Department of Medicine and Lung Biology Center, University of California, San Francisco, California 94143, USA. thiennu@itsa.ucsf.edu
Extracellular matrix (ECM) remodeling is critical to morphogenesis and homeostasis. The identification of inactivating mutations in a gene encoding one of its modifying enzymes, matrix metalloproteinase 2 (MMP-2), in people with a hereditary disorder in which the bones disintegrate, represents the first genetic evidence that the proteolysis of the ECM mediates human growth and development. It also underscores the need for an intricate balance between breakdown and deposition of the ECM.
The development of a multicellular organism is dependent upon an extracellular matrix, which facilitates the organization of cells into more complex functional units: tissues and organs. The extracellular matrix is the glue that holds cells together, and provides texture, strength and integrity to the tissues. Diversity in tissue function depends not only upon diversity in cell types but also upon diversity in the composition of the ECM. Bone, for example, is comprised of one type of ECM; that of lung and brain is quite different. It has become increasingly apparent that the ECM harbors informational cues that direct cell behavior. In fact, the interaction of a cell with its ECM regulates some of the most fundamental cellular processes, such as growth, survival, differentiation, motility, signal transduction and changing cell shape2.
The cleavage of ECM molecules is carried out by specialized proteinases. Among these are the matrix metalloproteinases3 (MMPs), a family of zinc- and calcium-dependent endopeptidases that are active at neutral pH. Each member has specificity for a subset of ECM molecules; collectively, they catalyze the proteolysis of all components of the ECM.
On page 261 of this issue, John Martignetti and colleagues report the surprising finding that a human disease with features commonly attributed to an enhanced degradation of the ECM is, in fact, caused by the lack of a single proteolytic enzyme, MMP-2. Linkage analysis led them to consider MMP2 as a candidate: mutations in four Saudi Arabian families, together with lack of MMP-2 activity in fibroblasts of affected individuals, implicated MMP2 as the 'causative' gene for familial osteolysis. People with this rare inherited disorder have characteristic facial features, many lytic lesions in their bones, arthritis, and subcutaneous nodules.
A model of MMP deficiency. Deficiency of MMPs leads to in an imbalance between the breakdown and deposition of the ECM. This leads to tissue fibrosis, recruitment of osteoclasts, and impairment of osteoblast function. Increased osteoclast activity results in tissue destruction such as arthritis, osteolysis and osteoporosis. Impaired osteoblast function leads to craniofacial dysmorphism and osteopenia.
Bob Crimi
Whereas Mmp2-null mice have no developmental defects (which may reflect genetic redundancy), mice with targeted inactivation of the MT1-MMP gene have many of the same features as people with multicentric osteolysis and arthritis syndrome4,
5,
6. As MT1-MMP activates MMP-2, it is not surprising that deficiency in either of these enzymes—albeit in different species—can result in similar defects. Deficiency of MT1-MMP in mice results in a decrease of collagen breakdown by fibroblasts in the skin and osteoblasts, a decrease in bone formation, and an increase in the number of osteoclasts (especially at ectopic sites). Tissue fibrosis may therefore be attributed to impaired function of fibroblasts; arthritis and osteolysis to increased osteoclastic activity; and craniofacial dysmorphism and osteopenia to impaired function of osteoblasts (see figure).
Tweaking TGF How does the lack of a single MMP effect such a range of phenotypes? One possibility is that it regulates the activity of a critical growth factor. TGF-signaling mediates the coupling of the 'reciprocal' activities of bone formation and resorption by influencing the maturation of osteoblasts into terminally differentiated osteocyte and enhancing the activity of osteoclasts.
Overexpression of TFG in osteoblasts causes deregulated bone remodeling, with increases in and imbalances between bone deposition and osteoclastic bone resorption7,
8. Inhibition of TGF-signaling in skeletal tissues leads to reduced osteoclastic activity and increased bone mass9. TGF signaling is also important for cartilage formation and maintenance. Inhibition of the TGF receptor or a downstream effector molecule, Smad3, results in cartilage destruction resembling osteoarthritis10,
11. So, many of the features found in people with familial osteolysis and mice lacking MT1-MMP resemble those caused by deregulated TGF signaling. TGF is synthesized and secreted as part of a complex; it is activated by its release from the latent associated peptide. MMP-2 activates TGF by proteolytically cleaving the latent associated peptide (ref. 12). Lack of MMP-2 may therefore affect bone formation and homeostasis through modulating the level of active TGF.
A balancing act Another way in which disruption of a single MMP could effect a pleiotropic phenotype is through tipping the balance of activity of MMPs and that of other proteinases. The serine proteinases also degrade components of the ECM, but with different consequences than those of MMP activity. For example, inhibition of MMP activity accelerates the differentiation of the adipocyte13, whereas the inhibition of a serine proteinase (plasminogen) inhibits adipocyte differentiation14. The cleavage of different components of the ECM by these two classes of enzymes may explain their different effects on the cell. It is possible that a balance between the local activity of serine proteinases and MMPs regulates the differentiation of osteoblasts, osteoclasts and fibroblasts.
In contrast with another class of ECM enzymes, the cysteine proteinases (which are necessary for the removal of cartilage and bone ECM), the MMPs seem to be required for the recruitment of cells to their sites of action. For example, MMP-9 recruits osteoclasts to the hypertrophic zone of the cartilage anlage to initiate the process of endochondral bone formation15. MMP-2 may also help to recruit osteoblasts. If so, its deficiency could result in decreased bone formation and increased bone resorption through the unopposed action of the cysteine proteinases.
It is apparent that the organism uses many and diverse mechanisms to ensure a precise control of the ECM microenvironment, and that a delicate balance among these mechanisms is essential to normal physiology. Too much or too little of any one component may lead to disease. Whereas it is tempting to consider interventionist strategies to treat diseases caused by imbalance of ECM components, such strategies could have unpredictable consequences. It is imperative that we fully understand the biology of the proteins that modify the ECM before considering them as therapeutic targets.
-signaling mediates the coupling of the 'reciprocal' activities of bone formation and resorption by influencing the maturation of osteoblasts into terminally differentiated osteocyte and enhancing the activity of osteoclasts.
