Characterization of mast cell secretory granules and their cell biology

Mast cell-mediated microRNA functioning in immune regulation and disease pathophysiology

Upon stimulation and activation, mast cells (MCs) release soluble mediators, including histamine, proteases, and cytokines. These mediators are often stored within cytoplasmic granules in MCs and may be released in a granulated form. The secretion of cytokines and chemokines occurs within hours following activation, with the potential to result in chronic inflammation. In addition to their role in allergic inflammation, MCs are components of the tumor microenvironment (TME). MicroRNAs (miRNAs) are small RNA molecules that do not encode proteins, but regulate post-transcriptional gene expression by binding to the 3' non-coding regions of mRNAs. This plays a crucial role in the function of MC, including the key processes of MC proliferation, maturation, apoptosis, and activation. It has been demonstrated that miRNAs are also present in extracellular vesicles (EVs) secreted by MCs. EVs derived from MCs mediate intercellular communication by carrying miRNAs, affecting various diseases including allergic diseases, intestinal disorders, neuroinflammation, and tumors. These findings provide important insights into the therapeutic mechanisms and targets of miRNAs in MCs that affect diseases. This review discusses the relevance of miRNA production by MCs in regulating their own activity and the effect of miRNAs putatively produced by other cells in the control of MC activity and their participation in selected pathologies.

Endolysosomal vesicles at the center of B cell activation

The endolysosomal system specializes in degrading cellular components and is crucial to maintaining homeostasis and adapting rapidly to metabolic and environmental cues. Cells of the immune system exploit this network to process antigens or promote cell death by secreting lysosome-related vesicles. In B lymphocytes, lysosomes are harnessed to facilitate the extraction of antigens and to promote their processing into peptides for presentation to T cells, critical steps to mount protective high-affinity antibody responses. Intriguingly, lysosomal vesicles are now considered important signaling units within cells and also display secretory functions by releasing their content to the extracellular space. In this review, we focus on how B cells use pathways involved in the intracellular trafficking, secretion, and function of endolysosomes to promote adaptive immune responses. A basic understanding of such mechanisms poses an interesting frontier for the development of therapeutic strategies in the context of cancer and autoimmune diseases.

Carboxypeptidase A3 in the structure of the protease phenotype of mast cells: cytophysiological aspects

Carboxypeptidase A3 (CPA3) is a specific protease of mast cells (MC) with variable expression and appears to be one of the preformed components of the secretome. CPA3 is involved in regulation of the state of a specifi tissue microenvironment and components of the integrative-buffer metabolic environment in adaptive and pathological processes; it affects implementation of the innate immunity, mechanisms of angiogenesis, processes of the extracellular matrix remodeling, etc. CPA3 identification using protocols of multiplex immunohistochemistry allows specifying details of the organ-specific mast cell population features, including the protease phenotype, mechanisms of biogenesis with cytoand histotopographic criteria, and features of secretory pathways. Numerous biological effects of CPA3, including participation in the regulation of the pulmonary parenchyma and systemic blood flow, in biogenesis and remodeling of the fibrous component of the extracellular matrix, in epigenetic reprogramming, determine the importance of fundamental investigation of the physiological activity of protease and its involvement in the implementation of pathological processes. Further studies will contribute to the detection of the translational value of the mast cell CPA3 expression features as a prognostic factor and a promising molecular target for treatment of socially significant diseases.

Carboxypeptidase A3 in the structure of the protease phenotype of mast cells: cytophysiological aspects

Carboxypeptidase A3 (CPA3) is a specific protease of mast cells (MC) with variable expression and appears to be one of the preformed components of the secretome. CPA3 is involved in regulation of the state of a specifi tissue microenvironment and components of the integrative-buffer metabolic environment in adaptive and pathological processes; it affects implementation of the innate immunity, mechanisms of angiogenesis, processes of the extracellular matrix remodeling, etc. CPA3 identification using protocols of multiplex immunohistochemistry allows specifying details of the organ-specific mast cell population features, including the protease phenotype, mechanisms of biogenesis with cyto- and histotopographic criteria, and features of secretory pathways. Numerous biological effects of CPA3, including participation in the regulation of the pulmonary parenchyma and systemic blood flow, in biogenesis and remodeling of the fibrous component of the extracellular matrix, in epigenetic reprogramming, determine the importance of fundamental investigation of the biological activity and regulation of pathological processes of CPA3. Further studies will contribute to the detection of the true value of the mast cell CPA3 expression features as a prognostic factor and a promising molecular target for treatment of socially significant diseases.

Carboxypeptidase A3—A Key Component of the Protease Phenotype of Mast Cells

Carboxypeptidase A3 (CPA3) is a specific mast cell (MC) protease with variable expression. This protease is one of the preformed components of the secretome. During maturation of granules, CPA3 becomes an active enzyme with a characteristic localization determining the features of the cytological and ultrastructural phenotype of MC. CPA3 takes part in the regulation of a specific tissue microenvironment, affecting the implementation of innate immunity, the mechanisms of angiogenesis, the processes of remodeling of the extracellular matrix, etc. Characterization of CPA3 expression in MC can be used to refine the MC classification, help in a prognosis, and increase the effectiveness of targeted therapy.

Inhibition of Histamine Release from RBL-2H3 Cells by Zoledronate Did Not Affect Rab27a/Doc2a Interaction

Mast cell (MC) exocytosis is organized by prenylated protein, including Rab families. Among Rab proteins, Rab3a, Rab27a, and Rab11 are responsible for exocytosis arrangement. Rab3a and Rab27a are contributed to exocytosis by interacting with other exocytosis proteins. Zoledronate administration disrupted the Rab prenylation process that affected its interaction with other proteins, and finally, its function. The present study has investigated the effect of zoledronate on the histamine release (HR) from RBL-2H3 cells. The main focus is to answer the question of whether zoledronate affects Rab27a/Doc2a interaction. Histamine release on RBL-2H3 cells after zoledronate or clodronate administration was measured using HPLC-fluorometry. Dinitrophenylated bovine serum albumin (DNP-BSA) (20 ng/mL) or ionomycin (1 µM) are used as secretagogues. Calcium (Ca²⁺) influx observation was performed using Fura-2A/M. In situ proximity ligation assay (PLA) is used to investigate Rab27a/Doc2a interaction after bisphosphonates (BPs) treatment. Histamine concentration measurement with HPLC-fluorometry showed that zoledronate (30, 100 µM) inhibited HR from antigen-activated RBL-2H3 cells. Zoledronate showed less inhibition in cells activated with ionomycin. Intracellular Ca²⁺ concentration and Ca²⁺ flux rate from the extracellular compartment was not changed by zoledronate administration. No changes in Rab27a/Doc2a interaction after zoledronate treatment. Histamine release inhibition by zoledronate in DNP-BSA-activated RBL-2H3 cells is not related to the disruption of Rab27a/Doc2a interaction and is not involve the change in Ca²⁺ influx.

Transcription Factors in the Development and Pro-Allergic Function of Mast Cells

Mast cells (MCs) are innate immune cells of hematopoietic origin localized in the mucosal tissues of the body and are broadly implicated in the pathogenesis of allergic inflammation. Transcription factors have a pivotal role in the development and differentiation of mast cells in response to various microenvironmental signals encountered in the resident tissues. Understanding the regulation of mast cells by transcription factors is therefore vital for mechanistic insights into allergic diseases. In this review we summarize advances in defining the transcription factors that impact the development of mast cells throughout the body and in specific tissues, and factors that are involved in responding to the extracellular milieu. We will further describe the complex networks of transcription factors that impact mast cell physiology and expansion during allergic inflammation and functions from degranulation to cytokine secretion. As our understanding of the heterogeneity of mast cells becomes more detailed, the contribution of specific transcription factors in mast cell-dependent functions will potentially offer new pathways for therapeutic targeting.

Rab44 isoforms similarly promote lysosomal exocytosis, but exhibit differential localization in mast cells

Rab44 is a large Rab GTPase containing a Rab GTPase domain and some additional N-terminal domains. We recently used Rab44-deficient mice to demonstrate that Rab44 regulates granule exocytosis in mast cells and IgE-mediated anaphylaxis. In mouse mast cells, Rab44 is expressed as two isoforms, namely, the long and short forms; however, the characteristics of these two isoforms remain unknown. Here, we investigated secretion and localization of the human long Rab44 isoform and the two mouse isoforms and their mutants expressed in rat basophilic leukemia (RBL)-2H3 cells. Expression of the human long isoform and both mouse isoforms caused an increase in β-hexosaminidase secretion. Confocal and quantitative analyses showed that both human and mouse long isoforms localized mainly to lysosomes while the mouse short isoform localized mainly to the ER. Live imaging with LysoTracker indicated that the size and number of LysoTracker-positive vesicles were altered by the various mutants. Ionomycin treatment partially altered localization of both long isoforms to the plasma membrane and cytosol, whereas it had little effect on colocalization of the short isoform with lysosomes. Mechanistically, both human and mouse Rab44 proteins interacted with vesicle-associated membrane protein 8 (VAMP8), a v-SNARE protein. Therefore, Rab44 isoforms similarly promote lysosomal exocytosis, but exhibit differential localization in mast cells.

Protease profile of normal and neoplastic mast cells in the human bone marrow with special emphasis on systemic mastocytosis

Mast cells (MC) are immune cells that produce a variety of mediators, such as proteases, that are important in the body's immune responses. MC proteases have pronounced multifunctionality and in many respects determine the biological characteristics of the organ-specific MC population. Although, increased numbers of MC are one of the objective mastocytosis signs, a detailed assessment of the proteases biogenesis and excretion mechanisms in the bone marrow (BM) has not yet been carried out. Here, we performed an analysis of the expression of proteases in patients with various forms of systemic mastocytosis. We presented data on intracellular protease co-localization in human BM MCs and discussed their implication in secretory pathways of MCs in the development of the disease. Systemic mastocytosis, depending on the course, is featured by the formation of definite profiles of specific proteases in various forms of atypical mast cells. Intragranular accumulation of tryptase, chymase and carboxypeptidases in the hypochromic phenotype of atypical mast cells is characterized. Characterization of MC proteases expression during mastocytosis can be used to refine the MC classification, help in a prognosis, and increase the effectiveness of targeted therapy.

Leukotriene D4 role in allergic asthma pathogenesis from cellular and therapeutic perspectives

Asthma is a chronic inflammatory and allergic disease that is mainly characterized by reversible airway obstruction and bronchial hyperresponsiveness. The incidence of asthma is increasing with more than 350 million people worldwide are affected. Up to now, there is no therapeutic option for asthma and most of the prescribed drugs aim to ameliorate the symptoms of the disease especially during the acute exacerbations after trigger exposure. Asthma is a heterogonous disease that involves interactions between inflammatory mediators and cellular components within the disease microenvironment including inflammatory and structural cells. Cysteinyl leukotrienes (cys-LTs) are inflammatory lipid mediators that have potent roles in asthma pathogenesis. CysLTs consisting of LTC₄, LTD₄, and LTE₄ are mainly secreted by leukocytes and act through three main G-protein coupled receptors (CysLT₁R, CysLT₂R, and CysLT₃R). LTD₄ is the most potent bronchoconstrictor which gives it the priority to be discussed in detail in this review. LTD₄ binds with high affinity to CysLT₁R and many studies showed that using CysLT₁R antagonists such as montelukast has a beneficial effect for asthmatics especially in corticosteroid refractory cases. Since asthma is a heterogeneous inflammatory disease of many cell types involved in the disease pathogenies and LTD₄ has a special role in inflammation and bronchoconstriction, this review highlights the role of LTD₄ on each cellular component in asthma and the benefits of using CysLT₁R antagonists in ameliorating LTD₄-induced effects.

In vivo non-invasive staining-free visualization of dermal mast cells in healthy, allergy and mastocytosis humans using two-photon fluorescence lifetime imaging

Mast cells (MCs) are multifunctional cells of the immune system and are found in skin and all major tissues of the body. They contribute to the pathology of several diseases including urticaria, psoriasis, atopic dermatitis and mastocytosis where they are increased at lesional sites. Histomorphometric analysis of skin biopsies serves as a routine method for the assessment of MC numbers and their activation status, which comes with major limitations. As of now, non-invasive techniques to study MCs in vivo are not available. Here, we describe a label-free imaging technique to visualize MCs and their activation status in the human papillary dermis in vivo. This technique uses two-photon excited fluorescence lifetime imaging (TPE-FLIM) signatures, which are different for MCs and other dermal components. TPE-FLIM allows for the visualization and quantification of dermal MCs in healthy subjects and patients with skin diseases. Moreover, TPE-FLIM can differentiate between two MC populations in the papillary dermis in vivo-resting and activated MCs with a sensitivity of 0.81 and 0.87 and a specificity of 0.85 and 0.84, respectively. Results obtained on healthy volunteers and allergy and mastocytosis patients indicate the existence of other MC subpopulations within known resting and activated MC populations. The developed method may become an important tool for non-invasive in vivo diagnostics and therapy control in dermatology and immunology, which will help to better understand pathomechanisms involving MC accumulation, activation and degranulation and to characterize the effects of therapies that target MCs.

Mast cell activation markers for in vitro study

Mast cells (MCs) are well known for their role in allergic conditions. This cell can be activated by various types of secretagogues, ranging from a small chemical to a huge protein. Mast cell activation by secretagogues triggers the increase in intracellular calcium (iCa²⁺) concentration, granule trafficking, and exocytosis. Activated mast cells release their intra-granular pre-stored mediator or the newly synthesized mediator in the exocytosis process, in the form of degranulation or secretion. There are at least three types of exocytosis in mast cells, which are suggested to contribute to the release of different mediators, i.e.,, piecemeal, kiss-and-run, and compound exocytosis. The status of mast cells, i.e., activated or resting, is often determined by measuring the concentration of the released mediator such as histamine or β-hexosaminidase. This review summarizes several mast cell components that have been and are generally used as mast cell activation indicator, from the classical histamine and β-hexosaminidase measurement, to eicosanoid and granule trafficking observation. Basic principle of the component determination is also explained with their specified research application and purpose. The information will help to predict the experiment results with a certain study design.

Mast Cell Biology at Molecular Level: a Comprehensive Review

Mast cells (MCs) are portions of the innate and adaptive immune system derived from bone marrow (BM) progenitors that are rich in cytoplasmic granules. MC maturation, phenotype, and function are determined by their microenvironment. MCs accumulate at inflammatory sites associated with atopy, wound healing, and malignancies. They interact with the external environment and are predominantly located in close proximity of blood vessels and sensory nerves. MCs are key initiators and modulators of allergic, anaphylactic, and other inflammatory reactions, by induction of vasodilation, promoting of vascular permeability, recruitment of inflammatory cells, facilitation of adaptive immune responses, and modulation of angiogenesis, and fibrosis. They express a wide range of receptors, e.g., for IgE (FcεRI), IgG (FcγR), stem cell factor (SCF) (KIT receptor or CD117), complement (including C5aR), and cytokines, that upon activation trigger various signaling pathways. The final consequence of such ligand receptor-based activation of MCs is the release of a broad array of mediators which are classified in three categories. While some mediators are preformed and remain stored in granules such as heparin, histamine, and enzymes mainly chymase and tryptase, others are de novo synthesized only after activation including LTB4, LTD4, PDG2, and PAF, and the cytokines IL-10, IL-8, IL-5, IL-3, IL-1, GM-CSF, TGF-β, VEGF, and TNF-α. Depending on the stimulus, MCs calibrate their pattern of mediator release, modulate the amplification of allergic inflammation, and are involved in the resolution of the immune responses. Here, we review recent findings and reports that help to understand the MC biology, pathology, and physiology of diseases with MC involvement.

Human endothelial cells size-select their secretory granules for exocytosis to modulate their functional output

BACKGROUND

The secretory granules of endothelial cells, Weibel-Palade bodies, are released in response to numerous extracellular signals. Their cargo is critical to many vascular functions including hemostasis and inflammation. This presents a fundamental problem: how can these cells initiate tailor-made responses from the release of a single type of organelle, each with similar cargo? Each cell contains Weibel-Palade bodies in a wide range of sizes, and we have shown that experimentally shortening these organelles disproportionately reduces their ability to initiate hemostasis in vitro, leaving leukocyte recruitment unaffected. Could the production of this range of sizes underpin differential responses?

OBJECTIVES

To determine whether different agonists drive the exocytosis of different sizes of Weibel-Palade bodies.

METHODS

We used a high-throughput automated unbiased imaging workflow to analyze the sizes of Weibel-Palade bodies within human umbilical vein endothelial cells (HUVECs) before and after agonist activation to determine changes in organelle size distributions.

RESULTS

We found that a subset of agonists differentially evoke the release of the longest, most pro-hemostatic organelles. Inhibiting the release of these longest organelles by just 15% gives a fall of 60% in an assay of secreted von Willebrand factor (vWF) function.

CONCLUSIONS

The size-selection of granules for exocytosis represents a novel layer of control, allowing endothelial cells to provide diverse responses to different signals via the release of a single type of organelle.

Endogenous Annexin-A1 Negatively Regulates Mast Cell-Mediated Allergic Reactions

Mast cell stabilizers like cromoglycate and nedocromil are mainstream treatments for ocular allergy. Biochemical studies in vitro suggest that these drugs prevent mast cell degranulation through the release of Annexin-A1 (Anx-A1) protein. However, the direct effect of Anx-A1 gene deletion on mast cell function in vitro and in vivo is yet to be fully investigated. Hence, we aim to elucidate the role of Anx-A1 in mast cell function, both in vivo and in vitro, using a transgenic mouse model where the Anx-A1 gene has been deleted. Bone marrow-derived mast cells (BMDMCs) were cultured from wild-type animals and compared throughout their development to BMDMCs obtained from mice lacking the Anx-A1 gene. The mast cell differentiation, maturity, mediator, and cytokine release were explored using multiple biochemical techniques, such as Western blots, ELISA, and flow cytometry analysis. Electron microscopy was used to identify metachromatic granules content of cells. For in vivo studies, Balb/C wild-type and Anx-A1-deficient mice were divided into the following groups: group 1, a control receiving only saline, and group 2, which had been sensitized by prior exposure to short ragweed (SRW) pollen by topical contact with the conjunctival mucosae. Allergic conjunctivitis was evaluated blind after 24 h by trained observers scoring clinical signs. Electron micrographs of BMDMCs from Anx-A1-null mice revealed more vacuoles overall and more fused vacuoles than wild-type cells, suggesting enhanced secretory activity. Congruent with these observations, BMDMCs lacking the Anx-A1 gene released significantly increased amounts of histamine both spontaneously as well as in response to Ig-E-FcεRI cross-linking compared to those from wild-type mice. Interestingly, the spontaneous release of IL-5, IL-6, IL-9, and monocyte chemoattractant protein-1 (MCP-1) were also markedly increased with a greater production observed upon IgE cross-linking. This latter finding is congruent with augmented calcium mobilization in BMDMCs lacking the Anx-A1 gene. In vivo, when compared to wild-type animals, Anx-A1-deficient mice exposed to SRW pollen displayed exacerbated signs and symptoms of allergic conjunctivitis. Taken together, these results suggest Anx-A1 is an important non-redundant regulator of mast cell reactivity and particularly in allergen mediated allergic reactions.

Calcium release-activated calcium modulator 1 as a therapeutic target in allergic skin diseases

Allergic skin disease is the most common skin condition, and considerably affects patients' life quality because of its recurrence and pruritus. Numbers of studies point out that immune cells, including mast cells and T cells, play pathogenic roles in allergic skin diseases, and share similarities in the activation and secretion of cytokines. Calcium Release-Activated Calcium Modulator 1(CRACM1/ORAI1) is a subtype of Ca²⁺ membrane channel, causing Ca²⁺ influx into the cells. As a second messenger, Ca²⁺ is an essential element that regulates immune responses, especially in the development and function of T and B cells. Thus, ORAI1 is considered to participate in allergic diseases. However, the specific mechanism of ORAI1 in skin disorders is still unclear. In order to investigate the roles of ORAI1 in allergic skin disorders, we reviewed the related articles and concluded that ORAI1 could be a potential therapeutic target for allergic skin diseases.

Morphological and functional analysis of beige (Chèdiak-Higashi syndrome) mouse mast cells with giant granules

Chèdiak-Higashi syndrome is a rare autosomal recessive disease that causes hypopigmentation, recurrent infections, mild coagulation defects and neurological problems. Beige mice carry a mutation in the lysosome trafficking regulator (LYST) gene and display some of the key characteristics of human Chèdiak-Higashi syndrome, in particular, a high susceptibility to infection due to aberrant natural killer (NK) cell and polymorphonuclear leucocyte function. Morphological analysis of beige mice reveals the presence of enlarged lysosomes in a variety of cell types, including leucocytes, hepatocytes, fibroblasts and renal tubule cells. To examine the process of granule maturation and degranulation in beige mice mast cells, morphological studies have been conducted using a combination of electrophysiological techniques; however, few functional studies have been conducted with mast cells, such as mediator release. The aim of the present study was to determine the morphological and functional characteristics of skin and peritoneal mast cells and bone marrow-derived mast cells of homozygous (bg/bg) and heterozygous (bg/+) beige mice and wild-type (+/+) mice. The histamine concentration was lower in the peritoneal and bone marrow-derived mast cells of bg/bg mice compared with those of bg/+ and +/+ mice, but the histamine release response was potentiated. In vivo studies of passive cutaneous anaphylaxis showed no differences between bg/bg mice and either bg/+ or +/+ mice. Although bg/bg mast cells with enlarged granules display specific exocytotic processes in vitro, the consequences of mast cell activation in beige mice were similar to those of wild-type mice in vivo.

Mastocytosis-derived extracellular vesicles exhibit a mast cell signature, transfer KIT to stellate cells, and promote their activation

Extracellular vesicles (EVs) have been implicated in the development and progression of hematological malignancies. We thus examined serum samples from patients with systemic mastocytosis (SM) and found EVs with a mast cell signature including the presence of tryptase, FcεRI, MRGX2, and KIT. The concentration of these EVs correlated with parameters of disease including levels of serum tryptase, IL-6, and alkaline phosphatase and physical findings including hepatosplenomegaly. Given reports that EVs from one cell type may influence another cell's behavior, we asked whether SM-EVs might affect hepatic stellate cells (HSCs), based on the abnormal liver pathology associated with mastocytosis. We found that KIT was transferred from SM-EVs into an HSC line eliciting proliferation, cytokine production, and differentiation, processes that have been associated with liver pathology. These effects were reduced by KIT inhibition or neutralization and recapitulated by enforced expression of KIT or constitutively active D816V-KIT, a gain-of-function variant associated with SM. Furthermore, HSCs in liver from mice injected with SM-EVs had increased expression of α-SMA and human KIT, particularly around portal areas, compared with mice injected with EVs from normal individuals, suggesting that SM-EVs can also initiate HSC activation in vivo. Our data are thus consistent with the conclusion that SM-EVs have the potential to influence cells outside the hematological compartment and that therapeutic approaches for treatment of SM may be effective in part through inhibition of effects of EVs on target tissues, findings important both to understanding complex disease pathology and in developing interventional agents for the treatment of hematologic diseases.

Mitogen-Activated Protein Kinase Signaling Regulates Proteoglycan Composition of Mast Cell Secretory Granules

Mast cells (MCs) are characterized by an abundance of lysosome-like secretory granules filled with immunomodulatory compounds including histamine, cytokines, lysosomal hydrolases, MC-restricted proteases, and serglycin proteoglycans. The latter are essential for promoting the storage of other granule compounds and are built up of the serglycin core protein to which highly sulfated and thereby negatively charged glycosaminoglycan (GAG) side chains of heparin or chondroitin sulfate type are attached. In the search for mechanisms operating in regulating MC granule homeostasis, we here investigated the role of mitogen-activated protein kinase (MAPK) signaling. We show that inhibition of MEK1/2 (a MAPK kinase) leads to increased metachromatic staining of MC granules, indicative of increased proteoglycan content. Indeed, MEK1/2 inhibition caused a profound increase in the expression of the gene coding for the serglycin core protein and of genes coding for various enzymes involved in the biosynthesis/sulfation of the GAGs attached to the serglycin core protein. This was accompanied by corresponding increases in the levels of the respective GAGs. Deletion of the serglycin core protein abrogated the induction of enzymes operative in proteoglycan synthesis, indicating that availability of the serglycin proteoglycan core protein has a regulatory function impacting on the expression of the various serglycin-modifying enzymes. MEK1/2 inhibition also caused a substantial increase in the expression of granule-localized, proteoglycan-binding proteases. Altogether, this study identifies a novel role for MAPK signaling in regulating the content of secretory granules in MCs.

Acidic pH is essential for maintaining mast cell secretory granule homeostasis

It has been recognized for a long time that the secretory granules of mast cells are acidic, but the functional importance of maintaining an acidic pH in the mast cell granules is not fully understood. Here we addressed this issue by examining the effects of raising the pH of the mast cell secretory granules. Mast cells were incubated with bafilomycin A1, an inhibitor of the vacuolar-type ATPase proton pump. Supporting a role of vacuolar-type ATPase in mast cell granule acidification, bafilomycin A1 treatment caused a robust increase in granule pH. This was accompanied by marked effects on mast cell granules, including swelling and acquisition of vacuole-like morphology. Moreover, bafilomycin A1 caused extensive, yet selective effects on the granule content. These included aberrant processing of pro-carboxypeptidase A3 and a reduction in the level of intracellular histamine, the latter being accompanied by an increase in extracellular histamine. In contrast, the storage of β-hexosaminidase, a prototype lysosomal hydrolase known to be stored in mast cell granules, was not affected by abrogation of granule acidification. Moreover, bafilomycin A1 caused a reduction of tryptase enzymatic activity and appearance of tryptase degradation products. Tryptase inhibition prevented the formation of such degradation products, suggesting that the pH elevation causes tryptase to undergo autoproteolysis. Taken together, our findings reveal that mast cell secretory granule homeostasis is critically dependent on an acidic milieu.

Imaging protective mast cells in living mice during severe contact hypersensitivity

Contact hypersensitivity (CHS) is a common skin disease induced by epicutaneous sensitization to haptens. Conflicting results have been obtained regarding pathogenic versus protective roles of mast cells (MCs) in CHS, and this has been attributed in part to the limitations of certain models for studying MC functions in vivo. Here we describe a fluorescent imaging approach that enables in vivo selective labeling and tracking of MC secretory granules by real-time intravital 2-photon microscopy in living mice, and permits the identification of such MCs as a potential source of cytokines in different disease models. We show using this method that dermal MCs release their granules progressively into the surrounding microenvironment, but also represent an initial source of the antiinflammatory cytokine IL-10, during the early phase of severe CHS reactions. Finally, using 3 different types of MC-deficient mice, as well as mice in which IL-10 is ablated specifically in MCs, we show that IL-10 production by MCs can significantly limit the inflammation and tissue pathology observed in severe CHS reactions.

Mast cells are directly activated by contact with cancer cells by a mechanism involving autocrine formation of adenosine and autocrine/paracrine signaling of the adenosine A3 receptor

Mast cells (MCs) constitute an important part of the tumor microenvironment (TME). However, their underlying mechanisms of activation within the TME remain poorly understood. Here we show that recapitulating cell-to-cell contact interactions by exposing MCs to membranes derived from a number of cancer cell types, results in MC activation, evident by the increased phosphorylation of the ERK1/2 MAP kinases and Akt, in a phosphatidylinositol 3-kinase dependent fashion. Activation is unidirectional since MC derived membranes do not activate cancer cells. Stimulated ERK1/2 phosphorylation is strictly dependent on the ecto enzyme CD73 that mediates autocrine formation of adenosine, and is inhibited by knockdown of the A3 adenosine receptor (A3R) as well as by an A3R antagonist or by agonist-stimulated down-regulation of the A3R. We also show that cancer cell mediated triggering upregulates expression and stimulates secretion of interleukin 8 from the activated MCs. These findings provide evidence for a novel mode of unidirectional crosstalk between MCs and cancer cells implicating direct activation by cancer cells in MC reprogramming into a pro tumorigenic profile.

Munc13-4 functions as a Ca2+ sensor for homotypic secretory granule fusion to generate endosomal exocytic vacuoles

Munc13-4 is a Ca2+-dependent SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)- and phospholipid-binding protein that localizes to and primes secretory granules (SGs) for Ca2+-evoked secretion in various secretory cells. Studies in mast cell-like RBL-2H3 cells provide direct evidence that Munc13-4 with its two Ca2+-binding C2 domains functions as a Ca2+ sensor for SG exocytosis. Unexpectedly, Ca2+ stimulation also generated large (>2.4 μm in diameter) Munc13-4+/Rab7+/Rab11+ endosomal vacuoles. Vacuole generation involved the homotypic fusion of Munc13-4+/Rab7+ SGs, followed by a merge with Rab11+ endosomes, and depended on Ca2+ binding to Munc13-4. Munc13-4 promoted the Ca2+-stimulated fusion of VAMP8-containing liposomes with liposomes containing exocytic or endosomal Q-SNAREs and directly interacted with late endosomal SNARE complexes. Thus Munc13-4 is a tethering/priming factor and Ca2+ sensor for both heterotypic SG-plasma membrane and homotypic SG-SG fusion. Total internal reflection fluorescence microscopy imaging revealed that vacuoles were exocytic and mediated secretion of β-hexosaminidase and cytokines accompanied by Munc13-4 diffusion onto the plasma membrane. The results provide new molecular insights into the mechanism of multigranular compound exocytosis commonly observed in various secretory cells.

Rabs on the fly: Functions of Rab GTPases during development

The organization of intracellular transport processes is adapted specifically to different cell types, developmental stages, and physiologic requirements. Some protein traffic routes are universal to all cells and constitutively active, while other routes are cell-type specific, transient, and induced under particular conditions only. Small GTPases of the Rab (Ras related in brain) subfamily are conserved across eukaryotes and regulate most intracellular transit pathways. The complete sets of Rab proteins have been identified in model organisms, and molecular principles underlying Rab functions have been uncovered. Rabs provide intracellular landmarks that define intracellular transport sequences. Nevertheless, it remains a challenge to systematically map the subcellular distribution of all Rabs and their functional interrelations. This task requires novel tools to precisely describe and manipulate the Rab machinery in vivo. Here we discuss recent findings about Rab roles during development and we consider novel approaches to investigate Rab functions in vivo.

A Role for Human Skin Mast Cells in Dengue Virus Infection and Systemic Spread

Dengue virus (DENV) is a mosquito-borne flavivirus that causes serious global human disease and mortality. Skin immune cells are an important component of initial DENV infection and systemic spread. Here, we show that mast cells are a target of DENV in human skin and that DENV infection of skin mast cells induces degranulation and alters cytokine and growth factor expression profiles. Importantly, to our knowledge, we also demonstrate for the first time that DENV localizes within secretory granules in infected skin mast cells. In addition, DENV within extracellular granules was infectious in vitro and in vivo, trafficking through lymph to draining lymph nodes in mice. We demonstrate an important role for human skin mast cells in DENV infection and identify a novel mechanism for systemic spread of DENV infection from the initial peripheral mosquito injection site.

Different activation signals induce distinct mast cell degranulation strategies

Mast cells (MCs) influence intercellular communication during inflammation by secreting cytoplasmic granules that contain diverse mediators. Here, we have demonstrated that MCs decode different activation stimuli into spatially and temporally distinct patterns of granule secretion. Certain signals, including substance P, the complement anaphylatoxins C3a and C5a, and endothelin 1, induced human MCs rapidly to secrete small and relatively spherical granule structures, a pattern consistent with the secretion of individual granules. Conversely, activating MCs with anti-IgE increased the time partition between signaling and secretion, which was associated with a period of sustained elevation of intracellular calcium and formation of larger and more heterogeneously shaped granule structures that underwent prolonged exteriorization. Pharmacological inhibition of IKK-β during IgE-dependent stimulation strongly reduced the time partition between signaling and secretion, inhibited SNAP23/STX4 complex formation, and switched the degranulation pattern into one that resembled degranulation induced by substance P. IgE-dependent and substance P-dependent activation in vivo also induced different patterns of mouse MC degranulation that were associated with distinct local and systemic pathophysiological responses. These findings show that cytoplasmic granule secretion from MCs that occurs in response to different activating stimuli can exhibit distinct dynamics and features that are associated with distinct patterns of MC-dependent inflammation.

A rapid and sensitive assay based on particle analysis for cell degranulation detection in basophils and mast cells

The degranulation of mast cells and basophils is often initiated by a number of pathophysiological responses, especially in allergic and inflammatory conditions. Efficient techniques and methods for determining the level of such degranulation are highly demanded for laboratory and clinical studies. In this work, a rapid and sensitive assay based on the particle analysis of granules in RBL-2H3 cells, a cell line widely used as a convenient model system to study the degranulation of mast cells and basophils, was developed to detect cell degranulation using a Nanosight NS300 in light scatter mode and dynamic light scattering (DLS) on a Malvern Zetasizer Nano-ZS instrument. Using this method, drug-induced mast cell degranulation and systemic anaphylaxis were efficiently determined both in cell culture medium and blood samples from animals in the current study. This promising method is expected to be widely used for screening anti-allergic and anti-inflammatory drugs both in vitro and in vivo models, as well as for determining the level of mast cell degranulation of the patients in the clinic.

Assessment of seminal mast cells in infertile men with varicocele after surgical repair

This study aimed to assess seminal mast cells in infertile men associated with varicocele (Vx) pre- and post-surgical repair. Forty-five infertile men associated with Vx were subjected to history taking and clinical examination. In addition, semen parameters and seminal mast cells stained with 1% toluidine blue were estimated pre-varicocelectomy and three months post-varicocelectomy. Vx surgical repair revealed a significant improvement in the mean sperm concentration, progressive sperm motility, total sperm motility and sperm abnormal morphology and a significant decrement in seminal mast cells (mean ± SD, 3.56 ± 2.23 cells per high-power field (HPF) vs. 2.22 ± 1.06 cells per HPF, p = .01). The pre-operative mean mast cell count demonstrated significant increases in cases with Vx grade III compared with other Vx grades and in cases with bilateral Vx compared with unilateral Vx cases. Seminal mast cells demonstrated a significant correlation with sperm concentration, progressive sperm motility and total sperm motility and a nonsignificant correlation with age and sperm abnormal morphology. It is concluded that seminal mast cells decrease significantly in infertile men with Vx after surgical repair showing a significant negative correlation with sperm concentration, progressive sperm motility and total sperm motility.

Rab12 Regulates Retrograde Transport of Mast Cell Secretory Granules by Interacting with the RILP-Dynein Complex

Secretory granule (SG) transport is a critical step in regulated exocytosis including degranulation of activated mast cells. The latter process results in the release of multiple inflammatory mediators that play key roles in innate immunity, as well as in allergic responses. In this study, we identified the small GTPase Rab12 as a novel regulator of mast cell SG transport, and we provide mechanistic insights into its mode of action. We show that Rab12 is activated in a stimulus-dependent fashion and promotes microtubule-dependent retrograde transport of the SGs in the activated cells. We also show that this minus end transport of the SGs is mediated by the RILP-dynein complex and identify RILP as a novel effector of Rab12. Finally, we show that Rab12 negatively regulates mast cell degranulation. Taken together, our results identify Rab12 as a novel regulator of mast cell responses and disclose for the first time, to our knowledge, the mechanism of retrograde transport of the mast cell SGs.