1. Cell migration is a highly regulated and complex mechanism that is essential for animal cell function.
2. External signaling cues converge on key signaling molecules like Rho and Rac to convert these signals into actin-dependent mechanical functions within the cell.
3. A growing number of cell migration mechanisms are described which are important in either individual or collective cell migration.
Cell migration, which is fundamental for life, is the directed movement of a cell or multiple cells in response to chemical, mechanical, physiological, and pathological signals(reviewed in 1, 2). Proper cell migration is essential for embryonic development, wound repair, tissue homeostasis, immune response, and many other processes. Conversely, dysfunctional migration contributes to diseases including cancer metastasis, inflammatory diseases, neurodegenerative diseases, cardiac diseases, and others(reviewed in 1, 2). Mesenchymal cell migration involving actin polymerization and adhesion-based mechanisms is the best characterized form of migration, but other migration mechanisms including amoeboid bleb-based, osmotic engine model, and nuclear piston models have also been described(reviewed in 1). Integrins, growth factors, cytokines, and other signaling molecules send cues that converge on signaling pathways to activate distinct modes of migration.
The Rho family of small GTPases, including RhoA, Cdc42, and Rac1, have all been shown to play an essential role in migration(reviewed in 2, 3). Broadly speaking, Rho GTPases are critical molecular switches that effectively transmit signaling to cytoskeletal activation to achieve proper cell migration4. Recent reports highlight Rac1’s role in several critical cell migration mechanisms which we discuss below.
While the process of cell migration is quite complex and can have distinct differences depending on the specific model, the process of mesenchymal migration can be divided into four general steps including the following: 1) cell polarization, 2) extension of the cell’s leading edge, 3) formation of new adhesions, and 4) retraction of the cell rear(reviewed in 2, 3, 5).
The polymerization step in particular is of great interest because it requires symmetry breaking whereby localized signaling occurs at the front versus back of the cell to prime it for movement. Rac1, Cdc42, and RhoA GTPases have been identified as key regulators of spatially patterning polarized cells during migration. In the mesenchymal migration model, Rac1 and Cdc42 are localized at the front of the cell and promote actin-based protrusions or lamellipodia structures. In a coordinated fashion, Rho is localized at the rear of the migrating cell, and it controls actomyosin contractility(reviewed in 3, 6, 7). In a recent study, disturbance of the active Rac1 gradient, which led to homogenous distribution across the cell, led to random lamellipodia extensions and dysfunctional directional migration8.
Localized regulation of Rac1 clearly occurs, with one example showing that the α4 integrin–paxillin–Arf-GAP complex driving Rac1 activation at the leading edge9, while in another study syndecan-4 and Tiam1 was necessary for proper active-Rac1 localization at the leading edge of migrating cells10. There is evidence that Rac and Rho can inhibit each other locally(reviewed in 6), which is an important mechanism to produce polarization.However, there is evidence that this alone is not sufficient to drive polarization and an emerging hypothesis is that long range crosstalk between Rac and Rho may also play a critical role11,12.
A recent study by De Belly et al., showed that there is long-range mutual activation between the back and the front of the cell to establish Rho and Rac polarity during cell migration13. Importantly, they identified a mechanism where the front stimulates the back of the cell via membrane tension, while the back utilizes cortical remodeling to convey signals to the front of the cell. This study provides evidence of long range communication between these small GTPases to ensure proper polarization form migration13.
Amoeboid bleb-based migration is characterized by hemispheric protrusions of plasma membranes, known as blebs, which are usually devoid of actin and often have weak adhesion properties(reviewed in 14). Bleb migration has been observed in many cell types, such as, embryonic cells, immune cells, and metastatic cancer cells.
Bleb-based migration also requires cell polarization as blebbing is often confined to the leading edge of the cells15. Several biochemical markers are present at either the front or rear of polarized bleb-based migrating cells(reviewed in 14). Importantly active Rac has been identified at the cell front, and was shown to restrict bleb formation to the leading edge16, 17. In one example bleb growth was controlled by adhesion induced Rac1 activity18, while in another instance, Rac1-activity appeared to drive bleb-inhibiting proteins to the rear of the cell19. In a third case, active Rac1 promoted bleb formation at the front of the cell in the absence of adhesins or actin regulators16.
There is evidence that extracellular signaling cues not only activates mesenchymal migration, but can also stimulate bleb-based migration(reviewed in 1, 14). In a recent study Jha et al., found that EGF signaling, via a PI3K-dependent mechanism, can maintain polarized leader blebs20.
Remarkably, they found that EGFR activities promoted Rac1 accumulation at the bleb rear, while RhoA-GTP concentrated at the bleb tip20. Precise disruption via optogenetic tools resulted in bleb retraction further implicating Rac1 in bleb-based migration. As many of these signaling pathways are involved in both types of migration, it will be interesting to define the critical signaling mechanisms that dictate which type of migration the cell chooses.
Collective cell migration has also emerged as a critical means of cell movement because understanding how multiple cells coordinate movement, as would occur in a developing or regenerating tissue, is of significant importance(reviewed in 5, 21, 22).
Coordinated cell migration, which is where a collective group of cells polarize from the front to the back of the collective to produce net movement in a common direction, is essential for many processes including wound healing, cancer metastasis, and organogenesis21. In many cases of collective cell migration, leader cells establish at the migration front and provide guidance to follower cells23-25. There has been important findings in the biochemical and mechanical signals that contribute to leader cell establishment and activation and has been reviewed in detail by Cheung and Horne-Badovinac21.
All boarder cells in theory have the ability to become leader cells , but it was shown through precise photoactivable Rac1, that cells with the highest Rac1 activity is sufficient to become leader cells and steer the collective group26. Physiologically, it was shown that collective cell migration is guided by external growth factor signaling like EGFR, which establish Rac1 gradients in leader cells27, 28. Additionally, it was found that a Ras-Rab5-Rac1 positive feedback loop localized at the front of the cell further amplifies Rac1 levels to promote collective cell polarization and migration29.
In a recent report, a group showed that leader cells displayed peripheral accumulation of lysosomes via microtubule directed movement30. Lysosomes controlled leader cell emergence through associating with Rac1 to control local Rac1 activity and actin polymerization. Interestingly, Rac1 also contributes to other parts of collective cell migration, as a Scribble-Cdcp-Rac1 pathway was shown to direct follower cell crawling and cohesion that mediated the collective movement31. Recently, Zhou et al., identified two switchable Rac1 pools at border cell protrusions and supracellular cables that require coordination for collective cell migration32. Collectively it is clear that Rac1 plays an integral role in collective cell migration, but further research is uncovering its multitude of functions in this key process.
Rac has an essential function in cell migration, and the studies above suggest that it may be more pervasive than was even realized. it is important to note that while we highlighted Rac1’s role in these mechanisms of cell migration as distinct processes, most cells can switch between mesenchymal versus bleb-bases migration, and the mechanisms that allow these types of switches to occur are being investigated.
Similarly, modification of a cell’s migration path in response to signaling cues also occurs, where a new leading edge in response to external signaling cues occurs. Recent work found that periodic pulses of Rac1 activity allowed for transitions between these run and tumble effects, and importantly, they found apolar blebbing during tumbling phases erased the memory of the previous front-back polarity of the migrating cell33.
Collectively, these results suggest that Rac is indispensable for cell migration, and additional research defining its precise spatial and temporal role to control these distinct, yet interconnected forms of migration will be invaluable.
Rac1 protein: His tagged: human wild type (Cat # RC01)
Rac1 Pull-down Activation Assay Biochem Kit (bead pull-down format) (Cat # BK035)
Rac1,2,3 G-LISA GTPase Activation Assay (Colorimetric format) 96 assays (Cat # BK125)
Rho/Rac/Cdc42 Activator I (Cat # CN04)
Anti-Rac1: mouse Mab (Cat # ARC03)
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