Breaking Down is Hard

 

Breaking down bone is a tough job. Yet our bones undergo remodeling every day of our lives, and old material must be cleared away so that new bone can form. The heavyweights of the breakdown team are cells called osteoclasts that specialize in digesting bone. In diseases such as osteoporosis, an imbalance in this process is responsible for the characteristic bone loss.

 

Osteoclasts have some unique features. They move around the bone until they sense that their services are required, at which point they undergo a transformation called polarization. The polarized osteoclast sticks itself tightly to the bone, while an impermeable ring forms around the cell perimeter. This ring functions to keep the bone-eating acids and enzymes produced between the cell and the bone confined to the demolition site. New research at the Weizmann Institute of Science, which recently appeared in the on-line journal PLoS ONE, has revealed, in unprecedented detail, how these roving, bone-dissolving cells seal off their work area as they get down to business.

 

Prof. Benjamin Geiger, Dean of Biology, and Prof. Lia Addadi of the Structural Biology Department, together with doctoral students Chen Luxenburg and Dafna Geblinger, and with the assistance of Dr. Eugenia Klein of the Electron Microscopy Unit, and Prof. Dorit Hanein and Karen Anderson of the Burnham Institute, San Diego, applied two different observation methods to samples of stripped-down, polarized osteoclasts: electron microscope imaging that allowed them to see fine details of the ring structure, and a light microscope method in which specific features were induced to glow. Because each method captures a different type of information and on a different scale, combining them was tricky, but the two together gave them a uniquely extensive picture. 

 

The team found that the ring is composed of dot-like structures called podosomes anchored to the cell membrane. When the osteoclast is on the move, these little dots amble randomly around the cell, but when the cell prepares to dissolve bone, they make a beeline for the edge. Scientists had been unsure of the podosomes’ role in ring formation. The research team’s findings showed clearly that the ring is made up of individual podosomes held together by interconnecting protein filaments they throw out to each other. “The podosomes are like folk-dancers,” says Geiger. “As soon as the music starts, they join hands and form a tight circle. From afar, a circle of dancers looks like a blur, but now we have managed to make out the individual dancers.”

 

Addadi points out that isolated podosomes look, from above, like big-top tents with “ropes” radiating out from a central pole. She says: “The podosomes may be more than just seals. They appear to act as highly connected nodes of communication between the inside and outside of the cell, enabling the cell to adjust its activity according to the condition of the bone underneath.”