Oxidation reactions are central to cellular metabolism, but when their balance is offset by excess free radicals extensive cellular damage can result. By keeping a tight control over oxidative modifications, cells are able to efficiently regulate signalling pathways involved in protein folding and cell proliferation. A study in the Journal of Cell Biology now shows that by localizing oxidation at the leading edge, cells can also improve their ability to migrate.

Wu and colleagues found that the nicotinamide adenine dinucleotide phosphate reduced (NADPH) oxidase subunit p47^phox binds to the orphan adaptor TRAF4 at the leading edge of migrating endothelial cells. Direct binding of TRAF4 and the paxillin paralogue Hic-5 seems to be responsible for targeting oxidase activity to focal complexes in nascent lamellar protrusions. RNAi-mediated knockdown of TRAF4 or Hic-5, disruption of the TRAF-Hic-5 interaction, or treatment with an oxidant scavenging agent, all inhibit cell migration, indicating that localized oxidation is important for cell motility.

To determine the mechanism through which these proteins regulate cell migration the effects of an active TRAF mutant (Myr-TRAF4) were analysed. Myr-TRAF4 activated RhoGTPases and the effector kinase PAK1. Using serine phosphorylation of p47^phox as a read out of oxidase activation, the authors show that Myr-TRAF4 activates NADPH oxidase by signalling through a Rac1 and PAK1- dependent pathway.

So, are any focal complex proteins are oxidized during cell migration? The authors found that Myr-TRAF4 leads to the oxidation and inactivation of the focal complex tyrosine phosphatase PTP-PEST. Interestingly, oxidized PTP-PEST can activate Rac1, which in turn activates NADPH oxidase, thus forming a positive feedback loop. Finally the authors show that localized oxidation causes robust membrane ruffling. This effect depends on Rac1, PAK1 and PTP-PEST oxidation. The mechanism through which PTP-PEST increases membrane ruffling is likely to involve hyperactivation of its targets: Src and Pyk2.

This study not only highlights a role for localized oxidation during cell migration, it also reveals that TRAF4 is important for restricting oxidative reactions to critical sites; this may help to explain the abnormalities in cell migration observed in TRAF4 mutant mice.

Author: Monica Hoyos-Flight