When low back pain strikes and the diagnosis is a ruptured disc, some patients face surgery while others recover without treatment. Vanderbilt University Medical Center investigators trying to understand how some herniated discs repair themselves have now discovered an intricate interplay of molecular signals.
Their findings, reported in the January 15th Journal of Clinical Investigation, could lead to the development of non-surgical treatments for herniated discs. Low back pain is a common problem, affecting between 60 and 80 percent of Americans during their lifetime, and most of this pain is presumed to be disc-related, said Dr. Dan M. Spengler, professor and chair of Orthopaedics and Rehabilitation.
"There are a lot of disc surgeries-up to 300,000-each year in the United States," Spengler said.
The intervertebral discs that separate the spine's vertebrae act as shock absorbers, allowing the spine to bend and flex. Physical trauma to the spine-for example from something as simple as bending forward while lifting a heavy object-can cause the outer cartilage ring of a disc to rupture. The semifluid disc filling then spills out and presses on nerves, resulting in low back pain and sciatica, shooting pain in the legs.
"We know that about half of patients with disc herniation will improve within six weeks, and that the herniation may be completely resolved over time," Spengler said. "The interesting question is: why doesn't everyone have this spontaneous resorption?
"Understanding what causes the resorption process is a first step to the future development of medication that would predictably remove the herniation." Dr. Hirotaka Haro, a visiting orthopaedic surgeon from Japan, initiated a collaboration with Lynn M. Matrisian, Ph.D., professor and interim chair of Cell Biology, to examine the molecules at work in herniated disc resorption. Haro had previously demonstrated that proteins called matrix metalloproteinases (MMPs) are present in herniated disc samples. Since MMPs chew through other proteins in Pacman-like fashion, it makes sense that they might participate in dissolving the protruding disc.
Other investigators had observed that different cells are present in herniated disc samples compared to normal discs. Macrophages, immune system cells that ingest microorganisms and foreign materials, are particularly prevalent. "In fact, the clinical literature suggests that contact of the herniated disc with the blood supply is important to the resorption process," Matrisian said. "So the more that the herniation extends and protrudes into the spinal cord-in essence the worse it is-the more likely it is that it will resorb by itself." Haro wanted to put these two observations together and probe the role of MMPs and disc/macrophage interactions in disc resorption, and Matrisian had the perfect tools: knockout mice that lacked the MMPs matrilysin and stromelysin. These two MMPs just happened to be the specific ones that Haro had found in herniated disc samples.
To study resorption, Haro developed a culture system that mimics what is happening in the herniated disc. He isolated discs and macrophages from mice and put them together in the same culture dish.
"Hiro was able to dissect the tiny discs from the tails of these mice and add macrophages to them to re-create the conditions in the herniated disc," Matrisian said, "and it worked just wonderfully. When discs are co-cultured with macrophages, the discs essentially dissolve. You can measure, very simply with a balance, that the discs lose weight."
By co-culturing various combinations of discs and macrophages from normal and MMP knockout mice, Haro was able to determine that for the resorption process to occur, the two types of cells require different MMPs.
"It turned out that if the macrophages didn't have matrilysin in them, the discs did not dissolve," Matrisian said. "And if the discs themselves were from the stromelysin knockout mice, they didn't dissolve. So we knew specifically that one MMP was important in the macrophages, and the other was involved in the disc cells."
What turned out to be very surprising was just how these MMP enzymes are involved in disc resorption. "We thought it was very straightforward-that these enzymes chew up the structure of the disc and cause it to disappear," Matrisian said. "But it turns out that they are involved in communication between the two cell types."
The choreography of this molecular dance is complicated. The MMP matrilysin is required in the macrophages to release a growth factor called TNF-alpha. This factor then induces the disc cells to make the MMP stromelysin, which in turn results in the production of a chemoattractant molecule that pulls macrophages into the disc. The new influx of macrophages dumps out other enzymes that degrade the disc. While both Spengler and Matrisian acknowledge that cell co-cultures are a long way from clinical treatments for human beings, the findings point in that direction.
"One possibility is to figure out what you can inject into the disc that would encourage penetration of the natural macrophages," Matrisian said. "Hiro would like to try stromelysin, with the idea that it would induce the production of more chemoattractant for the macrophages. "Once we figure out what the chemoattractant is, it might be possible to inject very small amounts of that."
The studies illustrate how a clinical observation can inform basic science research. "This really is translational research," Matrisian said. "We designed basic science experiments to understand an intriguing clinical question. And the discovery that these enzymes [MMPs] are important to processing and communication has really changed our thinking about how they function. "These enzymes are more complicated and interesting than we originally thought. Rather than being big bulldozers, they're a fine pair of scissors that only cut certain things."
Postdoctoral research associates Howard C. Crawford, Ph.D., Barbara Fingleton, Ph.D., and Kenichi Shinomiya, Ph.D. also participated in the studies. Merck Research Laboratories provided the stromelysin knockout mice. The work was supported by the NIH and by the department of Orthopaedics and Rehabilitation.
Journal
Journal of Clinical Investigation