Matrix metalloproteinase plays a significant role in disease and potentially its treatment

by Kenneth L. Marcella, D.V.M.

DOES your horse have laminitis or glucose intolerance? Perhaps you have been concerned about cancer in an older horse or developing arthritis in your equine athlete.

If you have tried to research any of these diseases or read any recent articles on these topics in technical journals or equine publications, chances are good you have encountered the term matrix metalloproteinase (MMP). Information about these enzymes is being discovered almost daily, and they are being linked in some way to almost every major disease or condition affecting horses.

Studies on problems ranging from allergic bronchitis to skin cancer to macular degeneration (a debilitating disease of the eye that often leaves animals and humans blind) have all shown links to MMP activity. Yet, while MMPs are seemingly involved in almost all new equine disease research, scientists still are trying to understand these powerful biological components.

The 2005 MMP Gordon Research Conference held in Montana during the winter opened with the statement, "We are at the end of the beginning in our understanding of MMP biology." Many researchers believe that continued understanding of MMPs and a growing ability to manipulate these enzymes may well be the best bet for treatments, and possibly cures, in the next few decades.

MMPs first were described in vertebrates in 1962 by Jerome Gross and Charles Lapiere. These researchers were investigating tail regeneration in tadpoles and uncovered the presence of unique enzymes necessary for both tail loss and the regeneration process. MMPs later were found to exist in invertebrates and plants, as well.

MMPs are a group of specific enzymes or endopeptides that either alone, or as co-factors, are capable of positive and negative effects within tissue. Currently 25 known MMPs exist, and they are distinguished from other body enzymes by their dependence on the metal ion zinc as a primary part of their structure. These metal-based enzymes are able to degrade all kinds of extracellular matrix proteins, any part of tissue in the body that is not part of a cell.

The extracellular matrix is one of the defining features of connective tissue and is essentially the tissue that holds the body together. It mainly is composed of molecules called glycoproteins and is a building block of collagen (joint surfaces), fibrin and elastin (tendons, ligaments, and muscles), blood, plasma, and related vessels. Extracellular matrix proteins provide support for tissue and cause cells to adhere together. These proteins regulate intracellular connections and provide a means of cellular regeneration and tissue growth.

MMPs specifically target extracellular matrix proteins and, as enzymes, can cause release of an entire array of cellular proteins that will effectively destroy extracellular matrix proteins. It is in this capacity that MMPs cause cell separation, the loss of cell surface receptors, and ultimately cell destruction.

Positive effects

While primarily being recognized for tissue destruction, MMPs can have positive effects on tissue by aiding in wound healing, aging, and tissue regeneration (as with tadpole tails). MMPs also can make cells or parts of cells agonists or antagonists within the body. By manipulating the surface proteins of cells and tissue, MMPs essentially can determine what tissues or components the body sees or recognizes as "good guys" (agonists) or "bad guys" (antagonists). This ability has placed MMP research at the forefront in diseases and conditions such as cancer, various infections, arthritis, laminitis, eye diseases, wound healing, some types of autoimmune-complex diseases, respiratory diseases, and many others.

Oncology researchers at the College of Veterinary Medicine at the University of Illinois have been investigating MMP activity and how it relates to metastasis of tumor cells. Because MMPs can affect cell connection and adhesion, it is currently thought that specific, inappropriate action of MMPs allows for the spread of some types of cancer.

MMPs also can cause cellular degeneration and connective tissue destruction, so some researchers are using this knowledge to learn more about the steps involved in the process of degeneration of eye tissues (macular degeneration) that often leads to blindness or the degeneration of tissue in the lung that leads to various types of respiratory disease.

According to leading researcher Gillian Murphy, B.Sc., Ph.D., at the University of Cambridge Institute for Medical Research in England, MMPs play a specific role in both development and repair in the arthritic process. She wrote: "Progressive degeneration of extracellular protein matrix that compromises joint tissue including articular cartilage, bone, and even intra-articular ligaments and tendons is a major feature of the arthritic diseases leading to permanent loss of function. Although proteinases of all classes play a role in the degeneration of connective tissue, it has long been thought that the major contributors involved in this process belong to the family of MMPs."

MMPs and laminitis

A tightly regulated balance appears to exist between MMP enzymes and their inhibitors, and when that balance is lost, disease processes result. This concept may be most evident in current research on laminitis.

Chris Pollitt, B.V.Sc., Ph.D., director of the Australian Equine Laminitis Research Unit at the University of Queensland, and his colleagues have been investigating the role of MMPs in laminitis. Many scientists now believe that the acute stage of swelling and pain in cases of laminitis has an enzymatic cause.

Recent research has shown that cell-to-cell and cell-to-basement-membrane connections within the normal equine hoof are constantly being broken and reformed because the foot is a growing, dynamic structure that flexes and changes with every step.

It is important that the cells that make up the supporting structures of the hoof (the lamella and the basement membrane to which they attach) are able to change, be modified, and regrow. In the normal hoof this constant change takes place through the action of MMPs, which enzymatically break down some tissue, dissolve extracellular proteins, and serve to promote new growth in other areas.

Specific bacterial proteinases or enzymes can activate MMPs, and inappropriate and aggressive MMP activity can offset the delicate balance of laminar breakdown and repair in the hoof. Increased bacterial enzymes can result from any number of causes of laminitis, ranging from too much spring grass consumption to overeating grain to uterine infection after foaling.

Pollitt and his colleagues have shown that the earliest cellular changes seen in laminitis more closely resemble enzyme destruction than abnormalities in blood flow, which was the previously favored explanation for laminitis. Aggressive MMPs that have been activated by bacterial enzymes cause destruction of the matrix proteins that hold the lamina of the hoof together. Then, all the specific changes seen in laminitis start to occur.

Research offers hope

Just as researchers are becoming aware of various bacterial enzymes that activate MMPs, they also are identifying agents that inhibit them. These drugs and biologic substances restrict the action of MMPs and have the potential to drastically reduce the damage caused by MMPs. More encouragingly, these agents may have the potential to make MMPs turn on processes that cause regeneration.

Many MMP inhibitors are known to exist, and more are being identified all the time. Some relatively common antibiotics such as oxytetracycline have been shown to function as MMP inhibitors at low dosages.

The potential clinical implications of this MMP manipulation, although only currently hinted at with a few completed studies, are enormous. Pollitt's studies provide us with an example:

Pollitt and his research team were able to grow horse hoof explants. These are cultures of the hoof wall, both dermal (inside layer) and epidermal (outside layer) lamella and the adjacent basement connective tissue that survive intact in culture medium for two days. These explants provided a unique means to investigate the role of MMPs in early laminitis.

Pollitt found that when pressure was applied to the hoof wall that had been cultured in the presence of MMPs activators, the lamella easily separated. If the MMP inhibitor Batimastat (BB-94) was added to the MMP-activated culture, lamellar separation was prevented when pressure was applied to the hoof wall.

MMP-2 and MMP-9 have been the two enzymes most associated with hoof damage in cases of laminitis, and Pollitt's study report concluded: "Activation of MMP (MMP-2 and MMP-9) may be responsible for the lamellar separation seen in laminitis and that MMP inhibition may be useful clinically for inhibition of this process."

The identification of MMPs and a brief look at their purpose and function have given researchers plenty of direction for future study and good reason to hope for some clinically applicable treatments on the horizon.

Kenneth L. Marcella, D.V.M., is a practicing veterinarian in Canton, Georgia.