Drosophila macrophages have a microtubule “arm” that points them in the right direction and pushes them away from their fellow leukocytes, Stramer et al. reveal.
Fly macrophages disperse themselves around the body during embryogenesis, ready to mount an immune response at the site of a wound. These cells can be observed relatively easily using confocal microscopy, so Stramer et al. developed a fluorescent probe to study the cells' microtubule dynamics as they migrated in living embryos.
The researchers saw that macrophages bundled their microtubules into an arm that pointed to the leading edge of each cell. Wounding the embryos caused macrophages to turn their arms toward the damage before the rest of the cell followed suit, suggesting that the bundles help macrophages polarize and migrate in the direction of their target. The arms also allow the cells to move away from each other: when two macrophages collided, their arms briefly aligned and then collapsed, spurring the cells to retreat in opposite directions. Removing the microtubule arm by expressing the filament-severing protein Spastin or removing the microtubule-stabilizing protein Orbit blocked macrophages' ability to repel each other. The cells still moved but they clumped together instead of dispersing throughout the embryo.
Lead author Brian Stramer now plans to screen for other proteins involved in the process to understand how the microtubule arms sense and redirect colliding macrophages. He also notes that these structures don't exist in cell culture, highlighting the importance of observing cell migration in vivo.