Protease-proteoglycan complexes of mouse and human mast cells and importance of their beta-tryptase-heparin complexes in inflammation and innate immunity

Bioengineered heparin: Advances in production technology

Heparin, a highly sulfated glycosaminoglycan, is considered an indispensable anticoagulant with diverse therapeutic applications and has been a mainstay in medical practice for nearly a century. Its potential extends beyond anticoagulation, showing promise in treating inflammation, cancer, and infectious diseases such as COVID-19. However, its current sourcing from animal tissues poses challenges due to variable structures and adulterations, impacting treatment efficacy and safety. Recent advancements in metabolic engineering and synthetic biology offer alternatives through bioengineered heparin production, albeit with challenges such as controlling molecular weight and sulfonation patterns. This review offers comprehensive insight into recent advancements, encompassing: (i) the metabolic engineering strategies in prokaryotic systems for heparin production; (ii) strides made in the development of bioengineered heparin; and (iii) groundbreaking approaches driving production enhancements in eukaryotic systems. Additionally, it explores the potential of recombinant Chinese hamster ovary cells in heparin synthesis, discussing recent progress, challenges, and future prospects, thereby opening up new avenues in biomedical research.

The Uptake of Heparanase into Mast Cells Is Regulated by Its Enzymatic Activity to Degrade Heparan Sulfate

Mast cells take up extracellular latent heparanase and store it in secretory granules. The present study examined whether the enzymatic activity of heparanase regulates its uptake efficiency. Recombinant mouse heparanase mimicking both the latent and mature forms (L-Hpse and M-Hpse, respectively) was internalized into mastocytoma MST cells, peritoneal cell-derived mast cells, and bone marrow-derived mast cells. The internalized amount of L-Hpse was significantly higher than that of M-Hpse. In MST cells, L-Hpse was continuously internalized for up to 8 h, while the uptake of M-Hpse was saturated after 2 h of incubation. L-Hpse and M-Hpse are similarly bound to the MST cell surface. The expression level of cell surface heparan sulfate was reduced in MST cells incubated with M-Hpse. The internalized amount of M-Hpse into mast cells was significantly increased in the presence of heparastatin (SF4), a small molecule heparanase inhibitor that does not affect the binding of heparanase to immobilized heparin. Enzymatically quiescent M-Hpse was prepared with a point mutation at Glu335. The internalized amount of mutated M-Hpse was significantly higher than that of wild-type M-Hpse but similar to that of wild-type and mutated L-Hpse. These results suggest that the enzymatic activity of heparanase negatively regulates the mast cell-mediated uptake of heparanase, possibly via the downregulation of cell surface heparan sulfate expression.

A type I interferon regulatory network for human plasmacytoid dendritic cells based on heparin, membrane-bound and soluble BDCA-2

Plasmacytoid dendritic cells (pDCs) produce type I interferons (IFNs) after sensing viral/bacterial RNA or DNA by toll-like receptor (TLR) 7 or TLR9, respectively. However, aberrant pDCs activation can cause adverse effects on the host and contributes to the pathogenesis of type I IFN-related autoimmune diseases. Here, we show that heparin interacts with the human pDCs-specific blood dendritic cell antigen 2 (BDCA-2) but not with related lectins such as DCIR or dectin-2. Importantly, BDCA-2-heparin interaction depends on heparin sulfation and receptor glycosylation and results in inhibition of TLR9-driven type I IFN production in primary human pDCs and the pDC-like cell line CAL-1. This inhibition is mediated by unfractionated and low-molecular-weight heparin, as well as endogenous heparin from plasma, suggesting that the local blood environment controls the production of IFN-α in pDCs. Additionally, we identified an activation-dependent soluble form of BDCA-2 (solBDCA-2) in human plasma that functions as heparin antagonist and thereby increases TLR9-driven IFN-α production in pDCs. Of importance, solBDCA-2 levels in the serum were increased in patients with scrub typhus (an acute infectious disease caused by Orientia tsutsugamushi) compared to healthy control subjects and correlated with anti-dsDNA antibodies titers. In contrast, solBDCA-2 levels in plasma from patients with bullous pemphigoid or psoriasis were reduced. In summary, this work identifies a regulatory network consisting of heparin, membrane-bound and solBDCA-2 modulating TLR9-driven IFN-α production in pDCs. This insight into pDCs function and regulation may have implications for the treatment of pDCs-related autoimmune diseases.

The Glycosaminoglycan Side Chains and Modular Core Proteins of Heparan Sulphate Proteoglycans and the Varied Ways They Provide Tissue Protection by Regulating Physiological Processes and Cellular Behaviour

This review examines the roles of HS-proteoglycans (HS-PGs) in general, and, in particular, perlecan and syndecan as representative examples and their interactive ligands, which regulate physiological processes and cellular behavior in health and disease. HS-PGs are essential for the functional properties of tissues both in development and in the extracellular matrix (ECM) remodeling that occurs in response to trauma or disease. HS-PGs interact with a biodiverse range of chemokines, chemokine receptors, protease inhibitors, and growth factors in immune regulation, inflammation, ECM stabilization, and tissue protection. Some cell regulatory proteoglycan receptors are dually modified hybrid HS/CS proteoglycans (betaglycan, CD47). Neurexins provide synaptic stabilization, plasticity, and specificity of interaction, promoting neurotransduction, neurogenesis, and differentiation. Ternary complexes of glypican-1 and Robbo-Slit neuroregulatory proteins direct axonogenesis and neural network formation. Specific neurexin-neuroligin complexes stabilize synaptic interactions and neural activity. Disruption in these interactions leads to neurological deficits in disorders of functional cognitive decline. Interactions with HS-PGs also promote or inhibit tumor development. Thus, HS-PGs have complex and diverse regulatory roles in the physiological processes that regulate cellular behavior and the functional properties of normal and pathological tissues. Specialized HS-PGs, such as the neurexins, pikachurin, and Eyes-shut, provide synaptic stabilization and specificity of neural transduction and also stabilize the axenome primary cilium of phototoreceptors and ribbon synapse interactions with bipolar neurons of retinal neural networks, which are essential in ocular vision. Pikachurin and Eyes-Shut interactions with an α-dystroglycan stabilize the photoreceptor synapse. Novel regulatory roles for HS-PGs controlling cell behavior and tissue function are expected to continue to be uncovered in this fascinating class of proteoglycan.

Construction of mouse cochlin mutants with different GAG-binding specificities and their use for immunohistochemistry

Glycosaminoglycan (GAG) is a polysaccharide present on the cell surface as an extracellular matrix component, and is composed of repeating disaccharide units consisting of an amino sugar and uronic acid except in the case of the keratan sulfate. Sulfated GAGs, such as heparan sulfate, heparin, and chondroitin sulfate mediate signal transduction of growth factors, and their functions vary with the type and degree of sulfated modification. We have previously identified human and mouse cochlins as proteins that bind to sulfated GAGs. Here, we prepared a recombinant cochlin fused to human IgG-Fc or Protein A at the C-terminus as a detection and purification tag and investigated the ligand specificity of cochlin. We found that cochlin can be used as a specific probe for highly sulfated heparan sulfate and chondroitin sulfate E. We then used mutant analysis to identify the mechanism by which cochlin recognizes GAGs and developed a GAG detection system using cochlin. Interestingly, a mutant lacking the vWA2 domain bound to various types of GAGs. The N-terminal amino acid residues of cochlin contributed to its binding to heparin. Pathological specimens from human myocarditis patients were stained with a cochlin-Fc mutant. The results showed that both tryptase-positive and tryptase-negative mast cells were stained with this mutant. The identification of detailed modification patterns of GAGs is an important method to elucidate the molecular mechanisms of various diseases. The method developed for evaluating the expression of highly sulfated GAGs will help understand the biological and pathological importance of sulfated GAGs in the future.

How "Neuronal" Are Human Skin Mast Cells?

Mast cells are evolutionarily old cells and the principal effectors in allergic responses and inflammation. They are seeded from the yolk sac during embryogenesis or are derived from hematopoietic progenitors and are therefore related to other leukocyte subsets, even though they form a separate clade in the hematopoietic system. Herein, we systematically bundle information from several recent high-throughput endeavors, especially those comparing MCs with other cell types, and combine such information with knowledge on the genes’ functions to reveal groups of neuronal markers specifically expressed by MCs. We focus on recent advances made regarding human tissue MCs, but also refer to studies in mice. In broad terms, genes hyper-expressed in MCs, but largely inactive in other myelocytes, can be classified into subcategories such as traffic/lysosomes (MLPH and RAB27B), the dopamine system (MAOB, DRD2, SLC6A3, and SLC18A2), Ca²⁺-related entities (CALB2), adhesion molecules (L1CAM and NTM) and, as an overall principle, the transcription factors and modulators of transcriptional activity (LMO4, PBX1, MEIS2, and EHMT2). Their function in MCs is generally unknown but may tentatively be deduced by comparison with other systems. MCs share functions with the nervous system, as they express typical neurotransmitters (histamine and serotonin) and a degranulation machinery that shares features with the neuronal apparatus at the synapse. Therefore, selective overlaps are plausible, and they further highlight the uniqueness of MCs within the myeloid system, as well as when compared with basophils. Apart from investigating their functional implications in MCs, a key question is whether their expression in the lineage is due to the specific reactivation of genes normally silenced in leukocytes or whether the genes are not switched off during mastocytic development from early progenitors.

Clinical relevance of inherited genetic differences in human tryptases: Hereditary alpha-tryptasemia and beyond

OBJECTIVE

To describe our current understanding of hereditary α-tryptasemia (HαT), how HαT fits into the evolutionary context of tryptases and contemporary framework of mast cell-associated disorders, and to discuss the future clinical and therapeutic landscape for symptomatic individuals with HαT.

DATA SOURCES

Primary peer-reviewed literature.

STUDY SELECTIONS

Basic, clinical, and translational studies describing tryptase gene composition, generation, secretion, and elevation and the associated clinical impacts of HαT and treatment of such individuals were reviewed.

RESULTS

HαT is a common autosomal dominant genetic trait caused by increased TPSAB1 copy number encoding α-tryptase. Approximately 1 in 20 White individuals have HαT, making it by far the most common cause for elevated basal serum tryptase levels. Although many individuals with HαT may not manifest associated symptoms, the prevalence of HαT is increased in patients with clonal and nonclonal mast cell-associated disorders wherein it is linked to more prevalent and/or severe anaphylaxis and increased mast cell mediator-associated symptoms. Increased generation of mature α/β-tryptase heterotetramers, and their unique physiochemical properties, may be responsible for some of these clinical findings.

CONCLUSION

HαT is a common modifier of mast cell-associated disorders and reactions. Nevertheless, whether HαT may be an independent cause of clinical phenotypes with which it has been associated remains unproven. Correct identification of HαT is critical to accurate interpretation of serum tryptase levels in the clinical evaluation of patients. Beyond HαT, we foresee tryptase genotyping as an important parameter in the standard workup of patients with mast cell-associated disorders and development of therapeutic modalities targeting these patients and associated clinical phenotypes.

Current Understanding of IL-37 in Human Health and Disease

IL-37 is a recently discovered cytokine in the IL-1 family exerting broad protective effects on inflammatory diseases, autoimmune diseases, and cancer. Immune and non-immune cells produce the IL-37 precursor upon pro-inflammatory stimuli. Intracellularly, caspase-1 cleaves and activates IL-37, and its mature form binds to Smad3; this complex translocates into the nucleus where it suppresses cytokine production, consequently reducing inflammation. Extracellularly, IL-37 forms a complex with IL-18Rα and IL-1R8 (formerly TIR8 or SIGIRR) that transduces anti-inflammatory signals by the suppression of NF-κB and MAPK and the activation of Mer-PTEN-DOK pathways. During inflammation, IL-37 suppresses the expression of several pro-inflammatory cytokine in favor to the expression of the anti-inflammatory ones by the regulation of macrophage polarization, lipid metabolism, inflammasome function, TSLP synthesis and miRNAs function. Moreover, IL-37 not only regulates the innate and acquired immunity, but also improves aging-associated immunosenescence. Furthermore, IL-37 exerts an inhibitory effect on tumor angiogenesis and metastasis, and progression. Finally, IL-37 may have a potential ability to reduce excessive inflammation since it is aberrantly expressed in patients with inflammatory diseases, autoimmune diseases, and cancer, thus, it may be used as a marker for different types of diseases. Therefore, this review provides an updated view of the role of IL-37 in human health and disease, and discusses the potential of IL-37 as a therapeutic target and biomarker in inflammatory diseases, autoimmune diseases, and cancer.

SNAP23 is essential for platelet and mast cell development and required in connective tissue mast cells for anaphylaxis

Degranulation, a fundamental effector response from mast cells (MCs) and platelets, is an example of regulated exocytosis. This process is mediated by SNARE proteins and their regulators. We have previously shown that several of these proteins are essential for exocytosis in MCs and platelets. Here, we assessed the role of the SNARE protein SNAP23 using conditional knockout mice, in which SNAP23 was selectively deleted from either the megakaryocyte/platelet or connective tissue MC lineages. We found that removal of SNAP23 in platelets results in severe defects in degranulation of all three platelet secretory granule types, i.e., alpha, dense, and lysosomal granules. The mutation also induces thrombocytopenia, abnormal platelet morphology and activation, and reduction in the number of alpha granules. Therefore, the degranulation defect might not be secondary to an intrinsic failure of the machinery mediating regulated exocytosis in platelets. When we removed SNAP23 expression in MCs, there was a complete developmental failure in vitro and in vivo. The developmental defects in platelets and MCs and the abnormal translocation of membrane proteins to the surface of platelets indicate that SNAP23 is also involved in constitutive exocytosis in these cells. The MC conditional deletant animals lacked connective tissue MCs, but their mucosal MCs were normal and expanded in response to an antigenic stimulus. We used this mouse to show that connective tissue MCs are required and mucosal MCs are not sufficient for an anaphylactic response.

Advances in the preparation and synthesis of heparin and related products

Heparin is a naturally occurring glycosaminoglycan from livestock, principally porcine intestine, and is clinically used as an anticoagulant drug. A limitation to heparin production is that it depends on a single animal species and potential problems have been associated with animal-derived heparin. The contamination crisis in 2008 led to a search for new animal sources and the investigation of non-animal sources of heparin. Over the past 5 years, new animal sources, chemical, and chemoenzymatic methods have been introduced to prepare heparin-based drugs. In this review, we describe advances in the preparation and synthesis of heparin and related products.

Argon Atmospheric Plasma Treatment Promotes Burn Healing by Stimulating Inflammation and Controlling the Redox State

Burns are a public health problem, with second-degree burns as one of the most common types. Although intense inflammation worsens burn healing, effective therapies are scarce. Thus, infections and hypertrophic scars may occur, which compromise patient quality of life and may delay healing. Argon atmospheric plasma (AP) has been shown to positively influence wound healing. In the context of identifying effective and alternative therapies for the treatment of second-degree burns, the present study evaluated AP in the treatment of second-degree burns in rats compared to that for sham treatment on the 2nd, 7th, 14th, and 21st days post-injury. Our results revealed proinflammatory effect for AP by recruiting predominantly neutrophils on the 7th day and macrophages on the 21st day compared to sham treatment, allowing a greater production of interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and IL-17, and also controlled the inflammation by IL-10 and transforming growth factor (TGF)-β1. AP also showed antioxidant activity important for controlling oxidative damage on the 2nd day. This favored the induction of angiogenesis from the 2nd day and induction fibroplasia and fibrillogenesis after the 14th day, which enhanced burn healing with the formation of a thinner burn eschar before the 21st day post-burn. Thus, AP effectively modulated the inflammatory phase of second-degree burn healing through the control of oxidative damage that favored the following phases. Therefore, AP is a relevant alternative in the treatment of second-degree burns.

Mast cells as a unique hematopoietic lineage and cell system: From Paul Ehrlich's visions to precision medicine concepts

The origin and functions of mast cells (MCs) have been debated since their description by Paul Ehrlich in 1879. MCs have long been considered 'reactive bystanders' and 'amplifiers' in inflammatory processes, allergic reactions, and host responses to infectious diseases. However, knowledge about the origin, phenotypes and functions of MCs has increased substantially over the past 50 years. MCs are now known to be derived from multipotent hematopoietic progenitors, which, through a process of differentiation and maturation, form a unique hematopoietic lineage residing in multiple organs. In particular, MCs are distinguishable from basophils and other hematopoietic cells by their unique phenotype, origin(s), and spectrum of functions, both in innate and adaptive immune responses and in other settings. The concept of a unique MC lineage is further supported by the development of a distinct group of neoplasms, collectively referred to as mastocytosis, in which MC precursors expand as clonal cells. The clinical consequences of the expansion and/or activation of MCs are best established in mastocytosis and in allergic inflammation. However, MCs have also been implicated as important participants in a number of additional pathologic conditions and physiological processes. In this article, we review concepts regarding MC development, factors controlling MC expansion and activation, and some of the fundamental roles MCs may play in both health and disease. We also discuss new concepts for suppressing MC expansion and/or activation using molecularly-targeted drugs.

Pharmacotherapy in Mast Cell Leukemia

INTRODUCTION

Mast cell leukemia (MCL) is one of the most aggressive forms of Systemic Mastocytosis (SM), a complex family of rare diseases, for which standard therapies are very few. MCL represents only <1% cases of SM and this is the reason why there are no specific clinical trials to better explore this disease. As a consequence, MCL is treated and grouped within other forms of SM, being all KIT-driven diseases; however, its KIT dysregulation leads to uncontrolled activation of mast cells (MCs), which correlates with forms of myeloid acute leukemia (AML).

AREAS COVERED

Different therapeutic approaches can be followed in the treatment of MCL. The authors look at both symptomatic therapies along with other approaches including targeted therapy. Further, the authors provide their expert opinion.

EXPERT OPINION

In the scenario of mast cell leukemia treatment, the key approach to achieve clinical results is, more than other similar pathologies, personalizing the therapy. It could be interesting or desirable to introduce for instance KIT mutant forms as minor criteria for the diagnosis of advanced SM, considering the small patient population with MCL and the relatively large panel of activating mutations for KIT and other important proteins involved in MCs' regulation.

Rupatadine, a dual antagonist of histamine and platelet-activating factor (PAF), attenuates experimentally induced diabetic nephropathy in rats

The role of histamine and platelet activating factor (PAF) as involved mediators in the pathophysiology of diabetic complications, in particular diabetic nephropathy (DN), has become a new focus of concern. Accordingly, the present study designed to explore the effect of rupatadine (RUP), a dual antagonist of histamine (H1) and PAF, on the progression of experimentally induced DN in rats. Rats were divided into five groups: control, RUP alone, streptozotocin (STZ)-diabetic model, STZ/RUP (3 mg/kg/day), and STZ/RUP (6 mg/kg/day). Treatment has continued for 4 weeks after diabetes confirmation. At the end of the study, serum was collected for measurement of glucose, insulin, urea, creatinine, histamine, and PAF. Renal tissue homogenates were prepared for measuring oxidative stress indices, tumor necrosis factor (TNF-α), cystatin C, and p21. Moreover, immunohistochemical expression of transforming growth factor-β1 (TGF-β1) and p53 along with histological pictures was also conducted. Antagonizing H1 and PAF receptors by RUP ameliorated the experimentally induced DN as evident by decreasing all serum parameters augmented by STZ together with improvement of the histopathological picture. RUP administration also improved oxidative-antioxidative agents with reduction in the anti-inflammatory marker, TNF-α. Additionally, the immunohistochemical expression of the fibrosis marker; TGF-β1, was also decreased. STZ-induced DN showed a p21/p53-dependent induction of premature senescence and RUP administration decreased the expression of p21 and p53 levels in injured renal tissue. RUP represents a novel promising drug to prevent DN complicated diabetes probably via its inhibitory effect on H1 and PAF receptors. The renal protection was also related to the anti-inflammatory and antioxidant roles and PAF-facilitated senescence effect via p21/p53 signaling.

KLF4 is required for suppression of histamine synthesis by polyamines during bone marrow-derived mast cell differentiation

Mast cells have secretory granules containing chemical mediators such as histamine and play important roles in the immune system. Polyamines are essential factors for cellular processes such as gene expression and translation. It has been reported that secretory granules contain both histamine and polyamines, which have similar chemical structures and are produced from the metabolism of cationic amino acids. We investigated the effect of polyamine depletion on mast cells using bone marrow-derived mast cells (BMMCs). Polyamine depletion was induced using α-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase. DFMO treatment resulted in a significant reduction of cell number and abnormal secretory granules in BMMCs. Moreover, the cells showed a 2.3-fold increase in intracellular histamine and up-regulation of histidine decarboxylase (HDC) at the transcriptional level during BMMC differentiation. Levels of the transcription factor kruppel-like factor 4 (KLF4) greatly decreased upon DFMO treatment; however, Klf4 mRNA was expressed at levels similar to controls. We determined the translational regulation of KLF4 using reporter genes encoding Klf4-luc2 fusion mRNA, for transfecting NIH3T3 cells, and performed in vitro translation. We found that the efficiency of KLF4 synthesis in response to DFMO treatment was enhanced by the existence of a GC-rich 5'-untranslated region (5'-UTR) on Klf4 mRNA, regardless of the recognition of the initiation codon. Taken together, these results indicate that the enhancement of histamine synthesis by DFMO depends on the up-regulation of Hdc expression, achieved by removal of transcriptional suppression of KLF4, during differentiation.

Metabolism in Acute-On-Chronic Liver Failure: The Solution More than the Problem

Chronic inflammatory liver disease with an acute deterioration of liver function is named acute-on-chronic inflammation and could be regulated by the metabolic impairments related to the liver dysfunction. In this way, the experimental cholestasis model is excellent for studying metabolism in both types of inflammatory responses. Along the evolution of this model, the rats develop biliary fibrosis and an acute-on-chronic decompensation. The acute decompensation of the liver disease is associated with encephalopathy, ascites, acute renal failure, an acute phase response and a splanchnic increase of pro- and anti-inflammatory cytokines. This multiorgan inflammatory dysfunction is mainly associated with a splanchnic and systemic metabolic switch with dedifferentiation of the epithelial, endothelial and mesothelial splanchnic barriers. Furthermore, a splanchnic infiltration by mast cells occurs, which suggests that these cells could carry out a compensatory metabolic role, especially through the modulation of hepatic and extrahepatic mitochondrial-peroxisome crosstalk. For this reason, we propose the hypothesis that mastocytosis in the acute-on-chronic hepatic insufficiency could represent the development of a survival metabolic mechanisms that mitigates the noxious effect of the hepatic functional deficit. A better understanding the pathophysiological response of the mast cells in liver insufficiency and portal hypertension would help to find new pathways for decreasing the high morbidity and mortality rate of these patients.

An evaluation of masitinib for treating systemic mastocytosis

Introduction: Systemic Mastocytosis (SM) is a complex family of rare diseases, against which pharmacological therapies are still very few. It is a c-kit driven disease, whose disregulation leads to uncontrolled activation and proliferation of mast cells (MCs) with consequent release of effector molecules which are responsible for its clinical manifestations. Areas covered: Masitinib is a relatively new potential drug against SM and its chemical structure strictly derives from imatinib, the first tyrosine kinase inhibitor which entered the pharmaceutical market about 15 years ago. In this review, the authors present masitinib in all its properties, from chemistry to pharmacology and toxicity to its potential clinical application in SM, focusing the discussion on the few clinical trials in which it has been involved, with a particular attention on the still open challenge to determine how to measure the response to therapy. Expert opinion: In spite of their similarity in chemistry and biological activity against submolecular targets, masitinib is much more selective towards c-kit receptors than other tyrosine kinases, such as Bcl-Abl. Furthermore, its ability to inhibit degranulation, cytokine production and MCs migration from bone marrow gives it a great chance to become an important therapeutic option for selected SM patients.

Mast Cells May Regulate The Anti-Inflammatory Activity of IL-37

Mast cells are unique immune cells involved in allergic reactions, but also in immunity and inflammation. Interleukin 37 (IL-37) has emerged as an important regulatory cytokine with ability to inhibit immune and inflammatory processes. IL-37 is made primarily by macrophages upon activation of toll-like receptors (TLR) leading to generation of mature IL-37 via the action of caspase 1. In this review, we advance the premise that mast cells could regulate the anti-inflammatory activity of the IL-37 via their secretion of heparin and tryptase. Extracellular IL-37 could either dimerize in the presence of heparin and lose biological activity, or be acted upon by proteases that can generate even more biologically active IL-37 forms. Molecules that could selectively inhibit the secretion of mast cell mediators may, therefore, be used together with IL-37 as novel therapeutic agents.

Mast cell tryptase - Marker and maker of cardiovascular diseases

Mast cells are tissue-resident cells, which have been proposed to participate in various inflammatory diseases, among them the cardiovascular diseases (CVDs). For mast cells to be able to contribute to an inflammatory process, they need to be activated to exocytose their cytoplasmic secretory granules. The granules contain a vast array of highly bioactive effector molecules, the neutral protease tryptase being the most abundant protein among them. The released tryptase may act locally in the inflamed cardiac or vascular tissue, so contributing directly to the pathogenesis of CVDs. Moreover, a fraction of the released tryptase reaches the systemic circulation, thereby serving as a biomarker of mast cell activation. Actually, increased levels of circulating tryptase have been found to associate with CVDs. Here we review the biological relevance of the circulating tryptase as a biomarker of mast cell activity in CVDs, with special emphasis on the relationship between activation of mast cells in their tissue microenvironments and the pathophysiological pathways of CVDs. Based on the available in vitro and in vivo studies, we highlight the potential molecular mechanisms by which tryptase may contribute to the pathogenesis of CVDs. Finally, the synthetic and natural inhibitors of tryptase are reviewed for their potential utility as therapeutic agents in CVDs.

Syntaxin 3, but not syntaxin 4, is required for mast cell-regulated exocytosis, where it plays a primary role mediating compound exocytosis

Mast cells (MCs) participate in allergy, inflammation, and defense against pathogens. They release multiple immune mediators via exocytosis, a process that requires SNARE proteins, including syntaxins (Stxs). The identity of the Stxs involved in MC exocytosis remains controversial. Here, we studied the roles of Stx3 and -4 in fully developed MCs from conditional knockout mice by electrophysiology and EM, and found that Stx3, and not Stx4, is crucial for MC exocytosis. The main defect seen in Stx3-deficient MCs was their inability to engage multigranular compound exocytosis, while leaving most single-vesicle fusion events intact. We used this defect to show that this form of exocytosis is not only required to accelerate MC degranulation but also essential to achieve full degranulation. The exocytic defect was severe but not absolute, indicating that an Stx other than Stx3 and -4 is also required for exocytosis in MCs. The removal of Stx3 affected only regulated exocytosis, leaving other MC effector responses intact, including the secretion of cytokines via constitutive exocytosis. Our in vivo model of passive systemic anaphylaxis showed that the residual exocytic function of Stx3-deficient MCs was sufficient to drive a full anaphylactic response in mice.

Dual functionality of β-tryptase protomers as both proteases and cofactors in the active tetramer

Human β-tryptase, a tetrameric trypsin-like serine protease, is an important mediator of the allergic inflammatory responses in asthma. During acute hypersensitivity reactions, mast cells degranulate, releasing active tetramer as a complex with proteoglycans. Extensive efforts have focused on developing therapeutic β-tryptase inhibitors, but its unique activation mechanism is less well-explored. Tryptase is active only after proteolytic removal of the pro-domain followed by tetramer formation via two distinct symmetry-related interfaces. We show that the cleaved I16G mutant cannot tetramerize, likely due to impaired insertion of its N terminus into its "activation pocket," indicating allosteric linkage at multiple sites on each protomer. We engineered cysteines into each of the two distinct interfaces (Y75C for small or I99C for large) to assess the activity of each tetramer and disulfide-locked dimer. Using size-exclusion chromatography and enzymatic assays, we demonstrate that the two large tetramer interfaces regulate enzymatic activity, elucidating the importance of this protein-protein interaction for allosteric regulation. Notably, the I99C large interface dimer is active, even in the absence of heparin. We show that a monomeric β-tryptase mutant (I99C*/Y75A/Y37bA, where C* is cysteinylated Cys-99) cannot form a dimer or tetramer, yet it is active but only in the presence of heparin. Thus heparin both stabilizes the tetramer and allosterically conditions the active site. We hypothesize that each β-tryptase protomer in the tetramer has two distinct roles, acting both as a protease and as a cofactor for its neighboring protomer, to allosterically regulate enzymatic activity, providing a rationale for direct correlation of tetramer stability with proteolytic activity.

Mucosal Mesenchymal Cells: Secondary Barrier and Peripheral Educator for the Gut Immune System

Stromal connective tissue contains mesenchymal cells, including fibroblasts and myofibroblasts, which line the tissue structure. However, it has been identified that the function of mesenchymal cells is not just structural-they also play critical roles in the creation and regulation of intestinal homeostasis. Thus, mucosal mesenchymal cells instruct intestinal immune cell education (or peripheral immune education) and epithelial cell differentiation thereby shaping the local environment of the mucosal immune system. Malfunction of the mesenchymal cell-mediated instruction system (e.g., fibrosis) leads to pathological conditions such as intestinal stricture.

Pressure overload leads to an increased accumulation and activity of mast cells in the right ventricle

Right ventricular (RV) remodeling represents a complex set of functional and structural adaptations in response to chronic pressure or volume overload due to various inborn defects or acquired diseases and is an important determinant of patient outcome. However, the underlying molecular mechanisms remain elusive. We investigated the time course of structural and functional changes in the RV in the murine model of pressure overload‐induced RV hypertrophy in C57Bl/6J mice. Using magnetic resonance imaging, we assessed the changes of RV structure and function at different time points for a period of 21 days. Pressure overload led to significant dilatation, cellular and chamber hypertrophy, myocardial fibrosis, and functional impairment of the RV. Progressive remodeling of the RV after pulmonary artery banding (PAB) in mice was associated with upregulation of myocardial gene markers of hypertrophy and fibrosis. Furthermore, remodeling of the RV was associated with accumulation and activation of mast cells in the RV tissue of PAB mice. Our data suggest possible involvement of mast cells in the RV remodeling process in response to pressure overload. Mast cells may thus represent an interesting target for the development of new therapeutic approaches directed specifically at the RV.

Munc13 proteins control regulated exocytosis in mast cells

Mast cells (MCs) are involved in host defenses against pathogens and inflammation. Stimulated MCs release substances stored in their granules via regulated exocytosis. In other cell types, Munc13 (mammalian homolog of Caenorhabditis elegans uncoordinated gene 13) proteins play essential roles in regulated exocytosis. Here, we found that MCs express Munc13-2 and -4, and we studied their roles using global and conditional knock-out (KO) mice. In a model of systemic anaphylaxis, we found no difference between WT and Munc13-2 KO mice, but global and MC-specific Munc13-4 KO mice developed less hypothermia. This protection correlated with lower plasma histamine levels and with histological evidence of defective MC degranulation but not with changes in MC development, distribution, numbers, or morphology. In vitro assays revealed that the defective response in Munc13-4-deficient MCs was limited to regulated exocytosis, leaving other MC secretory effector responses intact. Single cell capacitance measurements in MCs from mouse mutants differing in Munc13-4 expression levels in their MCs revealed that as levels of Munc13-4 decrease, the rate of exocytosis declines first, and then the total amount of exocytosis decreases. A requirement for Munc13-2 in MC exocytosis was revealed only in the absence of Munc13-4. Electrophysiology and EM studies uncovered that the number of multigranular compound events (i.e. granule-to-granule homotypic fusion) was severely reduced in the absence of Munc13-4. We conclude that although Munc13-2 plays a minor role, Munc13-4 is essential for regulated exocytosis in MCs, and that this MC effector response is required for a full anaphylactic response.

Sulfur mustard induced mast cell degranulation in mouse skin is inhibited by a novel anti-inflammatory and anticholinergic bifunctional prodrug

Sulfur mustard (SM, bis(2-chloroethyl sulfide) is a potent vesicating agent known to cause skin inflammation, necrosis and blistering. Evidence suggests that inflammatory cells and mediators that they generate are important in the pathogenic responses to SM. In the present studies we investigated the role of mast cells in SM-induced skin injury using a murine vapor cup exposure model. Mast cells, identified by toluidine blue staining, were localized in the dermis, adjacent to dermal appendages and at the dermal/epidermal junction. In control mice, 48-61% of mast cells were degranulated. SM exposure (1.4g/m³ in air for 6min) resulted in increased numbers of degranulated mast cells 1-14days post-exposure. Treatment of mice topically with an indomethacin choline bioisostere containing prodrug linked by an aromatic ester-carbonate that targets cyclooxygenases (COX) enzymes and acetylcholinesterase (1% in an ointment) 1-14days after SM reduced skin inflammation and injury and enhanced tissue repair. This was associated with a decrease in mast cell degranulation from 90% to 49% 1-3days post SM, and from 84% to 44% 7-14days post SM. These data suggest that reduced inflammation and injury in response to the bifunctional indomethacin prodrug may be due, at least in part, to abrogating mast cell degranulation. The use of inhibitors of mast cell degranulation may be an effective strategy for mitigating skin injury induced by SM.

Heparin: new life for an old drug

Heparin is one of the oldest drugs, which nevertheless remains in widespread clinical use as an inhibitor of blood coagulation. The history of its identification a century ago unfolded amid one of the most fascinating scientific controversies turning around the distribution of credit for its discovery. The composition, purification and structure-function relationship of this naturally occurring glycosaminoglycan regarding its classical role as anticoagulant will be dealt with before proceeding to discuss its therapeutic potential in, among other, inflammatory and infectious disease, cancer treatment, cystic fibrosis and Alzheimer's disease. The first bibliographic reference hit using the words 'nanomedicine' and 'heparin' is as recent as 2008. Since then, nanomedical applications of heparin have experienced an exponential growth that will be discussed in detail, with particular emphasis on its antimalarial activity. Some of the most intriguing potential applications of heparin nanomedicines will be exposed, such as those contemplating the delivery of drugs to the mosquito stages of malaria parasites.

Effect of tryptase inhibition on joint inflammation: a pharmacological and lentivirus-mediated gene transfer study

Background

Increasing evidences indicate that an unbalance between tryptases and their endogenous inhibitors, leading to an increased proteolytic activity, is implicated in the pathophysiology of rheumatoid arthritis. The aim of the present study was to evaluate the impact of tryptase inhibition on experimental arthritis.

Methods

Analysis of gene expression and regulation in the mouse knee joint was performed by RT-qPCR and in situ hybridization. Arthritis was induced in male C57BL/6 mice with mBSA/IL-1β. Tryptase was inhibited by two approaches: a lentivirus-mediated heterologous expression of the human endogenous tryptase inhibitor, sperm-associated antigen 11B isoform C (hSPAG11B/C), or a chronic treatment with the synthetic tryptase inhibitor APC366. Several inflammatory parameters were evaluated, such as oedema formation, histopathology, production of IL-1β, -6, -17A and CXCL1/KC, myeloperoxidase and tryptase-like activities.

Results

Spag11c was constitutively expressed in chondrocytes and cells from the synovial membrane in mice, but its expression did not change 7 days after the induction of arthritis, while tryptase expression and activity were upregulated. The intra-articular transduction of animals with the lentivirus phSPAG11B/C or the treatment with APC366 inhibited the increase of tryptase-like activity, the late phase of oedema formation, the production of IL-6 and CXCL1/KC. In contrast, neutrophil infiltration, degeneration of hyaline cartilage and erosion of subchondral bone were not affected.

Conclusions

Tryptase inhibition was effective in inhibiting some inflammatory parameters associated to mBSA/IL-1β-induced arthritis, notably late phase oedema formation and IL-6 production, but not neutrophil infiltration and joint degeneration. These results suggest that the therapeutic application of tryptase inhibitors to rheumatoid arthritis would be restrained to palliative care, but not as disease-modifying drugs. Finally, this study highlighted lentivirus-based gene delivery as an instrumental tool to study the relevance of target genes in synovial joint physiology and disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1326-9) contains supplementary material, which is available to authorized users.

Engineered heparins as new anticoagulant drugs

Heparin is an anionic polysaccharide that is widely used as a clinical anticoagulant. This glycosaminoglycan is prepared from animal tissues in metric ton quantities. Animal-sourced heparin is also widely used in the preparation of low molecular weight heparins that are gaining in popularity as a result of their improved pharmacological properties. The recent contamination of pharmaceutical heparin together with concerns about increasing demand for this life saving drug and the fragility of the heparin supply chain has led the scientific community to consider other potential sources for heparin. This review examines progress toward the preparation of engineered heparins through chemical synthesis, chemoenzymatic synthesis, and metabolic engineering.

Surprising absence of heparin in the intestinal mucosa of baby pigs

Heparin, a member of a family of molecules called glycosaminoglycans, is biosynthesized in mucosal mast cells. This important anticoagulant polysaccharide is primarily produced by extraction of the mast cell-rich intestinal mucosa of hogs. There is concern about our continued ability to supply sufficient heparin to support the worldwide growth of advanced medical procedures from the static population of adult hogs used as food animals. While the intestinal mucosa of adult pigs is rich in anticoagulant heparin (containing a few hundred milligrams per animal), little is known about how the content of heparin changes with animal age. Using sophisticated mass spectral analysis we discovered that heparin was largely absent from the intestinal mucosa of piglets. Moreover, while the related, nonanticoagulant heparan sulfate glycosaminoglycan was present in significant amounts we found little chondroitin sulfate E also associated with mast cells. Histological evaluation of piglet intestinal mucosa showed a very low mast cell content. Respiratory mast cells have been reported in baby pigs suggesting that there was something unique about the piglets used in the current study. These piglets were raised in the relatively clean environment of a university animal facility and treated with antibiotics over their lifetime resulting in a depleted microbiome that greatly reduced the number of mast cells and heparin content of the intestinal mucosal in these animals. Thus, from the current study it remains unclear whether the lack of intestinal mast cell-derived heparin results from the young age of these animals or their exposure to their depleted microbiome.

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.

Mast Cells: Key Players in the Shadow in Oral Inflammation and in Squamous Cell Carcinoma of the Oral Cavity

Although mast cells (MCs) have been discovered over 130 years ago, their function was almost exclusively linked to allergic affections. At the time being, it is well known that MCs possess a great variety of roles, in both physiologic and pathologic conditions. In the oral tissues, MCs release different proinflammatory cytokines, tumor necrosis factor alpha (TNF-α), that promote leukocyte infiltration in various inflammatory states of the oral cavity. These cells play a key role in the inflammatory process and, as a consequence, their number changes in different pathologic conditions of the oral cavity, like gingivitis, periodontitis, and so on. MCs also represent a rich source of proteases, especially of mast cell tryptase and chymase, which directly degrade the extracellular matrix through their proteolytic activity and thus indirectly stimulate angiogenesis and facilitate invasion and metastasis. It may be stated that mast cells could have an impact on primary tumor development, progression, and metastases in oral squamous cell carcinoma. By understanding the role of mast cells in the pathogenesis of different inflammatory and tumor diseases of the oral cavity, these cells may become therapeutic targets that could possibly improve the prognosis and survival of these patients.

Serglycin proteoglycans limit enteropathy in Trichinella spiralis-infected mice

BACKGROUND

Serglycin proteoglycans are essential for maturation of secretory granules and for the correct granular storage of cationic proteases in hematopoietic cells, e.g. mast cells. However, little is known about the in vivo functions of serglycin proteoglycans during infection. Here we investigated the potential role of serglycin proteoglycans in host defense after infection with the nematode Trichinella spiralis.

RESULTS

Twelve days post infection lack of serglycin proteoglycans caused significantly increased enteropathy. The serglycin-deficient mice showed significantly increased intestinal worm burden, reduced recruitment of mast cells to the intestinal crypts, decreased levels of the mast cell proteases MCPT5 and MCPT6 in intestinal tissue, decreased serum levels of TNF-α, IL-1β, IL-10 and IL-13, increased levels of IL-4 and total IgE in serum, and increased intestinal levels of the neutrophil markers myeloperoxidase and elastase, as compared to wild type mice. At five weeks post infection, increased larvae burden and inflammation were seen in the muscle tissue of the serglycin-deficient mice.

CONCLUSIONS

Our results demonstrate that the serglycin-deficient mice were more susceptible to T. spiralis infection and displayed an unbalanced immune response compared to wild type mice. These findings point to an essential regulatory role of serglycin proteoglycans in immunity.

Transcriptional regulation of mast cell and basophil lineage commitment

Basophils and mast cells have long been known to play critical roles in allergic disease and in immunity against parasitic infection. Accumulated evidence also supports that basophils and mast cells have important roles in immune regulations, host defense against bacteria and viruses, and autoimmune diseases. However, origin and molecular regulation of basophil and mast cell differentiation remain incompletely understood. In this review, we focus on recent advances in the understanding of origin and molecular regulation of mouse and human basophil and mast cell development. A more complete understanding of how basophils and mast cells develop at the molecular level will lead to development of interventions that are more effective in achieving long-term success.

IL-6 promotes an increase in human mast cell numbers and reactivity through suppression of suppressor of cytokine signaling 3

BACKGROUND

IL-6, levels of which are reported to be increased in association with mastocytosis, asthma, and urticaria, is used in conjunction with stem cell factor to generate CD34(+) cell-derived primary human mast cell (HuMC) cultures. Despite these associations, the effects on and mechanisms by which prolonged exposure to IL-6 alters HuMC numbers and function are not well understood.

OBJECTIVES

We sought to study the effect of IL-6 on HuMC function, the mechanisms by which IL-6 exerts its effects, and the relationship of these findings to mastocytosis.

METHODS

HuMCs were cultured in stem cell factor with or without IL-6. Responses to FcεRI aggregation and expression of proteases and receptors, including the soluble IL-6 receptor (sIL-6R), were then quantitated. Epigenetic changes in suppressor of cytokine signaling 3 (SOCS3) were determined by using methylation-specific PCR. Serum samples from healthy control subjects and patients with mastocytosis were assayed for IL-6, tryptase, and sIL-6R.

RESULTS

IL-6 enhanced mast cell (MC) proliferation, maturation, and reactivity after FcεRI aggregation. IL-6 reduced expression of SOCS3, which correlated with methylation of the SOCS3 promoter and increased expression and activation of signal transducer and activator of transcription 3. IL-6 also suppressed constitutive production of sIL-6R, and serum levels of sIL-6R were similarly reduced in patients with mastocytosis.

CONCLUSION

IL-6 increases MC proliferation and formation of a more reactive phenotype enabled by suppressing proteolytic cleavage of sIL-6R from IL-6R and downregulation of the SOCS3 autoinhibitory pathway. We suggest IL-6 blockade might ameliorate MC-related symptoms and pathology in patients with MC-related diseases associated with increased IL-6 levels, including mastocytosis.

Identification of a New Exo-Endocytic Mechanism Triggered by Corticotropin-Releasing Hormone in Mast Cells

The key role of mast cells (MC), either in development of inflammatory pathologies or in response to environmental stress, has been widely reported in recent years. Previous studies have described the effects of corticotropin-releasing hormone (CRH), which is released from inflamed tissues by cellular stress signals, on MC degranulation, a process possibly driven by selective secretion of mediators (piecemeal degranulation). In this study, we introduce a novel granular exo-endocytic pathway induced by CRH on peritoneal MC. We found that CRH triggers substantial exocytosis, which is even stronger than that induced by Ag stimulation and is characterized by large quantal size release events. Membrane fluorescence increases during stimulation in the presence of FM1-43 dye, corroborating the strength of this exocytosis, given that discrete upward fluorescence steps are often observed and suggesting that secretory granules are preferentially released by compound exocytosis. Additionally, the presence of a depot of large tubular organelles in the cytoplasm suggests that the exocytotic process is tightly coupled to a fast compound endocytosis. This CRH-stimulated mechanism is mediated through activation of adenylate cyclase and an increase of cAMP and intracellular Ca(2+), as evidenced by the fact that the effect of CRH is mimicked by forskolin and 8-bromo-cAMP. Thus, these outcomes constitute new evidence for the critical role of MC in pathophysiological conditions within a cellular stress environment and an alternative membrane trafficking route mediated by CRH.

Mast cell proteases as pharmacological targets

Mast cells are rich in proteases, which are the major proteins of intracellular granules and are released with histamine and heparin by activated cells. Most of these proteases are active in the granule as well as outside of the mast cell when secreted, and can cleave targets near degranulating mast cells and in adjoining tissue compartments. Some proteases released from mast cells reach the bloodstream and may have far-reaching actions. In terms of relative amounts, the major mast cell proteases include the tryptases, chymases, cathepsin G, carboxypeptidase A3, dipeptidylpeptidase I/cathepsin C, and cathepsins L and S. Some mast cells also produce granzyme B, plasminogen activators, and matrix metalloproteinases. Tryptases and chymases are almost entirely mast cell-specific, whereas other proteases, such as cathepsins G, C, and L are expressed by a variety of inflammatory cells. Carboxypeptidase A3 expression is a property shared by basophils and mast cells. Other proteases, such as mastins, are largely basophil-specific, although human basophils are protease-deficient compared with their murine counterparts. The major classes of mast cell proteases have been targeted for development of therapeutic inhibitors. Also, a human β-tryptase has been proposed as a potential drug itself, to inactivate of snake venins. Diseases linked to mast cell proteases include allergic diseases, such as asthma, eczema, and anaphylaxis, but also include non-allergic diseases such as inflammatory bowel disease, autoimmune arthritis, atherosclerosis, aortic aneurysms, hypertension, myocardial infarction, heart failure, pulmonary hypertension and scarring diseases of lungs and other organs. In some cases, studies performed in mouse models suggest protective or homeostatic roles for specific proteases (or groups of proteases) in infections by bacteria, worms and other parasites, and even in allergic inflammation. At the same time, a clearer picture has emerged of differences in the properties and patterns of expression of proteases expressed in human mast cell subsets, and in humans versus other mammals. These considerations are influencing prioritization of specific protease targets for therapeutic inhibition, as well as options of pre-clinical models, disease indications, and choice of topical versus systemic routes of inhibitor administration.

Development of a photoreactive probe-based system for detecting heparin

We previously identified a peptide heparin-associated peptide Y (HappY) that binds specifically to heparin. In this article, we report a novel heparin detection system using chemically modified HappY as a probe. The photoreactive HappY probe was serially diluted and dispensed into a 96-well plate coated with biotinylated heparin. After ultraviolet irradiation, the HappY probe crosslinked to the heparin on the plate was detected with fluorescein isothiocyanate-conjugated streptavidin. Furthermore, the photoreactive HappY probe was used to stain cutaneous tissue sections obtained from dermatitis-affected or mastocytoma-affected cats and dogs. The photoreactive HappY probe stained limited resident mast cells in the connective tissue of skin compared with the anti-heparan sulfate monoclonal antibody 10E4, suggesting that the probe can be used to distinguish the structure of heparin in tissues. The interactions between glycosaminoglycans and proteins in vivo tend to be weak. Therefore, our method for enhancing such weak interactions may be a promising tool for intermolecular interaction studies in glycobiology research.

Mast cells form antibody-dependent degranulatory synapse for dedicated secretion and defence

Mast cells are tissue-resident immune cells that play a key role in inflammation and allergy. Here we show that interaction of mast cells with antibody-targeted cells induces the polarized exocytosis of their granules resulting in a sustained exposure of effector enzymes, such as tryptase and chymase, at the cell-cell contact site. This previously unidentified mast cell effector mechanism, which we name the antibody-dependent degranulatory synapse (ADDS), is triggered by both IgE- and IgG-targeted cells. ADDSs take place within an area of cortical actin cytoskeleton clearance in the absence of microtubule organizing centre and Golgi apparatus repositioning towards the stimulating cell. Remarkably, IgG-mediated degranulatory synapses also occur upon contact with opsonized Toxoplasma gondii tachyzoites resulting in tryptase-dependent parasite death. Our results broaden current views of mast cell degranulation by revealing that human mast cells form degranulatory synapses with antibody-targeted cells and pathogens for dedicated secretion and defence.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

Vesicular trafficking and signaling for cytokine and chemokine secretion in mast cells

Upon activation mast cells (MCs) secrete numerous inflammatory compounds stored in their cytoplasmic secretory granules by a process called anaphylactic degranulation, which is responsible for type I hypersensitivity responses. Prestored mediators include histamine and MC proteases but also some cytokines and growth factors making them available within minutes for a maximal biological effect. Degranulation is followed by the de novo synthesis of lipid mediators such as prostaglandins and leukotrienes as well as a vast array of cytokines, chemokines, and growth factors, which are responsible for late phase inflammatory responses. While lipid mediators diffuse freely out of the cell through lipid bilayers, both anaphylactic degranulation and secretion of cytokines, chemokines, and growth factors depends on highly regulated vesicular trafficking steps that occur along the secretory pathway starting with the translocation of proteins to the endoplasmic reticulum. Vesicular trafficking in MCs also intersects with endocytic routes, notably to form specialized cytoplasmic granules called secretory lysosomes. Some of the mediators like histamine reach granules via specific vesicular monoamine transporters directly from the cytoplasm. In this review, we try to summarize the available data on granule biogenesis and signaling events that coordinate the complex steps that lead to the release of the inflammatory mediators from the various vesicular carriers in MCs.

Inositol phosphates induce DAPI fluorescence shift

The polymer inorganic polyP (polyphosphate) and inositol phosphates, such as IP6 (inositol hexakisphosphate; also known as phytic acid), share many biophysical features. These similarities must be attributed to the phosphate groups present in these molecules. Given the ability of polyP to modify the excitation-emission spectra of DAPI we decided to investigate whether inositol phosphates possess the same property. We discovered that DAPI-IP6 complexes emit at approximately 550 nm when excited with light of wavelength 410-420 nm. IP5 (inositol pentakisphosphate) is also able to induce a similar shift in DAPI fluorescence. Conversely, IP3 (inositol trisphosphate) and IP4 (inositol tetrakisphosphate) are unable to shift DAPI fluorescence. We have employed this newly discovered feature of DAPI to study the enzymatic activity of the inositol polyphosphate multikinase and to monitor phytase phosphatase reactions. Finally, we used DAPI-IP6 fluorescence to determine the amount of IP6 in plant seeds. Using an IP6 standard curve this straight-forward analysis revealed that among the samples tested, borlotti beans possess the highest level of IP6 (9.4 mg/g of dry mass), whereas the Indian urad bean the lowest (3.2 mg/g of dry mass). The newly identified fluorescence properties of the DAPI-IP5 and DAPI-IP6 complexes allow the levels and enzymatic conversion of these two important messengers to be rapidly and reliably monitored.

Ancient origin of mast cells

The sentinel roles of mammalian mast cells (MCs) in varied infections raised the question of their evolutionary origin. We discovered that the test cells in the sea squirt Ciona intestinalis morphologically and histochemically resembled cutaneous human MCs. Like the latter, C. intestinalis test cells stored histamine and varied heparin·serine protease complexes in their granules. Moreover, they exocytosed these preformed mediators when exposed to compound 48/80. In support of the histamine data, a C. intestinalis-derived cDNA was isolated that resembled that which encodes histidine decarboxylase in human MCs. Like heparin-expressing mammalian MCs, activated test cells produced prostaglandin D2 and contained cDNAs that encode a protein that resembles the synthase needed for its biosynthesis in human MCs. The accumulated morphological, histochemical, biochemical, and molecular biology data suggest that the test cells in C. intestinalis are the counterparts of mammalian MCs that reside in varied connective tissues. The accumulated data point to an ancient origin of MCs that predates the emergence of the chordates >500million years ago, well before the development of adaptive immunity. The remarkable conservation of MCs throughout evolution is consistent with their importance in innate immunity.

Two functionally distinct subsets of mast cells discriminated By IL-2-independent CD25 activities

We identified two mast cell subsets characterized by the differential expression of surface CD25 (IL-2Rα) and by different abilities to produce cytokines and to proliferate, both in vitro and in vivo. CD25 can be expressed on the surface of immune cells in the absence of the other chains of the IL-2R, which are indispensable for IL-2 signaling. We show that functional differences between the two mast cell populations were dependent on CD25 itself, which directly modulated proliferation and cytokine responses. These effects were completely independent from IL-2 or the expression of the other chains of the high-affinity IL-2R, indicating an autonomous and previously unappreciated role for CD25 in regulating cell functions. Cells genetically ablated for CD25 completely recapitulated the CD25-negative phenotype and never acquired the properties characteristic of CD25-positive mast cells. Finally, adoptive transfer experiments in the mouse demonstrated a different impact of these populations in models of anaphylaxis and contact sensitivity. Our findings indicate a general role for CD25 in contexts where IL-2 signaling is not involved, and may have important implications for all mast cell-related diseases, as well as in all cell types expressing CD25 independently of its IL-2-related functions.

Does β-hexosaminidase function only as a degranulation indicator in mast cells? The primary role of β-hexosaminidase in mast cell granules

β-Hexosaminidase, which is generally present in the lysosome, is essential for glycoprotein metabolism in the maintenance of cell homeostasis. In mast cells (MCs), large amounts of β-hexosaminidase are present in the granules as opposed to the lysosome, and the biological role of MC β-hexosaminidase has yet to be fully elucidated. Therefore, we investigated the biological role of β-hexosaminidase in MC granules. Bone marrow-derived MCs from C57BL/6 (BL/6-BMMC) or β-hexosaminidase gene-deficient (hexb(-/-)-BMMC) mice were transplanted into MC-deficient (WBB6F1/J-Kit(W)/Kit(W-v) [W/W(v)]) mice to generate MC-reconstituted models. In asthma model experiments, no differences were observed in the symptoms of BL/6, W/W(v), BL/6-BMMC-reconstituted W/W(v), or hexb(-/-)-BMMC-reconstituted W/W(v) mice. In Staphylococcus epidermidis experimental infection model experiments, the severity of symptoms and frequency of death were markedly higher in W/W(v) and hexb(-/-)-BMMC-reconstituted W/W(v) mice than in BL/6 and BL/6-BMMC-reconstituted W/W(v) mice. The growth of S. epidermidis in an in vitro study was clearly inhibited by addition of BL/6-BMMC lysate, but not by addition of hexb(-/-)-BMMC lysate. Moreover, suppression of bacterial proliferation was completely recovered when bacteria were incubated with hexb(-/-)-BMMC lysate plus β-hexosaminidase. Transmission electron microscopy indicated that the cell wall of S. epidermidis was heavily degraded following coincubation of bacteria with BL/6-BMMC lysate, but not following coincubation with hexb(-/-)-BMMC lysate. These findings strongly suggest that MC granule β-hexosaminidase is crucial for defense against bacterial invasion, but is not involved in the allergic response. Our results also suggest that the bactericidal mechanism of β-hexosaminidase involves degradation of bacterial cell wall peptidoglycan.

Intratracheal exposure to Fab fragments of an allergen-specific monoclonal antibody regulates asthmatic responses in mice

Fab fragments (Fabs) maintain the ability to bind to specific antigens but lack effector functions due to the absence of the Fc portion. In the present study, we tested whether Fabs of an allergen-specific monoclonal antibody (mAb) were able to regulate asthmatic responses in mice. Asthmatic responses were induced in BALB/c mice by passive sensitization with anti-ovalbumin (OVA) polyclonal antibodies (pAbs) (day 0) and by active sensitization with OVA (days 0 and 14), followed by intratracheal (i.t.) challenge with OVA on day 1 and days 28, 29, 30 and 35. Fabs prepared by the digestion of an anti-OVA IgG1 (O1-10) mAb with papain were i.t. administered only once 30 min before antigenic challenge on day 1 or day 35. The results showed that i.t. administration of O1-10 Fabs with OVA markedly suppressed the early and/or late phases of asthmatic responses caused by passive and active sensitization. Similar results were obtained when Fabs of anti-OVA IgG2b mAb (O2B-3) were i.t. administered. In contrast, neither i.t. injection of intact 01-10/O2B-3 nor systemic injection of O1-10 Fabs suppressed the asthmatic responses. In vitro studies revealed that the capture of OVA by O1-10 Fabs prevented the subsequent binding of intact anti-OVA pAbs to the captured OVA. These results suggest that asthmatic responses may be down-regulated by the i.t. exposure to Fabs of an allergen-specific mAb via a mechanism involving the capture of allergen by Fabs in the respiratory tract before the interaction of intact antibody and allergen essential for the induction of asthmatic responses.

Heparanase-mediated cleavage of macromolecular heparin accelerates release of granular components of mast cells from extracellular matrices

Heparanase cleaves macromolecular heparin in the secretory granules of connective tissue-type mast cells. We investigated roles of the cleavage under a microenvironment mimicking where the mast cells physiologically reside. A connective tissue-type mast cell line MST and mouse peritoneal cell-derived mast cells stored macromolecular heparin in the secretory granules. The cells expressing heparanase stored fragmented heparin (~10 kDa) due to heparanase-dependent cleavage of the heparin. We produced an artificial collagen-based extracellular matrix and placed the live cells or glycosaminoglycans purified from the cells in the matrix to measure the release of sulfated macromolecules into the medium. The sulfate-radiolabelled molecules from the degranulating heparanase-expressing cells and the purified glycosaminoglycans showed significantly greater release into the medium than those derived from mock cells, which was not the case in suspension culture. The mast cell granular enzyme chymase, but not β-hexosaminidase, showed significantly greater release from the degranulating heparanase-expressing cells than from mock cells. Purified chymase mixed with fragmented heparin derived from heparanase-expressing cells showed greater release from collagen gels than the enzyme alone or mixed with macromolecular heparin derived from mock cells. We propose that the cleavage of macromolecular heparin by heparanase accelerates the release of heparin and chymase from extracellular matrices.

Comparison of protein expression profiles of the hepatopancreas in Fenneropenaeus chinensis challenged with heat-inactivated Vibrio anguillarum and white spot syndrome virus

Fenneropenaeus chinensis (Chinese shrimp) culture industry, like other Penaeidae culture, has been seriously affected by the shrimp diseases caused by bacteria and virus. To better understand the mechanism of immune response of shrimp to different pathogens, proteome research approach was utilized in this study. Firstly, the soluble hepatopancreas protein samples in adult Chinese shrimp among control, heat-inactivated Vibrio-challenged and white spot syndrome virus-infected groups were separated by 2-DE (pH range, 4-7; sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and pH range, 3-10; tricine-SDS-PAGE). Then the differentially expressed protein spots (≥1.5-fold or ≤0.67-fold averagely of controls) were analyzed by LC-ESI-MS/MS. Using Mascot online database searching algorithm and SEQUEST searching program, 48 and 49 differentially expressed protein spots were successfully identified in response to Vibrio and white spot syndrome virus infection, respectively. Based on these results, we discussed the mechanism of immune response of the shrimp and shed light on the differences between immune response of shrimp toward Vibrio and white spot syndrome virus. This study also set a basis for further analyses of some key genes in immune response of Chinese shrimp.