Photoactivation of Rac in a single cell is sufficient to drive collective migration of a group of cells.
The image shows high magnification view of border cells. Photoactivatable Rac1 labelled with mCherry (red), actin filaments labelled with Alexa 488-phalloidin (green), and nuclei labelled with DAPI (blue). From Nature Cell Biology, 12, 591-597 (2010)
Collective cell migration is important during morphogenesis and in tumour metastasis. In Drosophila migratory border cells, the small GTPase Rac controls collective cell migration in vivo in response to guidance signals through platelet-derived growth factor/vascular endothelial growth factor receptor (PVR) and the epidermal growth factor receptor (EGFR). In Nature Cell Biology, Denise Montell and colleagues now report that localized activation of Rac in a single cell is sufficient for collective migration of cells in vivo and that guidance signals finely tune Rac activity, whereas Jun N-terminal kinase (JNK) signalling maintains communication between the cells.
Using transgenic flies with fluorescently labelled photoactivatable Rac, the authors first show that local activation of Rac in a single cell next to the leading cell is sufficient to redirect migration of the rest of the cells in a cluster. Photoactivation of Rac at the rear was also sufficient to stop forward protrusion and promote a slower rearward movement. Furthermore, the cells recovered forward movement when illumination was halted, showing that Rac activity is sufficient to polarize a group of cells and guide their collective movement.
But, why was Rac-induced rearward movement slower than normal? Experiments in border cells expressing dominant-negative forms of PVR and EGFR showed that in the absence of guidance signals, random protrusions formed in all directions and the cells moved very little. Photoactivation of Rac rescued these defects and restored direction sensing. Interestingly, illuminating the rescued cells at the front or the rear resulted in forward or reverse migration, respectively, now with similar speeds. Moreover, they could not recover forward movement when rear illumination stopped. This suggests that the guidance signals normally produce directional Rac activity. Consistently, time-lapse imaging of a Rac FRET biosensor revealed that Rac activity is normally asymmetric in border cells — highest at the front and lowest at the back — and this asymmetry was lost in the absence of PVR and EGFR.
Next, they tested what happens when Rac is locally inhibited. Photoinactivation of Rac in the leading cell stopped forward migration and promoted rearward protrusion, whereas photoinhibition of Rac at the rear enhanced forward protrusion. High magnification images revealed that local photoactivation or photoinactivation of Rac in a single cell is sufficient to cause morphological changes of all cells in the cluster.
So, what is the signalling mechanism downstream of Rac? As JNK signalling controls directional protrusions and collective movement of border cells, they tested whether JNK is important for the effects of Rac. Rac activity was sufficient to extend protrusions in the direction of light even in the absence of JNK signalling; however, retraction of protrusions in other directions was inhibited when JNK activity was reduced. This indicates that JNK is not required downstream of Rac to promote protrusion, but maintains the communication from the cell with highest Rac activity to other cells within the cluster.
Together, this study identifies a key role for Rac and JNK in collective cell migration, which has implications for tumour metastasis. It also demonstrates the power of using light-controlled activation of a Rho GTPase to control migration in vivo.
Original Research Paper
- Wang X. , He L. , Wu Y.I. , Hahn K.M & Montell D.J. Light-mediated activation reveals a key role for Rac in collective guidance of cell movement in vivo. Nature Cell Biology, 12, 591–597 (2010).
doi:10.1038/ncb2061 | Article