Cartilage is composed of a stiff, spongy matrix and the cells that produce that matrix. The matrix, in turn, is made up of a tough collagen mesh and a gel of molecules known as proteoglycans.
That gel, whose basic building block resembles the circular brushes used to clean bottles, is the focus of the current work. "For many years we've suspected from measurements at the tissue level that these molecules play an extremely important role in cartilage function because of electrical repulsive interactions between the brush bristles,' or glycosaminoglycan (GAG) chains," said Grodzinsky, who is also director of the Center for Biomedical Engineering.
In 1995 Grodzinsky's lab predicted via theoretical models what the repulsive forces should be between these bristles. To probe the bristles directly, the current team removed bristles from their protein backbone and chemically attached one end of each to a gold surface. They packed the bristles as closely together as possible to mimic the structure of native polymer (proteoglycan) brushes. The distance between each bristle: about two to four nanometers, or billionths of a meter.
A new nanomechanical instrument called the Molecular Force Probe was used to make the measurements. The team outfitted the machine's probe tip, which is only about 25 nanometers in radius, with known molecules. They were then able to measure the forces between the tip and "bristled" polymer surface as the tip was moved up and down at a constant rate. "To have a window into how these GAG molecules deform just hasn't been done before in the cartilage field," Grodzinsky said.
The results of this first experiment, in addition to new molecular-level theoretical simulations, have helped quantify the contribution of different forces between bristles. The electrical repulsive, or electrostatic, forces were found, as expected, to be dominant, but ste
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Contact: Elizabeth Thomson
thomson@mit.edu
617-258-5402
Massachusetts Institute of Technology
11-Jun-2002