Sopping up chemokine

A ctin fi laments lie in a wide range of orientations in the leading edge of a migrating cell, according to images from Stefan Koestler, Victor Small (Austrian Academy of Sciences, Vienna, Austria), and colleagues. What looks like disorder might help organize both protrusion at the front and retraction behind. The work casts doubt on the textbook view of the network in the very front of a migrating cell, known as the lamellipod. In this view, the actin fi laments branch off one another at consistent 70° angles. Small’s group has been questioning this model for years, worried that fi xation techniques might introduce branch-like artifacts by causing the fi laments to collapse. In the new work, the authors tried to avoid these problems by using a stronger fi xative and drying the samples directly in a negative stain. Live cells were fi rst viewed using light microscopy to determine whether lamellipodia were protruding, pausing, or retracting. They were then immediately fi xed and examined by EM. Protruding lamellipodia, the images revealed, contained actin fi laments that hit the plasma membrane at a wide range of angles, varying from 15° to 90°. During pauses, more fi laments aligned more nearly parallel to the membrane. Small thinks the rearrangement comes naturally as the cell front slows its forward movement. “As cells slow, some actin fi laments stop growing, but others are still polymerizing. Those fi laments have to change orientation,” he says, since their growing plus ends are tracking along the membrane. The change might create an organization of fi laments that maintains the retracting cell edge by lying parallel to it. It might also help construct the cytoskeleton just behind the lamellipod, in the lamella, where contractile bundles are built from myosin and antiparallel actin arrays. In an upcoming JCB paper, Small and colleagues show that actin bundles from fi lopodia also contribute to contractile bundles in the lamella. “What we’re saying,” Small explains, “is that lamellipodia and fi lopodia are fi lament factories not only for protrusion but also for constructing the cytoskeleton behind. One way to get antiparallel fi laments in the lamella is by reorienting those at the front.” Although Small hopes to change how people envision the lamellipodial actin array, he knows more work is yet to be done. “We haven’t disproven branching yet,” he says. “We’ll need 3D imaging to put the nail in that coffi n. But what we’ve seen makes the branching model unlikely.”