UCSF

Cells must migrate for neurons to make connections with distant partners, immune cells to find their prey and cancer cells to metastasize. Actin filament assembly at the leading edge and actomyosin contraction at the trailing edge cause protrusion and retraction, respectively, which enable most human cells to crawl through their environment. We do not understand how the cell spatially restricts the signals promoting actin assembly to the leading edge.

In my first project, I described the polarization behavior of an essential actin assembly signal (WAVE complex) in response to external chemical cues (chemoattractants). At that time, there were contradictory theories for the establishment of polarized signals. One theory proposed that the signals initiate throughout the cell and then get carved down to just the leading edge. A competing theory proposed that the signals only initiate at the leading edge. We found that both modes of polarization can occur, depending on the circumstances. At high chemoattractant concentrations, WAVE signaling initiates uniformly and subsequently polarizes. At lower concentrations, WAVE signaling was polarized from the outset. Polarized initiation even occurred in response to uniform chemoatractant, although gradients made it more likely.

In my second project, I investigated the spatial restriction mechanism itself. Numerous theoretical studies proposed that the signals at the front limit themselves either by depleting essential signaling components via diffusion and capture or by producing inhibitory molecules which diffuse throughout the cell. I used cell-severing experiments and severe morphological perturbations to demonstrate that the leading edge produces an inhibitory signal that operates when the cell morphology reduces diffusion by 1000-fold. We suspected that tension in the membrane or cytoskeleton provides an inhibitory signal that does not spread via diffusion. Consistent with this, we found that tension in the membrane increases during polarization. Furthermore, we demonstrated that artificial tension increases, induced by micropipette suction, inhibited WAVE signaling throughout the cell. Finally, artificially reducing membrane tension with hyperosmotic buffer expands signaling throughout the cell leading to uniform protrusion. Thus, membrane tension spatially confines signals to the leading edge to enable migration.