Mast cell tryptase, a still enigmatic enzyme

Top-Down Proteomics of Human Saliva Discloses Significant Variations of the Protein Profile in Patients with Mastocytosis

Mastocytosis is a myeloproliferative neoplasm causing abnormal clonal mast cell accumulation in different tissues, such as skin and bone marrow. A cutaneous subtype (CM) is distinguished from a systemic one (SM); SM patients can be grouped into SM with (SM+C) or without (SM-C) additional cutaneous lesions, and their classification is often challenging. This study was purposed to highlight variations in the salivary proteome of patients with different mastocytosis subtypes and compared to healthy controls. A top-down proteomics approach coupled to a label-free quantitation revealed salivary profiles in patients different from those of controls and a down-regulation of peptides/proteins involved in the mouth homeostasis and defense, such as statherin, histatins, and acidic proline-rich proteins (aPRPs), and in innate immunity and inflammation, such as the cathepsin inhibitors, suggesting a systemic condition associated with an exacerbated inflammatory state. The up-regulation of antileukoproteinase and S100A8 suggested a protective role against the disease status. The two SM forms were distinguished by the lower levels of truncated forms of aPRPs, statherin, P-B peptide, and cystatin D and the higher levels of thymosin β4 and α-defensins 1 and 4 in SM-C patients with respect to SM+C. Data are available via ProteomeXchange with identifier PXD017759.

Post mortem tryptase: A review of literature on its use, sampling and interpretation in the investigation of fatal anaphylaxis

Post mortem tryptase is a commonly-used ancillary test in the investigation of possible anaphylactic deaths. Ante mortem tryptase interpretation differs from post mortem interpretation due to differing priorities, biochemical behaviours and capacity for follow-up. Additionally, post mortem tryptase sampling site, method and even cut-off levels are not standardised between facilities. This review of the literature investigates the existing research and recommendations on the use of post mortem tryptase in suspected anaphylactic deaths. Currently, autopsy recommendations suggest early sampling, standardised sampling technique with clamping of and aspiration from the femoral vein, and for the results to be interpreted within the wider autopsy and clinical context. Areas in need of further research include the effects of cytolysis on tryptase levels and studies to stratify differing tryptase levels based on type of death and anaphylactic trigger.

Top-Down Proteomics of Human Saliva Discloses Significant Variations of the Protein Profile in Patients with Mastocytosis

Mastocytosis is a myeloproliferative neoplasm causing abnormal clonal mast cell accumulation in different tissues, such as skin and bone marrow. A cutaneous subtype (CM) is distinguished from a systemic one (SM); SM patients can be grouped into SM with (SM+C) or without (SM-C) additional cutaneous lesions, and their classification is often challenging. This study was purposed to highlight variations in the salivary proteome of patients with different mastocytosis subtypes and compared to healthy controls. A top-down proteomics approach coupled to a label-free quantitation revealed salivary profiles in patients different from those of controls and a down-regulation of peptides/proteins involved in the mouth homeostasis and defense, such as statherin, histatins, and acidic proline-rich proteins (aPRPs), and in innate immunity and inflammation, such as the cathepsin inhibitors, suggesting a systemic condition associated with an exacerbated inflammatory state. The up-regulation of antileukoproteinase and S100A8 suggested a protective role against the disease status. The two SM forms were distinguished by the lower levels of truncated forms of aPRPs, statherin, P-B peptide, and cystatin D and the higher levels of thymosin β4 and α-defensins 1 and 4 in SM-C patients with respect to SM+C. Data are available via ProteomeXchange with identifier PXD017759.

Potential Role of Cytochrome c and Tryptase in Psoriasis and Psoriatic Arthritis Pathogenesis: Focus on Resistance to Apoptosis and Oxidative Stress

Psoriasis (PsO) is an autoimmune disease characterized by keratinocyte proliferation, chronic inflammation and mast cell activation. Up to 42% of patients with PsO may present psoriatic arthritis (PsA). PsO and PsA share common pathophysiological mechanisms: keratinocytes and fibroblast-like synoviocytes are resistant to apoptosis: this is one of the mechanism facilitating their hyperplasic growth, and at joint level, the destruction of articular cartilage, and bone erosion and/or proliferation. Several clinical studies regarding diseases characterized by impairment of cell death, either due to apoptosis or necrosis, reported cytochrome c release from the mitochondria into the extracellular space and finally into the circulation. The presence of elevated cytochrome c levels in serum has been demonstrated in diseases as inflammatory arthritis, myocardial infarction and stroke, and liver diseases. Cytochrome c is a signaling molecule essential for apoptotic cell death released from mitochondria to the cytosol allowing the interaction with protease, as the apoptosis protease activation factor, which lead to the activation of factor-1 and procaspase 9. It has been demonstrated that this efflux from the mitochondria is crucial to start the intracellular signaling responsible for apoptosis, then to the activation of the inflammatory process. Another inflammatory marker, the tryptase, a trypsin-like serine protease produced by mast cells, is released during inflammation, leading to the activation of several immune cells through proteinase-activated receptor-2. In this review, we aimed at discussing the role played by cytochrome c and tryptase in PsO and PsA pathogenesis. To this purpose, we searched pathogenetic mechanisms in PUBMED database and review on oxidative stress, cytochrome c and tryptase and their potential role during inflammation in PsO and PsA. To this regard, the cytochrome c release into the extracellular space and tryptase may have a role in skin and joint inflammation.

A Novel, Nonpeptidic, Orally Active Bivalent Inhibitor of Human β-Tryptase

β-Tryptase is released from mast cells upon degranulation in response to allergic and inflammatory stimuli. Human tryptase is a homotetrameric serine protease with 4 identical active sites directed toward a central pore. These active sites present an optimized scenario for the rational design of bivalent inhibitors, which bridge 2 adjacent active sites. Using (3-[1-acylpiperidin-4-yl]phenyl)methanamine as the pharmacophoric core and a disiloxane linker to span 2 active sites we have successfully produced a novel bivalent tryptase inhibitor, compound 1a, with a comparable profile to previously described inhibitors. Pharmacological properties of compound 1a were studied in a range of in vitro enzymic and cellular screening assays, and in vivo xenograft models. This non-peptide inhibitor of tryptase demonstrated superior activity (IC50 at 100 pmol/L tryptase = 1.82 nmol/L) compared to monomeric modes of inhibition. X-ray crystallography validated the dimeric mechanism of inhibition, and 1a demonstrated good oral bioavailability and efficacy in HMC-1 xenograft models. Furthermore, compound 1a demonstrated extremely slow off rates and high selectivity against-related proteases. This highly potent, orally bioavailable and selective inhibitor of human tryptase will be an invaluable tool in future studies to explore the therapeutic potential of attenuating the activity of this elusive target.

Mast Cell-Derived Tryptase in Geographic Atrophy

Purpose

Our previous study demonstrated significantly more degranulating mast cells (MCs) in choroids from subjects with age-related macular degeneration compared to aged controls. This study examined the immunolocalization of tryptase, the most abundant MC secretory granule-derived serine protease, in aged control eyes and eyes with geographic atrophy (GA).

Methods

Postmortem human eyes with and without GA were obtained from the National Disease Research Interchange. Tissue was fixed, cryopreserved, sectioned, and immunostained with a monoclonal antibody against tryptase. Sections were imaged on a Zeiss 710 Confocal Microscope.

Results

In the posterior pole of all aged control eyes, tryptase was confined to choroidal MCs, which were located primarily in Sattler's layer. In eyes with GA, many MCs were located in the inner choroid near choriocapillaris and Bruch's membrane (BM). Tryptase was found not only in MCs but also diffusely around them in stroma, suggesting they had degranulated. In contrast with aged control eyes, eyes with GA also had strong tryptase staining in BM. Tryptase was observed within BM in regions of RPE atrophy, at the border of atrophy, and extending well into the nonatrophic region.

Conclusions

Our results demonstrate that tryptase, released during choroidal MC degranulation, binds to BM in GA in advance of RPE atrophy. Tryptase activates MMPs that can degrade extracellular matrix (ECM) and basement membrane components found in BM. ECM modifications are likely to have a profound effect on the function and health of RPE and choroidal thinning in GA.

Tryptase inhibitors: a patent review

INTRODUCTION

Tryptase is one of the main serine-proteinases located in the secretory granules of mast cells, and is released through degranulation, which is involved in the pathogenesis of allergic inflammatory disease, cardiovascular diseases, lung fibrosis and tumor. Therefore, inhibitors targeting tryptase may represent a new direction for the treatment of allergic inflammatory disease and other diseases. Areas covered: In this article, we discussed the history and development of tryptase inhibitors and described a variety of tryptase inhibitors via their structures and biological importance in clinical studies and drug development for tryptase-related diseases. Expert opinion: Initial tryptase inhibitors based on indole structure as the hydrophobic substituent on a benzylamine-piperidine template have low specificity and poor bioavailability. Therefore, designing new and specific inhibitors targeting tryptase should be involved in future clinical studies. Modifications toward indoles with varying N-substitution, introducing an amide bond, and growing the chain length contribute to an increase in the specific selectivity and potency of tryptase inhibitors. Tryptase has become the research hotspot to explore many related diseases. Therefore, there has been growing appreciation for the potential importance of the tryptase inhibitors as a target for treating these diseases.

Mast cell glycosaminoglycans

Mast cells contain granules packed with a mixture of proteins that are released on degranulation. The proteoglycan serglycin carries an array of glycosaminoglycan (GAG) side chains, sometimes heparin, sometimes chondroitin or dermatan sulphate. Tight packing of granule proteins is dependent on the presence of serglycin carrying these GAGs. The GAGs of mast cells were most intensively studied in the 1970s and 1980s, and though something is known about the fine structure of chondroitin sulphate and dermatan sulphate in mast cells, little is understood about the composition of the heparin/heparan sulphate chains. Recent emphasis on the analysis of mast cell heparin from different species and tissues, arising from the use of this GAG in medicine, lead to the question of whether variations within heparin structures between mast cell populations are as significant as variations in the mix of chondroitins and heparins.

Serum Concentrations of Tryptase in Children

Background: Mast cells are dispersed in many tissues, especially in digestive and respiratory-tract mucosal membranes. Tryptase, considered a marker of mast-cell activity, is the most important protease released from these cells during degranulation. Tryptase concentration is mainly accessed in anaphylaxis and mastocytosis, being one diagnostic criteria of this disease. There are no data concerning tryptase activity in healthy children in the current literature. Aim: The aims of this study were the analysis of concentrations of serum tryptase in healthy children, and determining reference values of the enzyme at different developmental ages. Materials and methods: The investigated group consisted of 131 healthy children (75 girls, 56 boys) aged 3 months-18 years. The concentration of tryptase in the studied samples was evaluated by the fluoro-immuno-enzymatic method with UniCAP. Results: The mean concentration of serum tryptase in the studied group was 2.8±2.2 ng/dL: 2.5±2.2 ng/dL in girls and 3.2±2.1 ng/dL in boys. Conclusion: The upper reference limit of 7.2 ng/dL was lower than in adults.

Allosteric control of βII-tryptase by a redox active disulfide bond

The S1A serine proteases function in many key biological processes such as development, immunity, and blood coagulation. S1A proteases contain a highly conserved disulfide bond (Cys(191)-Cys(220) in chymotrypsin numbering) that links two β-loop structures that define the rim of the active site pocket. Mast cell βII-tryptase is a S1A protease that is associated with pathological inflammation. In this study, we have found that the conserved disulfide bond (Cys(220)-Cys(248) in βII-tryptase) exists in oxidized and reduced states in the enzyme stored and secreted by mast cells. The disulfide bond has a standard redox potential of -301 mV and is stoichiometrically reduced by the inflammatory mediator, thioredoxin, with a rate constant of 350 m(-1) s(-1). The oxidized and reduced enzymes have different substrate specificity and catalytic efficiency for hydrolysis of both small and macromolecular substrates. These observations indicate that βII-tryptase activity is post-translationally regulated by an allosteric disulfide bond. It is likely that other S1A serine proteases are similarly regulated.

Expression of mast cell proteases correlates with mast cell maturation and angiogenesis during tumor progression

Tumor cells are surrounded by infiltrating inflammatory cells, such as lymphocytes, neutrophils, macrophages, and mast cells. A body of evidence indicates that mast cells are associated with various types of tumors. Although role of mast cells can be directly related to their granule content, their function in angiogenesis and tumor progression remains obscure. This study aims to understand the role of mast cells in these processes. Tumors were chemically induced in BALB/c mice and tumor progression was divided into Phases I, II and III. Phase I tumors exhibited a large number of mast cells, which increased in phase II and remained unchanged in phase III. The expression of mouse mast cell protease (mMCP)-4, mMCP-5, mMCP-6, mMCP-7, and carboxypeptidase A were analyzed at the 3 stages. Our results show that with the exception of mMCP-4 expression of these mast cell chymase (mMCP-5), tryptases (mMCP-6 and 7), and carboxypeptidase A (mMC-CPA) increased during tumor progression. Chymase and tryptase activity increased at all stages of tumor progression whereas the number of mast cells remained constant from phase II to III. The number of new blood vessels increased significantly in phase I, while in phases II and III an enlargement of existing blood vessels occurred. In vitro, mMCP-6 and 7 are able to induce vessel formation. The present study suggests that mast cells are involved in induction of angiogenesis in the early stages of tumor development and in modulating blood vessel growth in the later stages of tumor progression.

Dimerization of β-tryptase inhibitors, does it work for both basic and neutral P1 groups?

The tetrameric folding of β-tryptase and the pair-wise distribution of its substrate binding sites offer a unique opportunity for development of inhibitors that span two adjacent binding sites. A series of dimeric inhibitors with two basic P1 moieties was discovered using this design strategy and exhibited tight-binder characteristics. Using the same strategy, an attempt was made to design and synthesize dimeric inhibitors with two neutral-P1 groups in hope to exploit the dimeric binding mode to achieve a starting point for further optimization. The unsuccessful attempt, however, demonstrated the important role played by Ala190 in neutral-P1 binding and casted further doubt on the possibility of developing neutral-P1 inhibitors for β-tryptase.

The design, structures and therapeutic potential of protein epitope mimetics

Using a biologically relevant peptide or protein structure as a starting point for lead identification represents one of the most powerful approaches in modern drug discovery. Here, we focus on the protein epitope mimetic (PEM) approach, where folded 3D structures of peptides and proteins are taken as starting points for the design of synthetic molecules that mimic key epitopes involved in protein-protein and protein-nucleic acid interactions. By transferring the epitope from a recombinant to a synthetic scaffold that can be produced by parallel combinatorial methods, it is possible to optimize target affinity and specificity as well as other drug-like ADMET properties. The PEM technology is a powerful tool for target validation, and for the development of novel PEM-based drugs.

Tryptase activates TGFbeta in human airway smooth muscle cells via direct proteolysis

Transforming growth factor beta (TGFbeta) is a key remodelling factor in asthma. It is produced as a latent complex and the main limiting step in TGFbeta bioavailability is its activation. Mast cell tryptase has been shown to stimulate the release of functionally active TGFbeta from human airway smooth muscle (ASM) cells [P. Berger, P.O. Girodet, H. Begueret, O. Ousova, D.W. Perng, R. Marthan, A.F. Walls, J.M. Tunon de Lara, Tryptase-stimulated human airway smooth muscle cells induce cytokine synthesis and mast cell chemotaxis, FASEB J. 17 (2003) 2139-2141]. The aim of this study was to determine if tryptase could cause TGFbeta activation as well as expression in ASM cells via its receptor, proteinase-activated receptor 2 (PAR2). Tryptase caused TGFbeta activation without affecting levels of total TGFbeta. This effect was inhibited by the selective tryptase inhibitor FUT175 and leupeptin but not mimicked by the PAR2 activating peptide SLIGKV-NH(2). Furthermore, the ASM cells used in the study did not express PAR2. The results indicate that tryptase activates TGFbeta via a PAR2-independent proteolytic mechanism in human ASM cells and may help understanding the role of tryptase in asthma.

Novel, potent, selective, and orally bioavailable human βII-tryptase inhibitors

The synthesis of novel [1,2,4]oxadiazoles and their structure-activity relationship (SAR) for the inhibition of tryptase and related serine proteases is presented. Elaboration of the P'-side afforded potent, selective, and orally bioavailable tryptase inhibitors.

Design and synthesis of selective keto-1,2,4-oxadiazole-based tryptase inhibitors

Using a scaleable, directed library approach based on orthogonally protected advanced intermediates, we have prepared a series of potent keto-1,2,4-oxadiazoles designed to explore the P(2) binding pocket of human mast cell tryptase, while building in a high degree of selectivity over human trypsin and other serine proteases.

Functional Characterization and Expression Analysis of the Proteinase-Activated Receptor-2 in Human Cutaneous Mast Cells

Proteinase-activated receptor-2 (PAR2) belongs to a new G protein-coupled receptor subfamily activated by serine proteinases. PAR2 has been demonstrated to play a role during inflammation and immune response in different tissues including the skin. We examined whether PAR2 is functionally expressed by cutaneous human primary skin mast cells (HPMC) and the human mast cell line 1 (HMC-1). Reverse transcription-polymerase chain reaction and FACS analysis show expression of PAR2 both at the RNA and protein level. HPMCs and HMC-1 also express PAR1, PAR3, and PAR4. Ca-mobilization studies demonstrate functional PAR2 expressed by human skin mast cells, as shown by natural and synthetic PAR2 agonists. PAR2 agonists induced histamine release from HPMC indicating a role of PAR2 in regulating inflammatory and immune responses by skin mast cells. Double-immunofluorescence staining reveals colocalization of PAR2 with tryptase in the majority of human skin mast cells. In conclusion, trypsin and tryptase as well as specific agonists for PAR2 were able to induce Ca2+ mobilization in HPMCs, and agonists of PAR2 induce the release of histamine from these cells. Thus, PAR2 may be an important regulator of skin mast cell function during cutaneous inflammation and hypersensitivity.

Identification and characterization of human polyserase-3, a novel protein with tandem serine-protease domains in the same polypeptide chain

Background

We have previously described the identification and characterization of polyserase-1 and polyserase-2, two human serine proteases containing three different catalytic domains within the same polypeptide chain. Polyserase-1 shows a complex organization and it is synthesized as a membrane-bound protein which can generate three independent serine protease domains as a consequence of post-translational processing events. The two first domains are enzymatically active. By contrast, polyserase-2 is an extracellular glycosylated protein whose three protease domains remain embedded in the same chain, and only the first domain possesses catalytic activity.

Results

Following our interest in the study of the human degradome, we have cloned a human liver cDNA encoding polyserase-3, a new protease with tandem serine protease domains in the same polypeptide chain. Comparative analysis of polyserase-3 with the two human polyserases described to date, revealed that this novel polyprotein is more closely related to polyserase-2 than to polyserase-1. Thus, polyserase-3 is a secreted protein such as polyserase-2, but lacks additional domains like the type II transmembrane motif and the low-density lipoprotein receptor module present in the membrane-anchored polyserase-1. Moreover, analysis of post-translational mechanisms operating in polyserase-3 maturation showed that its two protease domains remain as integral parts of the same polypeptide chain. This situation is similar to that observed in polyserase-2, but distinct from polyserase-1 whose protease domains are proteolytically released from the original chain to generate independent units. Immunolocalization studies indicated that polyserase-3 is secreted as a non-glycosylated protein, thus being also distinct from polyserase-2, which is a heavily glycosylated protein. Enzymatic assays indicated that recombinant polyserase-3 degrades the α-chain of fibrinogen as well as pro-urokinase-type plasminogen activator (pro-uPA). Northern blot analysis showed that polyserase-3 exhibits a unique expression pattern among human polyserases, being predominantly detected in testis, liver, heart and ovary, as well as in several tumor cell lines.

Conclusion

These findings contribute to define the growing group of human polyserine proteases composed at present by three different proteins. All of them share a complex structural design with several catalytic units in a single polypeptide but also show specific features in terms of enzymatic properties, expression patterns and post-translational maturation mechanisms.

X-ray structures of free and leupeptin-complexed human alphaI-tryptase mutants: indication for an alpha-->beta-tryptase transition

Tryptases alpha and beta are trypsin-like serine proteinases expressed in large amounts by mast cells. Beta-tryptase is a tetramer that has enzymatic activity, but requires heparin binding to maintain functional and structural stability, whereas alpha-tryptase has little, if any, enzymatic activity but is a stable tetramer in the absence of heparin. As shown previously, these differences can be mainly attributed to the different conformations of the 214-220 segment. Interestingly, the replacement of Asp216 by Gly, which is present in beta-tryptase, results in enzymatically active but less stable alpha-tryptase mutants. We have solved the crystal structures of both the single (D216G) and the double (K192Q/D216G) mutant forms of recombinant human alphaI-tryptase in complex with the peptide inhibitor leupeptin, as well as the structure of the non-inhibited single mutant. The inhibited mutants exhibited an open functional substrate binding site, while in the absence of an inhibitor, the open (beta-tryptase-like) and the closed (alpha-tryptase-like) conformations were present simultaneously. This shows that both forms are in a two-state equilibrium, which is influenced by the residues in the vicinity of the active site and by inhibitor/substrate binding. Novel insights regarding the observed stability differences as well as a potential proteolytic activity of wild-type alpha-tryptase, which may possess a cryptic active site, are discussed.

Proteolytic susceptibility of the serine protease inhibitor trappin-2 (pre-elafin): evidence for tryptase-mediated generation of elafin

A number of serine, cysteine, metallo- and acid proteases were evaluated for their ability to proteolytically cleave the serine protease inhibitor trappin-2, also known as pre-elafin, and to release elafin from its precursor. None of the metalloproteases or acid proteases examined cleaved trappin-2, while serine and cysteine proteases preferentially cleaved trappin-2 within its non-inhibitory N-terminal moiety. Cathepsin L, cathepsin K, plasmin, trypsin and tryptase were able to release elafin by cleaving the Lys 38 -Ala 39 peptide bond in trappin-2. However, purified tryptase appeared to be efficient at releasing elafin. Incubation of trappin-2 with purified mast cells first challenged with anti-immunoglobulin E or calcium ionophore A23187 resulted in the rapid generation of elafin. This proteolytic release of elafin from trappin-2 was inhibited in the presence of a tryptase inhibitor, suggesting that this mast cell enzyme was involved in the process. Finally, ex vivo incubation of trappin-2 with sputum from cystic fibrosis patients indicated the production of a proteolytic immunoreactive fragment with the same mass as that of native elafin. This cleavage did not occur when preincubating the sputum with polyclonal antibodies directed against tryptase. Taken together, these findings indicate that tryptase could likely be involved in the maturation of trappin-2 into elafin under physiological conditions.

Combinatorial approaches towards the discovery of new tryptase inhibitors

The synthesis and evaluation as tryptase inhibitors of a library of 2,5-diketopiperazine derivatives containing guanidine or amidine functional groups is reported. Among the compounds evaluated, derivatives 6{CG4-CG8} and 6{CG4-CG9} are the most active compounds and have marked selectivity towards tryptase in front of trypsin.

Structural requirements and mechanism for heparin-dependent activation and tetramerization of human betaI- and betaII-tryptase

Tryptase, a tetrameric serine protease, is a main constituent of the secretory granules in human mast cells, where it is stored in complex with heparin or chondroitin sulfate proteoglycan. Human tryptase has been implicated in a variety of clinical conditions including asthma, but the mechanisms that lead to its tetramerization/activation have not been extensively investigated. Here we addressed the activation mechanisms for human betaI and betaII-tryptase, which differ in that betaI-tryptase is N-glycosylated at Asn102 whereas betaII-tryptase has a Lys residue at position 102, and consequently lacks the corresponding N-glycosylation. We found that both tryptases were dependent on heparin for activation/tetramerization, but whereas betaI-tryptase activation preferentially occurred at acidic pH, betaII-tryptase activation was less pH-dependent. Both betaI and betaII-tryptase bound strongly to heparin-Sepharose at acidic pH but with lower affinity at neutral pH. Further, while addition of heparin to betaI-tryptase predominantly resulted in formation of active tetrameric enzyme, betaII-tryptase showed a tendency to form inactive aggregates. betaI and betaII-tryptase were similar in that the minimal heparin size to induce activation was an octasaccharide and in that the interaction with heparin and structurally related polysaccharides was dependent on high anionic charge density rather than on specific structural motifs. Addition of decasaccharides to both betaI and betaII-tryptase resulted in the formation of active monomeric enzyme, whereas intact heparin promoted assembly of tetrameric enzyme. This, together with a bell-shaped dose response curve for heparin-induced activation, suggests that the mechanism for tetramerization involves bridging of individual tryptase monomers by heparin. Taken together, this study indicates a key role for heparin in the activation of human beta-tryptase.