Cell migration is a highly orchestrated process that involves intricate reorganization of the cytoskeleton to drive membrane protrusion and retraction. Rho GTPases are well established regulators of the cytoskeleton during migration, but the precise mechanisms that regulate the formation of membrane protrusions at the cell's front and the retraction at the cell's rear are still unclear. In a new study in the Journal of Biological Chemistry, Sastry and colleagues find that the cytosolic protein tyrosine phosphatase PTP-PEST is involved in both membrane protrusion and tail retraction by regulating the activity of Rac1 and RhoA through VAV2 and p190RhoGAP.

Previous work from this laboratory has shown that PTP-PEST overexpression suppresses cell motility by inhibiting Rac1, thus impairing membrane protrusion. Now, the authors examine the behaviour of PTP-PEST-null fibroblasts plated on fibronectin. Not only do these cells form exaggerated lamellipodia at the leading edge, consistent with the idea that PTP-PEST regulates membrane protrusions, they also develop long thin tails that are unable to detach from the substrate. Biochemical analysis reveals that while Rac1 activity is enhanced, RhoA activity, which is crucial for tail retraction, is suppressed. Interestingly, expression of either a dominant negative Rac1 or a constitutively active RhoA, rescued the PTP-PEST-null phenotype.

Using a substrate trapping assay, Sastry and colleagues found that the guanine nucleotide-exchange factor, VAV2, is a direct target of PTP-PEST and show that the levels of tyrosine phosphorylated VAV2 are higher in PTP-PEST-null cells compared with null-cells re-expressing PTP-PEST. The authors took advantage of the higher binding affinity of active GEFs for nucleotide-free Rho proteins to indirectly measure VAV2 activity, and found that higher levels of VAV2 were present when PTP-PEST was absent. Furthermore, expression of a dominant negative form of VAV2 is able to rescue the PTP-PEST phenotype, supporting the idea that PTP-PEST negatively regulates VAV2 activity.

The authors also identify the negative regulator of RhoA activity, p190RhoGAP as a PTP-PEST substrate. Again, higher levels of tyrosine phosphorylated active p190RhoGAP were found in the PTP-PEST-null cells and expression of an inactive form of p190RhoGAP restored the effect of PTP-PEST deletion on cell morphology.

The authors suggest a model whereby PTP-PEST keeps membrane protrusion under control, inhibiting the activation of Rac1 by VAV2, while also facilitating tail retraction by inhibiting p190RhoGAP and therefore RhoA activity. However, as expression of inactive forms of either VAV2 or p190RhoGAP restores the effects of PTP-PEST deletion on both membrane protrusion and tail retraction these pathways may not be completely independent. To gain a better understanding of how PTP-PEST functions in migration, it will be interesting to examine how PTP-PEST modulates the local activation of Rho GTPases as well as the contribution of other PTP-PEST substrates to its effects on membrane protrusion and tail retraction.

Author: Monica Hoyos-Flight