂ hydrolysis were blocked on PLC inhibition, suggesting that EGF-induced protrusion may result from PLC-catalysed hydrolysis of PtdIns(4,5)P₂.

The authors found that non-phosphorylated cofilin colocalized with PtdIns(4,5)P₂ at the plasma membrane in fixed cells, but that following a reduction in PtdIns(4,5)P₂ levels, cofilin became activated and enriched at the leading edge of newly formed lamellipodia. When PLC activity was blocked, EGF was unable to induce the drop in PtdIns(4,5)P₂ levels and cofilin remained at the plasma membrane. These data suggest that non-phosphorylated cofilin is sequestered and inactivated at the plasma membrane by PtdIns(4,5)P₂, and that EGF-induced PtdIns(4,5)P₂ hydrolysis causes the release and activation of cofilin, which then translocates to the filamentous (F)-actin compartment.

Using fluorescence imaging approaches, the authors observed the rapid translocation of cofilin from the plasma membrane that was dependent on PLC-mediated PtdIns(4,5)P₂ hydrolysis. Cofilin was also released from the membrane on PLC-independent PtdIns(4,5)P₂ reduction.

They further showed that endogenous cofilin binds to actin filaments shortly after EGF stimulation in fixed MTLn3 cells. Binding was reduced on PLC inhibition, indicating that F-actin-bound cofilin originated from the plasma membrane. Local application of EGF resulted in a local translocation of cofilin from the plasma membrane compartment to the F-actin compartment. Finally, the authors found that the actin-severing activity of cofilin increased after EGF treatment or PtdIns(4,5)P₂ reduction.

Taken together, this work provides the first in vivo evidence that cofilin is locally activated on release from the plasma membrane as a result of a reduction in PtdIns(4,5)P₂ levels. Free active non-phosphorylated cofilin can then rapidly translocate to the cell front where it binds and severs F-actin to promote cell protrusion.

Editor, Cell Migration Gateway

Kim Baumann

References

van Rheenen, J. et al. EGF-induced PIP₂ hydrolysis releases and activates cofilin locally in carcinoma cells. J. Cell Biol. 179, 1247–1259 (2007) | Article | PubMed |