The timely breakdown of extracellular matrix (ECM) 1 is essential for embryonic development, morphogenesis, reproduction, and tissue resorption and remodeling. The matrix metalloproteinases (MMPs), also called matrixins, are thought to play a central role in these processes. The expression of most matrixins is transcriptionally regulated by growth factors, hormones, cytokines, and cellular transformation (1, 2). The proteolytic activities of MMPs are precisely controlled during activation from their precursors and inhibition by endogenous inhibitors, ␣-macroglobulins, and tissue inhibitors of metalloproteinases (TIMPs). Table I lists currently known vertebrate matrixins. In addition, non-vertebrate members have been identified in sea urchins (3), Caenorhabditis elegans (4), soybean (5), and Arabidopsis thaliana (6). Most of these MMPs are the subject of individual chapters in the Handbook of Proteolytic Enzymes (7). This minireview focuses on recent progress in regulation of matrixin activities and their biological and pathological implications. Domain Structure and FunctionAll matrixins are synthesized as prepro-enzymes and secreted as inactive pro-MMPs in most cases. The primary structures of 20 vertebrate matrixins comprise several domain motifs, as illustrated in Fig. 1; the domain composition for each MMP is listed in Table I.The propeptide domain (about 80 amino acids) has a conserved unique PRCG(V/N)PD sequence. The Cys within this sequence (the "cysteine switch") ligates the catalytic zinc to maintain the latency of pro-MMPs (8, 9). This sequence is missing in MMP-23 (10). Stromelysin 3 (MMP-11), MT-MMPs, Xenopus MMP, and MMP-23 have a proprotein processing sequence RX(K/R)R at the C-terminal end of the propeptide, and MMP-11 (11) and MMP-14 (12) were shown to be activated intracellularly by furin.The catalytic domain (about 170 amino acids) contains a zinc binding motif HEXXHXXGXXH and a conserved methionine, which forms a unique "Met-turn" structure (13). This domain consists of a five-stranded ␤-sheet, three ␣-helices, and bridging loops (14). These backbone structures including the "Met-turn" are similar to those of the members from other metalloproteinase families, i.e. astacins, reprolysins (ADAMs), and serralysins; these four families constitute the "metzincins" (13). The catalytic domains of matrixins have an additional structural zinc ion and 2-3 calcium ions, which are required for the stability and the expression of enzymic activity. MMP-2 and MMP-9 have three repeats of fibronectin-type II domain inserted in the catalytic domain. These repeats interact with collagens and gelatins (15, 16).The C-terminal hemopexin-like domain (about 210 amino acids) has an ellipsoidal disk shape with a four bladed ␤-propeller structure; each blade consists of four antiparallel ␤-strands and an ␣-helix (17). The hemopexin domain is an absolute requirement for collagenases to cleave triple helical interstitial collagens (18), although the catalytic domains alone retain proteolytic activity toward other substra...

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CD44 is a facultative proteoglycan implicated in cell adhesion and trafficking, as well as in tumor survival and progression. We demonstrate here that CD44 heparan sulfate proteoglycan (CD44HSPG) recruits proteolytically active matrix metalloproteinase 7 (matrilysin, MMP-7) and heparin-binding epidermal growth factor precursor (pro-HB-EGF) to form a complex on the surface of tumor cell lines, postpartum uterine and lactating mammary gland epithelium, and uterine smooth muscle. The HB-EGF precursor within this complex is processed by MMP-7, and the resulting mature HB-EGF engages and activates its receptor, ErbB4, leading to, among other events, cell survival. In CD44 −/− mice, postpartum uterine involution is accelerated and maintenance of lactation is impaired. In both uterine and mammary epithelia of these mice, MMP-7 localization is altered and pro-HB-EGF processing as well as ErbB4 activation are decreased. Our observations provide a mechanism for the assembly and function of a cell surface complex composed of CD44HSPG, MMP 7, HB-EGF, and ErbB4 that may play an important role in the regulation of physiological tissue remodeling.

Cartilage specimens from tibial plateaus, obtained from 13 osteoarthritic (OA) patients and seven controls, were selected from three regions: zone A, center of fibrillated area; zone B, area adjacent to fibrillation, and zone C, remote region of plateau. Acid and neutral metalloproteinases and tissue inhibitor of metalloproteinase (TIMP) were extracted with 2 M guanidine. Methods were developed to selectively destroy either proteinases or TIMP to prevent cross-reaction during assay. Acid and neutral proteinases were elevated 150% in OA; TIMP was elevated 50%. A positive correlation (r = 0.50) was found between acid and neutral proteinase activities in OA, but not in controls. Both proteinases were elevated two-to threefold in zones A, B, and C. However, the self-active form of the acid metalloproteinase was elevated only in zones A and B (200%); it correlated well with the Mankin scores, whereas the total activities did not. TIMP was elevated (50%) only in zones A and B. Both the proteinase levels and the Mankin score were elevated to a greater extent in the medial, than in the lateral, compartment. Titration of TIMP against the two metalloproteinases indicates that there is a small excess of inhibitor over enzymes in normal cartilage. In OA, TIMP does not increase to the same extent as the proteinases; the resultant excess of proteinases over TIMP may contribute to cartilage breakdown.

Of the four known tissue inhibitors of metalloproteinases (TIMPs), TIMP-3 is distinguished by its tighter binding to the extracellular matrix. The present results show that glycosaminoglycans such as heparin, heparan sulfate, chondroitin sulfates A, B, and C, and sulfated compounds such as suramin and pentosan efficiently extract TIMP-3 from the postpartum rat uterus. Enzymatic treatment by heparinase III or chondroitinase ABC also releases TIMP-3, but neither one alone gives complete release. Confocal microscopy shows colocalization of heparan sulfate and TIMP-3 in the endometrium subjacent to the lumen of the uterus. Immunostaining of TIMP-3 is lost upon digestion of tissue sections with heparinase III and chondroitinase ABC. The N-terminal domain of human TIMP-3 was expressed and found to bind to heparin with affinity similar to that of full-length mouse TIMP-3. The A and B ␤-strands of the N-terminal domain of TIMP-3 contain two potential heparin-binding sequences rich in lysine and arginine; these strands should form a double track on the outer surface of TIMP-3. Synthetic peptides corresponding to segments of these two strands compete for heparin in the DNase II binding assay. TIMP-3 binding may be important for the cellular regulation of activity of the matrix metalloproteinases.The extracellular matrix (ECM) 1 provides mechanical support to cells and regulates signals reaching the cell that govern cell localization, differentiation, proliferation, and apoptosis. Components of the ECM, particularly the glycosaminoglycans (GAGs), are able to sequester bioactive molecules such as growth factors (1), proteases (2), and inhibitors. Turnover of the ECM is a highly regulated process necessary for movement of cells and for release of growth factors. Matrix metalloproteases (MMPs) are believed to be key participants in this remodeling; there are at least 20 MMPs, all able to digest various ECM components (3, 4).The MMPs, in turn, are regulated by tissue inhibitors of metalloproteinases or TIMPs. The major function of the TIMPs is to inhibit MMPs; any imbalance in which the activities of MMPs outweigh the TIMP levels will favor tissue destruction and pathological processes (5, 6). The TIMPs also possess growth stimulatory and regulatory activities (7,8). The four members of the TIMP family all have similar secondary structures of six loops stabilized by six highly conserved disulfide bonds. The TIMPs all bind tightly, albeit with widely varying affinity, to the various MMPs. The x-ray structure (9) shows that the N-terminal cysteine chelates the active site zinc. TIMPs have N-and C-terminal domains, each with three loops. The N-terminal domain of TIMP-1 folds readily and displays full inhibitory activity (10).TIMP-3 has several features that distinguish it from the other TIMPs. First, it is the only TIMP to bind tightly to the ECM: it was first observed as a transformation-sensitive protein bound to the ECM of chick embryo fibroblasts (11) and extractable with SDS or guanidine. This protein was subsequently shown ...

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MMP inhibitors are partially protective against cartilage and subchondral bone damage induced by iodoacetate. These results support an important role for MMPs in mediating the joint damage in this model of arthritis.