Regulation and function of mast cell proteases in inflammation

The mast cell in wound healing

This review describes the role of the mast cell in the pathobiology of skin healing. After illustrating its main morphofunctional characteristics, with special reference to the dog and cat, we consider the involvement of the mast cell in the various phases of skin repair. With the aid of a wide array of newly formed or preformed mediators released by degranulation, the activated mast cell controls the key events of the healing phases: triggering and modulation of the inflammatory stage, proliferation of connective cellular elements and final remodelling of the newly formed connective tissue matrix. The importance of the mast cell in regulating healing processes is also demonstrated by the fact that a surplus or deficit of degranulated biological mediators causes impaired repair, with the formation of exuberant granulation tissue (e.g. keloids and hypertrophic scars), delayed closure (dehiscence) and chronicity of the inflammatory stage.

Structural requirements and mechanism for heparin-induced activation of a recombinant mouse mast cell tryptase, mouse mast cell protease-6: formation of active tryptase monomers in the presence of low molecular weight heparin

Mast cell tryptase is stored as an active tetramer in complex with heparin in mast cell secretory granules. Previously, we demonstrated the dependence on heparin for the activation/tetramer formation of a recombinant tryptase. Here we have investigated the structural requirements for this activation process. The ability of heparin-related saccharides to activate a recombinant murine tryptase, mouse mast cell protease-6 (mMCP-6), was strongly dependent on anionic charge density and size. The dose-response curve for heparin-induced mMCP-6 activation displayed a bell-shaped appearance, indicating that heparin acts by binding to more than one tryptase monomer simultaneously. The minimal heparin oligosaccharide required for binding to mMCP-6 was 8-10 saccharide units. Gel filtration analyses showed that such short oligosaccharides were unable to generate tryptase tetramers, but instead gave rise to active mMCP-6 monomers. The active monomers were inhibited by bovine pancreatic trypsin inhibitor, whereas the tetramers were resistant. Furthermore, monomeric (but not tetrameric) mMCP-6 degraded fibronectin. Our results suggest a model for tryptase tetramer formation that involves bridging of tryptase monomers by heparin or other highly sulfated polysaccharides of sufficient chain length. Moreover, our results raise the possibility that some of the reported activities of tryptase may be related to active tryptase monomers that may be formed according to the mechanism described here.

Altered processing of fibronectin in mice lacking heparin. a role for heparin-dependent mast cell chymase in fibronectin degradation

We have previously generated a mouse strain with a defect in its heparin biosynthesis by targeting the gene for N-deacetylase/N-sulfotransferase-2 (NDST-2). The NDST-2(-/-) mice show reduced levels of various mast cell mediators such as histamine and various heparin-binding mast cell proteases, including chymases, tryptases, and carboxypeptidase A. In this work we have addressed the possible functional consequences of the lack of sulfated heparin. Peritoneal cells were harvested from normal and NDST-2(-/-) mice. After culturing the cells, conditioned media were collected and were subjected to SDS-polyacrylamide gel electrophoresis under reducing conditions. Several differences in the protein patterns were observed, including the presence of large amounts of a approximately 250-kDa protein in medium from NDST-2(-/-) mice that was absent in normal controls. Peptide microsequencing revealed identity of this protein with fibronectin. Western blot analysis showed the presence of fibronectin degradation products in cell cultures from normal mice, which were absent in cultures from NDST-2(-/-) animals. Further experiments showed that the degradation of fibronectin observed in cell cultures from NDST-2(+/+) mice was catalyzed by mast cell chymase in a strongly heparin-dependent manner. This report thus indicates a biological function for chymase/heparin proteoglycan complexes in fibronectin turnover.

In vivo immunological function of mast cells derived from embryonic stem cells: an approach for the rapid analysis of even embryonic lethal mutations in adult mice in vivo

An important goal of tissue engineering is to achieve reconstitution of specific functionally active cell types by transplantation of differentiated cell populations derived from normal or genetically altered embryonic stem cells in vitro. We find that mast cells derived in vitro from wild-type or genetically manipulated embryonic stem cells can survive and orchestrate immunologically specific IgE-dependent reactions after transplantation into mast cell-deficient Kit(W)/Kit(W-v) mice. These findings define a unique approach for analyzing the effects of mutations of any genes that are expressed in mast cells, including embryonic lethal mutations, in vitro or in vivo.

The diverse potential effector and immunoregulatory roles of mast cells in allergic disease

Mast cells are of hematopoietic origin but typically complete their maturation in peripheral connective tissues, especially those near epithelial surfaces. Mast cells express receptors that bind IgE antibodies with high affinity (FcepsilonRI), and aggregation of these FcepsilonRI by the reaction of cell-bound IgE with specific antigens induces mast cells to secrete a broad spectrum of biologically active preformed or lipid mediators, as well as many cytokines. Mast cells are widely thought to be essential for the expression of acute allergic reactions, but the importance of mast cells in late-phase reactions and chronic allergic inflammation has remained controversial. Although it is clear that many cell types may be involved in the expression of late-phase reactions and chronic allergic inflammation, studies in genetically mast cell-deficient and congenic normal mice indicate that mast cells may be critical for the full expression of certain features of late-phase reactions and may also contribute importantly to clinically relevant aspects of chronic allergic inflammation. Moreover, the pattern of cytokines that can be produced by mast cell populations, and the enhancement of such cytokine production in mast cells that have undergone IgE-dependent up-regulation of their surface expression of FcepsilonRI, suggests that mast cells may contribute to allergic diseases (and host defense) by acting as immunoregulatory cells, as well as by providing effector cell function.

Endotoxin and mast cell granule proteases synergistically activate human coronary artery endothelial cells to generate interleukin-6 and interleukin-8

Mast cells (MC) are strategically located along blood vessels and, on activation, exocytose granules that contain many vasoactive mediators. Endothelial cell (EC) activation, which includes the production of such cytokines as interleukin-6 (IL-6) and IL-8, is a key event in vascular inflammation. In this study, the effects of purified MC granules (MCG) on the production of IL-6 and IL-8 by human coronary artery EC (HCAEC) were examined. HCAEC were cocultured with MCG in the presence or absence of lipopolysaccharide (LPS), and IL-6 and IL-8 levels in the culture medium were assayed by ELISA. Unactivated HCAEC produced only low levels of IL-6 or IL-8, and the addition of MCG alone resulted in little or no increase in production of these cytokines. LPS-activated HCAEC produced significant amounts of IL-6 and IL-8 in a dose-dependent and time-dependent fashion, which was amplified 2-3-fold by MCG at EC/MC ratios of 16:1-2:1. Scanning electron microscopy revealed direct communication between MCG and HCAEC. The enhancement of IL-6 and IL-8 production by MCG was abrogated when MCG were pretreated with the serine protease inhibitor phenylmethylsulfonyl fluoride (PMSF). These results demonstrate that MCG interaction with HCAEC causes amplification of endotoxin-stimulated cytokine production via serine proteases present in MCG. The synergistic activation of EC by endotoxin and MCG proteases emphasizes the role of MC in amplifying vascular inflammation.

Synthesis and sorting of proteoglycans

Proteoglycans are widely expressed in animal cells. Interactions between negatively charged glycosaminoglycan chains and molecules such as growth factors are essential for differentiation of cells during development and maintenance of tissue organisation. We propose that glycosaminoglycan chains play a role in targeting of proteoglycans to their proper cellular or extracellular location. The variability seen in glycosaminoglycan chain structure from cell type to cell type, which is acquired by use of particular Ser-Gly sites in the protein core, might therefore be important for post-synthesis sorting. This links regulation of glycosaminoglycan synthesis to the post-Golgi fate of proteoglycans.

Mast cells and basophils

Mast cells and basophils are effector cells in IgE-associated immune responses, such as those that contribute to asthma and other allergic diseases and to host resistance to parasites. Recent work shows that mast cells can also participate in innate immunity to bacterial infection and that the expression of such mast cell-dependent natural immunity can be significantly enhanced by long-term treatment of mice with the kit ligand, stem cell factor. However, mast cells may also influence many other biologic responses, including tissue remodeling and angiogenesis. This review discusses certain recent findings about the differentiation, phenotype, and function of basophils and mast cells, as well as briefly considering evolving concepts about the roles of these cells in health and disease.

Identification of a new member of the tryptase family of mouse and human mast cell proteases which possesses a novel COOH-terminal hydrophobic extension

Mapping of the tryptase locus on chromosome 17 revealed a novel gene 2.3 kilobase 3' of the mouse mast cell protease (mMCP) 6 gene. This 3.7-kilobase gene encodes the first example of a protease in the tryptase family that contains a membrane-spanning segment located at its COOH terminus. Comparative structural studies indicated that the putative transmembrane tryptase (TMT) possesses a unique substrate-binding cleft. As assessed by RNA blot analyses, mTMT is expressed in mice in both strain- and tissue-dependent manners. Thus, different transcriptional and/or post-transcriptional mechanisms are used to control the expression of mTMT in vivo. Analysis of the corresponding tryptase locus in the human genome resulted in the isolation and characterization of the hTMT gene. The hTMT transcript is expressed in numerous tissues and is also translated. Analysis of the tryptase family of genes in mice and humans now indicates that a primordial serine protease gene duplicated early and often during the evolution of mammals to generate a panel of homologous tryptases in each species that differ in their tissue expression, substrate specificities, and physical properties.

Mechanism by which heparin proteoglycan modulates mast cell chymase activity

Chymases are highly basic chymotrypsin-like serine proteases expressed exclusively by mast cells. Large amounts of chymases complexed with heparin proteoglycan (PG) are released in vivo during mast cell activation. The tight binding of chymase to heparin PG results in increased activity of the protease toward certain substrates, e.g., thrombin and MeO-Suc-Arg-Pro-Tyr-pNA (S-2586). In this study, the mechanism by which heparin PG modulates chymase activity was investigated, using thrombin and various chromogenic peptide substrates as model substrates. Incubation of thrombin with oligonucleotides that block the heparin-binding site of thrombin abolished the stimulatory effect of heparin PG on thrombin inactivation. Further, thrombin mutants with defects in their heparin-binding regions were less efficiently inactivated by chymase-heparin PG than wild type thrombin. These findings suggest a model for chymase stimulation where heparin PG may promote the chymase-catalyzed cleavage of heparin-binding substrates by simultaneously binding to both chymase and substrate. Experiments in which various chromogenic peptide substrates were utilized showed that heparin PG enhanced the activity of chymase toward positively charged peptide substrates such as S-2586, whereas the cleavage of uncharged substrates was not affected by the presence of heparin PG. On the basis of the latter findings, an alternative stimulation mechanism is discussed where heparin PG may stimulate chymase activity by blocking positively charged regions in chymase, thereby reducing the level of electrostatic repulsion between chymase and positively charged substrates.

Abnormal mast cells in mice deficient in a heparin-synthesizing enzyme

Heparin is a sulphated polysaccharide, synthesized exclusively by connective-tissue-type mast cells and stored in the secretory granules in complex with histamine and various mast-cell proteases. Although heparin has long been used as an antithrombotic drug, endogenous heparin is not present in the blood, so it cannot have a physiological role in regulating blood coagulation. The biosynthesis of heparin involves a series of enzymatic reactions, including sulphation at various positions. The initial modification step, catalysed by the enzyme glucosaminyl N-deacetylase/N-sulphotransferase-2, NDST-2, is essential for the subsequent reactions. Here we report that mice carrying a targeted disruption of the gene encoding NDST-2 are unable to synthesize sulphated heparin. These NDST-2-deficient mice are viable and fertile but have fewer connective-tissue-type mast cells; these cells have an altered morphology and contain severely reduced amounts of histamine and mast-cell proteases. Our results indicate that one site of physiological action for heparin could be inside connective-tissue-type mast cells, where its absence results in severe defects in the secretory granules.