During cell migration, Rho GTPases spontaneously form spatial gradients that define the front and back of cells. At the front, active Cdc42 forms a steep gradient whereas active Rac1 forms a more extended pattern peaking a few microns away. What are the mechanisms shaping these gradients, and what is the functional role of the shape of these gradients? Here we report, using a combination of optogenetics and micropatterning, that Cdc42 and Rac1 gradients are set by spatial patterns of activators and deactivators and not directly by transport mechanisms. Cdc42 simply follows the distribution of Guanine nucleotide Exchange Factors, whereas Rac1 shaping requires the activity of a GTPase-Activating Protein, β2-chimaerin, which is sharply localized at the tip of the cell through feedbacks from Cdc42 and Rac1. Functionally, the spatial extent of Rho GTPases gradients governs cell migration, a sharp Cdc42 gradient maximizes directionality while an extended Rac1 gradient controls the speed.

Many studies have investigated the processes that support polarity establishment and maintenance in cells. On the one hand, polarity complexes at the cell cortex and their downstream signaling pathways have been assigned as major regulators of polarity. On the other hand, intracellular organelles and their polarized trafficking routes have emerged as important components of polarity. In this Review, we argue that rather than trying to identify the prime ‘culprit’, now it is time to consider all these players as a collective. We highlight that understanding the intimate coordination between the polarized cell cortex and the intracellular compass that is defined by organelle positioning is essential to capture the concept of polarity. After briefly reviewing how polarity emerges from a dynamic maintenance of cellular asymmetries, we highlight how intracellular organelles and their associated trafficking routes provide diverse feedback for dynamic cell polarity maintenance. We argue that the asymmetric organelle compass is an indispensable element of the polarity network.

Migrating cells present a variety of paths, from random to highly directional ones. While random movement can be explained by basal intrinsic activity, persistent movement requires stable polarization. Here, we quantitatively address emergence of persistent migration in RPE1 cells over long timescales. By live-cell imaging and dynamic micropatterning, we demonstrate that the Nucleus-Golgi axis aligns with direction of migration leading to efficient cell movement. We show that polarized trafficking is directed towards protrusions with a 20 min delay, and that migration becomes random after disrupting internal cell organization. Eventually, we prove that localized optogenetic Cdc42 activation orients the Nucleus-Golgi axis. Our work suggests that polarized trafficking stabilizes the protrusive activity of the cell, while protrusive activity orients this polarity axis, leading to persistent cell migration. Using a minimal physical model, we show that this feedback is sufficient to recapitulate the quantitative properties of cell migration in the timescale of hours.

During migration, cells present a polarized activity that is aligned with the direction of motion. This cell polarity is established by an internal molecular circuitry, without the requirement of extracellular cues. At the heart of this circuitry, Rho GTPases spontaneously form spatial gradients that define the front and back of migrating cells. At the front of the cell, active Cdc42 forms a steep gradient whereas active Rac1 forms a more extended pattern peaking a few microns away from the cell tip. What are the mechanisms shaping these gradients, and what is the functional role of the shape of these gradients? Combining optogenetics and cell micopatterning, we show that Cdc42 and Rac1 gradients are set by spatial patterns of activators and deactivators and not directly by advection or diffusion mechanisms. Cdc42 simply follows the distribution of GEFs thanks to a uniform GAP activity, whereas Rac1 shaping requires the activity of an additional GAP, β2-chimaerin, which is sharply localized at the tip of the cell. We find that β2-chimaerin recruitment depends on feedbacks from Cdc42 and Rac1. Functionally, the extent -neither the slope nor the amplitude-of RhoGTPases gradients governs cell migration. A Cdc42 gradient with a short spatial extent is required to maximize directionality during cell migration while an extended Rac1 gradient controls the speed of the cell.

Migrating cells present a variety of paths, from non-persistent random walks to highly directional trajectories. While random movement can be easily explained by an intrinsic basal activity of the cell, persistent movement requires the cell to be stably polarized. It remains unclear how this is achieved from the regulation of underlying subcellular processes. In the context of mesenchymal migration, the ability of cells to migrate persistently over several hours require a mechanism stabilizing their protruding activity at their front. Here, we address this mechanism using human RPE1 cell line as our model. We measure, manipulate, and quantitatively perturb cell protrusive activity of the cortex as well as intracellular organization of the endomembrane trafficking system using dynamic micropatterning, pharmacological and trafficking assays, optogenetics and live-cell imaging with tracking. First, we demonstrate that the Nucleus-Golgi axis aligns with the direction of migration and its alignment with the protrusive activity leads to efficient cell movement. Then, using low doses of Nocodazole to disrupt internal cell organization, we show that long-lived polarity breaks down and migration becomes random. Next, we indicate that a flow of vesicles is directed towards the protrusive activity with a delay of 20 min. Eventually, by applying a sustained optogenetic activation, we prove that a localized Cdc42 gradient is able to orient the Nucleus-Golgi axis over a couple of hours. Taken together, our results suggest that the internal polarity axis, provided by the polarized trafficking of vesicles, is stabilizing the protrusive activity of the cell, while the protrusive activity biases this polarity axis. Using a novel minimal physical model, we show that this feedback is sufficient by itself to recapitulate the quantitative properties of cell migration in the timescale of hours. Our work highlights the importance of the coupling between high-level cellular functions in stabilizing the direction of migration over long timescales.

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