Invadopodia are proteolytic actin-rich protrusions involved in cancer cell invasion. Condeelis and colleagues now identify a pH-dependent regulatory step in invadopodia maturation (J. Cell Biol. 28, 903–920; 2011).

The binding of cortactin to cofilin is known to inhibit cofilin's actin-severing activity, and cortactin phosphorylation releases this inhibition in a cycle that is essential for invadopodia function. The authors used FRET analysis of invadopodia to confirm that cortactin phosphorylation correlates with reduced interaction between the proteins. However, phosphorylation of cortactin did not affect this interaction in vitro; it was instead reduced by an increase in buffer pH. The authors found the pH enhanced in mature invadopodia in direct correlation with cortactin intensity, and cells stable at high pH displayed increased amounts of free barbed actin ends at invadopodia.

In accordance with a role for pH in invadopodium formation, interaction of the sodium hydrogen exchanger NHE1 with cortactin was promoted by cortactin phosphorylation, and NHE1 depletion influenced the cortactin–cofilin interaction at invadopodia. Finally, 3D invasion assays revealed the requirement for cortactin phosphorylation, cofilin and NHE1 in the formation of long invadopodia, and for a dynamic invadopodia protrusion–retraction cycle.

Thus, the authors propose that cortactin phosphorylation recruits NHE1 to invadopodia, where a local increase in pH releases cofilin to activate actin barbed-end generation for invadopodia elongation.