Researchers in the US have discovered that the structural protein collagen can switch from its usual rigid form into a much floppier, more flexible state and back again, opening the door to the idea that targeting collagen itself rather than the enzymes that degrade it could be a way forward for developing drugs that prevent collagen from rupturing, for example in arterial plaques to reduce the risk of heart attack.

You can read about how MIT associate professor Collin Stultz and colleagues discovered collagen’s floppy side online in an April issue of the journal Biochemistry.

Barbara Brodsky of Robert Wood Johnson Medical School, a collagen researcher who was not involved in Stultz’s project, described the finding as “intriguing”.

“The concept of multiple conformations is a big contribution to our thinking,” she added.

About 30 per cent of the tissue in the human body is collagen: it is an important component of muscle, bone, cartilage, and skin. Classed as a structural protein, collagen plays many roles.

For instance, when injuries heal, the scar tissue that forms is mostly made of collagen, as is the protective layer around the plaques that build up in arteries as a result of cholesterol and other fatty deposits.

It is collagen’s role as a plaque protector that most interests Stultz, a cardiologist and biomedical engineer, who started looking into it about eight years ago. When the collagen layer fails to hold the plaque together, its contents, which also include blood clotting agents, spill out and vastly increase the risk of a blood clot and heart attack.

Stultz, told the media that rupture of the collagen layer is the event most often linked to catastrophic heart attacks, “the kind where you’re walking down the street, or watching TV, and then keel over and die”.

One of the puzzles that has been intriguing scientists like Stultz for some time is how enzymes access the rigid, triple helix structure of collagen and break it down: a process that is carefully controlled in the body.

The enzymes that break down collagen are called collagenases and to break down the structural protein they have to be able to get a foothold. But Stultz said if you look at the structure, you can’t see how it is possible, and “you would say collagen should never be broken up by these enzymes”.

To study the protein, scientists usually crystallize it at low temperatures (10 degrees Celsius), which is when you can see the rigid triple helix form. Stultz then had a hunch: perhaps temperature was the problem. So with the help of computer models, he looked at what might happen to collagenase at higher temperatures such as room or body temperature. The models suggested that some sections of the molecule unwind and become floppy, and this allows collagenase to get a foothold and start breaking it down.

Having tested the idea with a computer model, Stultz and colleagues tested it in the lab. At room temperature, they exposed collagen to a form of collagenase that only recognizes unfolded collagen. When this was tried before, the collagenase failed to break down the collagen, but this time they waited longer, for several days, and found that some of the collagen did indeed break down.

In another part of the experiment they found that the ratio of rigid to floppy collagen is temperature sensitive, with about 1 floppy one to every 1,000 rigid ones at room temperature, with a higher ratio at body temperature.

These findings suggest that drugs to prevent arterial plaques from rupturing should perhaps target the collagen directly rather than try to inhibit the enzymes the break it down, an approach that as yet has not led to any approved drugs. For instance, a drug that makes collagen more rigid could be a way to stop the collagenases being able to break it down.

There are other diseases where this knowledge could lead to new treatments that rely on preserving collagen, such as arthritis and preventing tumors from spreading (tumors spread by breaking down collagen, for instance in the lining of nearby organ surfaces and blood vessels).

“Cleavage Site Specificity and Conformational Selection in Type I Collagen Degradation.”
Ramon Salsas-Escat, Paul S. Nerenberg, Collin M. Stultz.
Biochemistry, 2010, 49 (19), 4147-4158.
DOI: 10.1021/bi9021473

Source: MIT.

Written by: Catharine Paddock, PhD