Biological function of mast cell chymase

Mast cells (MCs) are key effector cells in various types of inflammatory conditions. The MC secretory granules contain inflammatory mediators such as histamine, heparin proteoglycan (PG), cytokines and various heparin-binding proteases, including tryptases, chymases and carboxypeptidase A. Previously, a mouse strain with a defect in its heparin biosynthesis was produced by targeting the gene for NDST-2 (N-deacetylase/N-sulfotransferase-2). These mice showed reduced levels of MC inflammatory mediators such as histamine and various heparin-binding proteases, including chymases, tryptases, and carboxypeptidase A. By using this mouse strain, we found that chymase in complex with heparin PG degraded fibronectin, suggesting a role for chymase in the regulation of connective tissue composition. Further, we found that chymase/heparin PG complexes degraded and thereby inactivated both thrombin and plasmin, suggesting an additional role for chymase in regulation of extravascular coagulation and fibrinolysis. However, although our findings implicated chymase in these processes, it was not possible to exclude the contribution to the observed activities by other MC components that are influenced by the knockout of NDST-2. Out of the different mouse chymases, mouse MC protease 4 (mMCP-4) has the most similar tissue distribution, heparin-binding and angiotensin I-converting properties as the only identified human chymase. Thus, mMCP-4 may be the closest homologue to human chymase and we therefore chose to target the gene for mMCP-4. A mouse strain with a targeted inactivation of the mMCP-4 gene was generated. This mouse strain displayed defects in the regulation of thrombin and in fibronectin turnover, demonstrating a key role for mMCP-4 in these processes. To address the role of MC proteases in the activation of matrix metalloproteases (MMPs) we used both the NDST-2 and mMCP-4 deficient mice. An analysis of peritoneal cells and tissue extracts from these mice revealed the presence of both pro-MMP-9 and active enzyme (MMP-9) in WT mice, but only the proform of MMP-9 was found in knockout mice. We also found that mMCP-4 can regulate the activation of pro-MMP-2 in the same manner. Our findings suggest that mMCP-4 plays a critical role in the activation of both pro-MMP-2 and pro-MMP-9 in vivo. The mMCP-4 knockout led to the accumulation of the fibronectin and collagen, resulting in fibrotic signs in the skin of the mMCP-4-/- mice. Possibly, mMCP-4-mediated activation of pro-MMP-2 and -9 may provide a link between MCs and processes that are regulated by MMPs