To migrate rapidly, many cells, including immune cells and invasive cancers, adopt a polarized amoeboid morphology. In this morphology, sustained forward motion requires the coordination of two actin networks: polymerizing branched actin at the front and contractile actomyosin at the back. Their assembly is guided by signaling molecules like activated Ras-GTPase and PI(3,4,5)P3 at the front and PI(4,5)P2. Yet, while these molecules turn over within seconds, cells maintain processive motion for minutes to hours. Understanding the principles that underlie this network is key to explaining how cells can migrate long distances in complex environments. We recently showed that front and back actin networks locally regulate signaling: branched actin activates Ras at the front, while actomyosin suppresses Ras at the rear. This feedback alone is sufficient to generate and maintain polarity. To understand the source and nature of cytoskeletal feedback on signaling, we engineered tools to manipulate and monitor actin mechanics. Expressing a series of synthetic actin crosslinkers dramatically polarizes cells and locally suppresses the activation of cell front markers like Ras. These effects on cell polarity and signaling are dependent on crosslinker properties and can be tuned by altering linker length. Additionally, we can directly map where and when actin networks come under strain using protein domains that bind specifically to compressed and tensed actin filaments. Interestingly, we determined that actin stress travels as a propagating wave across the cell membrane, closely correlated in time and space with branched actin nucleation and Ras activation. Together, our results uncover key biophysical differences in the actin populations at the cell front and back and demonstrate that these differences control signaling activity. This feedback may help explain how cells detect and negotiate obstacles in complicated tissue environments.
Kuhn et al. (Sun,) studied this question.