Poking and Prodding a Living Cell

The atomic force microscope (AFM) has largely been a physicists' tool, scanning atomic landscapes with its ultrafine probe. But today a group of scientists from Johns Hopkins University announced at the American Heart Association meeting in New Orleans that they have put the AFM to work in biomedicine. They used it to poke and prod living heart cells to find their resilient spots, much as a doctor palpates a patient's abdomen.

Ordinarily an AFM images the bumps and hollows of a surface by recording the deflections of its stylus. But the Hopkins group, which included force microscopist Jan Hoh, probed their cells with an AFM designed to work in reverse. Instead of passively recording forces, its stylus actively presses down, recording how hard the cell presses back. That enabled the researchers to map variations in the resilience of the cell in half-micrometer squares--each one less than a thousandth of the area of the cell.

Such fine-scale prodding could help researchers test the strength with which proteins and other molecules bind to the cell membrane, for example. ``What's new is the application of [force microscopy] to the nanomechanical properties of biological structure,'' says Sergei Magonov, a staff scientist at Digital Instruments, a company that makes force microscopes.

As a first demonstration, the Hopkins group examined cells before and after treating them with a compound that inhibits the polymerization of a protein called actin. After the treatment, they found that the cells became uniformly soft--the first quantitative evidence that actin forms the ``skeleton'' that gives cells their structure and enables them to contract and move, say the researchers.