Chemotaxis of cells involves the translation of an external gradient of attractants into directed motion; the process is often considered either globally, by assessing front-to-tail polarisation of the cell, or with a focus on local aspects, such as signalling within forming protrusions. In their work (p. 3462), Günther Gerisch and colleagues investigate the mechanisms of chemotaxis in oversized Dictyostelium cells that they generate by electro-pulse-induced fusion in order to challenge the process by increasing the cell length scale. Following application of a chemoattractant behind their tail to force the cells to reorient themselves, the authors analyse cell responses both locally, by visualising Ras activation at the front of the reorienting cells, and globally, by monitoring the actin flow from front to tail. They find that large cells tend to form multiple protrusions that often co-exist and independently respond to the stimulant before the cell eventually establishes protrusion at the new front and retraction at the tail. Interestingly, growing protrusions tend to turn into the direction of the gradient and show asymmetric Ras activation, arguing for a local mechanism of gradient sensing. Conversely, the establishment of global polarity is likely controlled by the flow of actin from the front of the cell to the tail. Taken together, this study presents a number of interesting new insights into chemotactic responses that also challenge some widely held views, thus contributing to further debate in the field.
- © 2016. Published by The Company of Biologists Ltd