Mast cells. Receptors, secretagogues, and signaling

Contribution of FcɛRI-associated vesicles to mast cell-macrophage communication following Francisella tularensis infection

Understanding innate immune intercellular communication following microbial infection remains a key biological issue. Using live cell imaging, we demonstrate that mast cells actively extend cellular projections to sample the macrophage periphery during Francisella tularensis LVS infection. Mast cell MHCII(hi) expression was elevated from less than 1% to 13% during LVS infection. Direct contact during co-culture with macrophages further increased mast cell MHCII(hi) expression to approximately 87%. Confocal analyses of the cellular perimeter revealed mast cell caspase-1 was localized in close proximity with FcɛRI in uninfected mast cells, and repositioned to clustered regions upon LVS infection. Importantly, mast cell FcɛRI-encompassed vesicles are transferred to macrophages by trogocytosis, and macrophage caspase-1 expression is further up-regulated upon direct contact with mast cells. Our study reveals direct cellular interactions between innate cells that may impact the function of caspase-1, a known sensor of microbial danger and requirement for innate defense against many pathogenic microbes including F. tularensis.

Engineered Nanostructures of Haptens Lead to Unexpected Formation of Membrane Nanotubes Connecting Rat Basophilic Leukemia Cells

A recent finding reports that co-stimulation of the high-affinity immunoglobulin E (IgE) receptor (FcεRI) and the chemokine receptor 1 (CCR1) triggered formation of membrane nanotubes among bone-marrow-derived mast cells. The co-stimulation was attained using corresponding ligands: IgE binding antigen and macrophage inflammatory protein 1α (MIP1 α), respectively. However, this approach failed to trigger formation of nanotubes among rat basophilic leukemia (RBL) cells due to the lack of CCR1 on the cell surface (Int. Immunol. 2010, 22 (2), 113-128). RBL cells are frequently used as a model for mast cells and are best known for antibody-mediated activation via FcεRI. This work reports the successful formation of membrane nanotubes among RBLs using only one stimulus, a hapten of 2,4-dinitrophenyl (DNP) molecules, which are presented as nanostructures with our designed spatial arrangements. This observation underlines the significance of the local presentation of ligands in the context of impacting the cellular signaling cascades. In the case of RBL, certain DNP nanostructures suppress antigen-induced degranulation and facilitate the rearrangement of the cytoskeleton to form nanotubes. These results demonstrate an important scientific concept; engineered nanostructures enable cellular signaling cascades, where current technologies encounter great difficulties. More importantly, nanotechnology offers a new platform to selectively activate and/or inhibit desired cellular signaling cascades.

Pharmacology and signaling of MAS-related G protein-coupled receptors

Signaling by heptahelical G protein-coupled receptors (GPCR) regulates many vital body functions. Consequently, dysfunction of GPCR signaling leads to pathologic states, and approximately 30% of all modern clinical drugs target GPCR. One decade ago, an entire new GPCR family was discovered, which was recently named MAS-related G protein-coupled receptors (MRGPR) by the HUGO Gene Nomenclature Committee. The MRGPR family consists of ∼40 members that are grouped into nine distinct subfamilies (MRGPRA to -H and -X) and are predominantly expressed in primary sensory neurons and mast cells. All members are formally still considered "orphan" by the Committee on Receptor Nomenclature and Drug Classification of the International Union of Basic and Clinical Pharmacology. However, several distinct peptides and amino acids are discussed as potential ligands, including β-alanine, angiotensin-(1-7), alamandine, GABA, cortistatin-14, and cleavage products of proenkephalin, pro-opiomelanocortin, prodynorphin, or proneuropeptide-FF-A. The full spectrum of biologic roles of all MRGPR is still ill-defined, but there is evidence pointing to a role of distinct MRGPR subtypes in nociception, pruritus, sleep, cell proliferation, circulation, and mast cell degranulation. This review article summarizes findings published in the last 10 years on the phylogenetic relationships, pharmacology, signaling, physiology, and agonist-promoted regulation of all MRGPR subfamilies. Furthermore, we highlight interactions between MRGPR and other hormonal systems, paying particular attention to receptor multimerization and morphine tolerance. Finally, we discuss the challenges the field faces presently and emphasize future directions of research.

Nanostructures of designed geometry and functionality enable regulation of cellular signaling processes

Extracellular matrices (ECM) triggered cellular signaling processes often begin with the clustering of the cellular receptors such as integrin and FcεRI. The sizes of these initial protein complexes or clusters are tens to 100 nm in dimension; therefore, engineered nanostructures could provide effective mimics of ECM for investigation and control of the initial and downstream specific signaling processes. This current topic discusses recent advances in nanotechnology in the context of design and production of matching chemical functionality and geometry for control of specific cellular signaling processes. Two investigations are reported to demonstrate this concept: (a) how the presentation of antigen at the nanometer scale would influence the aggregation of FcεRI, which would impact the formation of activation complexes, leading to the rearrangement of actin in cytoskeleton and degranulation or activation of mast cells; (b) how the engineered nanostructure could guide the initial integrin clustering, which would impact the formation of focal adhesion and downstream cell signaling cascades, leading to polarization, migration, and morphological changes. Complementary to engineered ECMs using synthetic ligands or peptides, or topographic control at the micrometer scale, nanostructures of designed geometry and chemical functionality provide new and effective biochemical cues for regulation of cellular signaling processes and downstream behaviors.

Engineered nanostructures of antigen provide an effective means for regulating mast cell activation

Nanostructures containing 2,4-dinitrophenyl (DNP) as antigen were designed and produced to investigate antibody-mediated activation of mast cells. The design consists of nanogrids of DNP termini inlaid in alkanethiol self-assembled monolayers (SAMs). Using scanning probe-based nanografting, nanometer precision was attained for designed geometry, size, and periodicity. Rat basophilic leukemia (RBL) cells exhibited high sensitivity to the geometry and local environment of DNP presented on these nanostructures. The impact included cellular adherence, spreading, membrane morphology, cytoskeleton structure, and activation. The highest level of spreading and activation was induced by nanogrids of 17 nm line width and 40 nm periodicity, with DNP haptens 1.4 nm above the surroundings. The high efficacy is attributed to two main factors. First, DNP sites in the nanostructure are highly accessible by anti-DNP IgE during recognition. Second, the arrangement or geometry of DNP termini in nanostructures promotes clustering of FcεRI receptors that are prelinked to IgE. The clustering effectively initiates Lyn-mediated signaling cascades, ultimately leading to the degranulation of RBL cells. This work demonstrates an important concept: that nanostructures of ligands provide new and effective cues for directing cellular signaling processes.

Impact of actin rearrangement and degranulation on the membrane structure of primary mast cells: a combined atomic force and laser scanning confocal microscopy investigation

Degranulation of bone marrow-derived mast cells (BMMCs) triggered by antigens (e.g., 2,4-dinitrophenylated bovine serum albumin (DNP-BSA) and secretagogues (e.g., poly-L-lysine) was investigated by combined atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). This combination enables the simultaneous visualization and correlation of membrane morphology with cytoskeletal actin arrangement and intracellular granules. Two degranulation mechanisms and detailed membrane structures that directly corresponded to the two stimuli were revealed. In DNP-BSA triggered activation, characteristic membrane ridges formed in accordance with the rearrangement of underlying F-actin networks. Individual granules were visualized after they released their contents, indicating a "kiss-and-run" pathway. In BMMCs stimulated by poly-L-lysine, lamellopodia and filopodia were observed in association with the F-actin assemblies at and near the cell periphery, whereas craters were observed on the central membrane lacking F-actin. These craters represent a new membrane feature resulting from the "kiss-and-merge" granule fusion. This work provides what we believe is important new insight into the local membrane structures in correlation with the cytoskeleton arrangement and detailed degranulation processes.

Anti-inflammatory potential of CB1-mediated cAMP elevation in mast cells

Cannabinoids are broadly immunosuppressive, and anti-inflammatory properties have been reported for certain marijuana constituents and endogenously produced cannabinoids. The CB2 cannabinoid receptor is an established constituent of immune system cells, and we have recently established that the CB1 cannabinoid receptor is expressed in mast cells. In the present study, we sought to define a role for CB1 in mast cells and to identify the signalling pathways that may mediate the suppressive effects of CB1 ligation on mast cell activation. Our results show that CB1 and CB2 mediate diametrically opposed effects on cAMP levels in mast cells. The observed long-term stimulation of cAMP levels by the Galpha(i/o)-coupled CB1 is paradoxical, and our results indicate that it may be attributed to CB1-mediated transcriptional regulation of specific adenylate cyclase isoenzymes that exhibit superactivatable kinetics. Taken together, these results reveal the complexity in signalling of natively co-expressed cannabinoid receptors and suggest that some anti-inflammatory effects of CB1 ligands may be attributable to sustained cAMP elevation that, in turn, causes suppression of mast cell degranulation.

Exposure to tobacco-derived materials induces overproduction of secreted proteinases in mast cells

Mast cells reside at interfaces with the environment, including the mucosa of the respiratory and gastrointestinal tracts. This localization exposes mast cells to inhaled, or ingested, environmental challenges. In the airways of smokers, resident immune cells will be in contact with the condensed components of cigarette smoke. Mast cells are of particular interest due to their ability to promote airway remodeling and mucus hypersecretion. Clinical data show increased levels of mast cell-secreted tryptase and increased numbers of degranulated mast cells in the lavage and bronchial tissue of smokers. Since mast cell-secreted proteinases (MCPTs), including tryptases, contribute to pathological airway remodeling, we investigated the relationship between mast cell proteinases and smoke exposure. We exposed a mast cell line to cigarette smoke condensate (CSC). We show that CSC exposure increases MCPT levels in mast cells using an assay for tryptase-type MCPT activity. We hypothesized that this increase in MCPT activity reflects a CSC-induced increase in the cytosolic pool of proteinase molecules, via stimulation of MCPT transcription. Transcript array data suggested that mRNA changes in response to CSC were limited in number and peaked after 3 h of CSC exposure. However, we noted marked transcriptional regulation of several MCPT genes. CSC-induced changes in the mRNA levels for MCPTs were confirmed using quantitative RT-PCR. Taken together, our data suggest that chronic exposure to cigarette smoke up-regulates MCPT levels in mast cells at both the protein and the mRNA level. We suggest that the pathological airway remodeling that has been described in clinical studies of smoke inhalation may be attributable to MCPT overproduction in vivo.

A TRPV2-PKA signaling module for transduction of physical stimuli in mast cells

Cutaneous mast cell responses to physical (thermal, mechanical, or osmotic) stimuli underlie the pathology of physical urticarias. In vitro experiments suggest that mast cells respond directly to these stimuli, implying that a signaling mechanism couples functional responses to physical inputs in mast cells. We asked whether transient receptor potential (vanilloid) (TRPV) cation channels were present and functionally coupled to signaling pathways in mast cells, since expression of this channel subfamily confers sensitivity to thermal, osmotic, and pressure inputs. Transcripts for a range of TRPVs were detected in mast cells, and we report the expression, surface localization, and oligomerization of TRPV2 protein subunits in these cells. We describe the functional coupling of TRPV2 protein to calcium fluxes and proinflammatory degranulation events in mast cells. In addition, we describe a novel protein kinase A (PKA)–dependent signaling module, containing PKA and a putative A kinase adapter protein, Acyl CoA binding domain protein (ACBD)3, that interacts with TRPV2 in mast cells. We propose that regulated phosphorylation by PKA may be a common pathway for TRPV modulation.