Skeletons in Space

"The cytoskeleton perceives gravity-or any force- through special proteins known as integrins, which poke through the cell's surface membrane," explains Ingber. Inside the cell, they're hooked to the cytoskeleton. Outside, they latch onto a framework known as the extracellular matrix - a fibrous scaffolding to which cells are anchored in our bodies.

Ingber and his colleagues have shown that when integrins move, the cytoskeleton stiffens. They did it by coating small magnetic beads, about 1 to 10 microns in size, with special molecules that bind to integrins. They attached the beads to the integrins and then applied a magnetic field.

"The beads turned and tried to align with the field, just like a compass needle would want to align with the earth's magnetic field," explains Ingber. The beads twisted the integrins and, in turn, tweaked the cytoskeleton. As more stress was applied, the cytoskeleton became stiffer and stiffer. In fact, it become so stiff that the beads couldn't be turned much past a few degrees!

Tugging on integrins not only caused the cytoskeleton to stiffen, it also activated certain genes. "Activating a gene" means coaxing a gene to generate RNA and proteins. That's important because proteins are little messages that signal the cell to take action. Tickling the cytoskeleton, it seems, can make cells switch between different genetic programs.

Even before the magnetic bead experiment, Ingber's group at Harvard had already discovered a link between cell geometry and cell behaviour. In one experiment they forced living cells to take on different shapes - spherical or flattened, square or round - by placing them on tiny adhesive islands of extracellular matrix. Cells that were flat and stretched tended to divide. Cells that were round and cramped tended to die.