New insights into the structural changes associated with osteoarthritis

The cartilage in the joints, along with a lubricant known as the synovial fluid, provides a smooth surface that helps withstand weight-bearing movements. The fluid contains several molecules, including hyaluronan (HA) and phospholipids. Since the cartilage environment cannot be quickly healed or repaired, researchers have tried to diagnose the early stages of joint disease by monitoring the molecular weight and concentration of HA.

"Although we know that in healthy joints there is very low friction, it is unclear which other molecules are involved and how they change during osteoarthritis," said Rosa Espinosa-Marzal (EIRH), Donald Biggar Willett Faculty Scholar and a professor of environmental engineering & science, and materials science & engineering. "During the early stages of osteoarthritis, cartilage starts degrading, and previous research has shown that the molecular composition of the synovial fluid changes. We wanted to see if the two changes are related to each other."

In a healthy joint, the molecular weight of HA varies between 2-20 MDa with a concentration ranging from 1-4 mg/ml. However, in diseased joints, HA is broken down resulting in a lower molecular weight. Additionally, its concentration is also reduced by ten times. Based on these observations, made by other researchers, the study looked at how the concentration and molecular weight of HA influences the structure of healthy and diseased joints.

To do so, the researchers combined vesicles with high and low molecular weight HA. Using neutron scattering and light scattering, they discovered that the molecular weight of HA can vastly change the structure of the vesicles. Lower molecular weight HA, which mimics osteoarthritis-diseased joints, results in larger vesicle size. They also observed that the molecular weight of HA changes the thickness of the phospholipid layers in the joints.

The researchers also studied how these differences can influence the formation of a protective film; in joints this film is responsible for the very low friction we need for unhindered motion. Once again, they used a combination of techniques, quartz crystal microbalance and atomic force microscopy, to examine how these molecules assemble on gold surfaces.

"The formation of a film is possible only when there is an optimal concentration of HA and phospholipids. Even though the gold surfaces have very little in common with cartilage, our studies indicate that there could also be an optimum concentration under biological conditions," Espinosa-Marzal said. "This is an important observation because we can use the concentration changes as a diagnostic tool."

"We are at a point where you need to use multiple techniques on a complex system like this," said Mark Rutland, a professor of surface science at the KTH Royal Institute of Technology. "None of these techniques alone would have given us any insight. The key was to look at all the different effects and put the pieces together to show that the molecular weight of HA has a huge effect on the characteristics of the layer that is formed with phospholipids."

The researchers are now working on using cartilage to understand whether their observations with gold surfaces also hold true in a biologically relevant system. They are also interested in studying the other molecular components that are found in joints to build a more comprehensive model of the changes that are associated with osteoarthritis.