| Protrusion formation at the cell leading edge is an essential step during cell migration. Although most studies focus on actin-filled protrusions such as lamellipodia and filopodia, increasing evidence points to the importance of spherical membrane protrusions called blebs. Blebs are initially devoid of actin and their growth is driven by intracellular pressure generated in the cytoplasm by the contractile actin cortex. Blebbing migration appears to be a common alternative to lamellipodia-based locomotion, particularly in vivo and in 3-dimensional environments. In spite of their importance, very little is known about the mechanisms of bleb formation and of bleb-based migration. I will first present our findings on the mechanics of bleb expansion. I will then describe two experimental systems where we study how the type of protrusion formed by a migrating cell is controlled: 1/ Migrating mesendoderm progenitor cells during zebrafish gastrulation. We could show that these cells migrate in vivo using a combination of blebs, lamellipodia and filopodia. We could then demonstrate that membrane-to-cortex attachment plays a central role in controlling the relative proportions of the different protrusion types, which are in turn critical for directed migration. 2/ Cultured Walker carcinosarcoma cells, which can be induced to form either blebs or lamellipodia. We use these cells as a model system to explore the mechanical and molecular requirements for bleb and lamellipodia formation. |
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