How Actin Networks Overcome Obstacles

Researchers have gained new insights into how the fibrous scaffolding within our cells, which is made up of the protein actin, responds to obstacles and barriers in its environment.

The above image shows organelles and an actin network inside an immune cell known as a macrophage as it pursues bacterial invaders.

Scientists have known that actin networks, unlike many other cell components, alter their growth in response to forces, not just chemical signals. Among other responsibilities, actin provides the structural support for cells and the growth force necessary for certain cellular activities.

Daniel Fletcher, a National Science Foundation researcher from the University of California, Berkeley, and colleagues hypothesized that when faced with a barrier, the elongating filaments in a growing actin network adds more branches to counter the resistance. When the barrier is removed, the added filaments remain and the network grows at a new, faster rate.

To track the growth rate and force generated by actin, the bioengineers modified an atomic force microscope (AFM), an instrument that can manipulate single atoms.

The researchers used the AFM to apply a force to a growing actin network, then applied a larger force for as long as 30 minutes. They then applied the original force again, at every stage tracking how fast the network grew.

They found that when the force was ratcheted back down, the actin filaments grew faster, and when the experiment was repeated with multiple forces, the network grew at a rate that was dependent upon all of the forces it had previously experienced.

This means the structure of a cell has some "memory" of its physical interactions, Fletcher said.

"For a given load, proteins assume a certain network architecture," he said. "This architecture then remodels under a new load. So, if you go back to the original load, the architecture is still tuned for a higher load, resulting in explosive growth."

The finding was detailed in the December issue of the journal Nature Cell Biology.