Interaction between activated chemokine receptor 1 and FcepsilonRI at membrane rafts promotes communication and F-actin-rich cytoneme extensions between mast cells

Secretome and Tunneling Nanotubes: A Multilevel Network for Long Range Intercellular Communication between Endothelial Cells and Distant Cells

As a cellular interface between the blood and tissues, the endothelial cell (EC) monolayer is involved in the control of key functions including vascular tone, permeability and homeostasis, leucocyte trafficking and hemostasis. EC regulatory functions require long-distance communications between ECs, circulating hematopoietic cells and other vascular cells for efficient adjusting thrombosis, angiogenesis, inflammation, infection and immunity. This intercellular crosstalk operates through the extracellular space and is orchestrated in part by the secretory pathway and the exocytosis of Weibel Palade Bodies (WPBs), secretory granules and extracellular vesicles (EVs). WPBs and secretory granules allow both immediate release and regulated exocytosis of messengers such as cytokines, chemokines, extracellular membrane proteins, coagulation or growth factors. The ectodomain shedding of transmembrane protein further provide the release of both receptor and ligands with key regulatory activities on target cells. Thin tubular membranous channels termed tunneling nanotubes (TNTs) may also connect EC with distant cells. EVs, in particular exosomes, and TNTs may contain and transfer different biomolecules (e.g., signaling mediators, proteins, lipids, and microRNAs) or pathogens and have emerged as a major triggers of horizontal intercellular transfer of information.

A possible way to prevent the progression of bone lesions in multiple myeloma via Src-homology-region-2-domain-containing-phosphatase-1 activation

On the basis of our recent findings, in which multiple receptor-mediated mast cell functions are regulated via a common signaling cascade, we posit that the formation and functioning of osteoclasts are also controlled by a similar common mechanism. These cells are derived from the same granulocyte/monocyte progenitors and share multiple receptors except those that are cell-specific. In both types of cells, all known receptors reside in lipid rafts, form multiprotein complexes with recruited signaling molecules, and are internalized upon receptor engagement. Signal transduction proceeds in a chain of protein phosphorylations, where adaptor protein LAT (linker-for-activation-of-T-cells) plays a central role. The key kinase that associates LAT phosphorylation and lipid raft internalization is Syk (spleen-tyrosine-kinase) and/or an Src-family-kinase, most probably Lck (lymphocyte-specific-protein-tyrosine-kinase). Dephosphorylation of phosphorylated Syk and Lck by activated SHP-1 (Src-homology-region-2-domain-containing-phosphatase-1) terminates the signal transduction and endocytosis of receptors, resulting in inhibition of osteoclast differentiation and other functions. In malignant plasma cells (MM cells) too, SHP-1 plays a similar indispensable role in controlling signal transduction required for survival and proliferation, though BLNK (B-cell-linker-protein), a functional equivalent of LAT and SLP-76 (SH2-domain-containing-leukocyte-protein-of-76-kDa) in B cells, is used instead of LAT. In both osteoclasts and MM cells, therefore, activated SHP-1 acts negatively in receptor-mediated cellular functions. In osteoblasts, however, activated SHP-1 promotes differentiation, osteocalcin generation, and mineralization by preventing both downregulation of transcription factors, such as Ostrix and Runx2, and degradation of β-catenin required for activation of the transcription factors. SHP-1 is activated by tyrosine phosphorylation and micromolar doses (M-dose) of CCRI-ligand-induced SHP-1 activation. Small molecular compounds, such as A770041, Sorafenib, Nitedanib, and Dovitinib, relieve the autoinhibitory conformation. Activation of SHP-1 by M-dose CCRI ligands or the compounds described may prevent the progression of bone lesions in MM.

The actin cytoskeleton and mast cell function

The application of high and super-resolution microscopy techniques has extended the possibilities of studying actin dynamics in mast cells (MCs). These studies demonstrated the close correlation between actin-driven changes in cell morphology and the functions that MC perform during their life cycle. Dynamic conversions between actin polymerization and depolymerization support MC degranulation and leading to the release of the preformed, secretory granule (SG)-contained, inflammatory mediators. Cell flattening inflicting an actin porous geometry and clearing of cortical actin, characterize the secretory actin phenotype. In contrast, pericentral actin clusters, that entrap the SGs, characterize the migratory actin phenotype, which supports MC migration, but restricts MC degranulation. Multiple actin binding and actin interacting proteins regulate these actin rearrangements, in compliance with the signals elicited by the respective activating receptors. Here, we review recent findings on the interplay between the actin cytoskeleton and MC migration and degranulation.

ELAVL1 Role in Cell Fusion and Tunneling Membrane Nanotube Formations with Implication to Treat Glioma Heterogeneity

Homotypic and heterotypic cell fusions via permanent membrane fusions and temporal tunneling nanotube formations in the glioma microenvironment were recently documented in vitro and in vivo and mediate glioma survival, plasticity, and recurrence. Chronic inflammation, a hypoxic environment, aberrant mitochondrial function, and ER stress due to unfolded protein accumulation upregulate cell fusion events, which leads to tumor heterogeneity and represents an adaptive mechanism to promote tumor cell survival and plasticity in cytotoxic, nutrient-deprived, mechanically stressed, and inflammatory microenvironments. Cell fusion is a multistep process, which consists of the activation of the cellular stress response, autophagy formation, rearrangement of cytoskeletal architecture in the areas of cell-to-cell contacts, and the expression of proinflammatory cytokines and fusogenic proteins. The mRNA-binding protein of ELAV-family HuR is a critical node, which orchestrates the stress response, autophagy formation, cytoskeletal architecture, and the expression of proinflammatory cytokines and fusogenic proteins. HuR is overexpressed in gliomas and is associated with poor prognosis and treatment resistance. Our review provides a link between the HuR role in the regulation of cell fusion and tunneling nanotube formations in the glioma microenvironment and the potential suppression of these processes by different classes of HuR inhibitors.

How Do mAbs Make Use of Complement to Kill Cancer Cells? The Role of Ca2

We examined the kinetics and mechanisms by which monoclonal antibodies (mAbs) utilize complement to rapidly kill targeted cancer cells. Based on results from flow cytometry, confocal microscopy and high-resolution digital imaging experiments, the general patterns which have emerged reveal cytotoxic activities mediated by substantial and lethal Ca2+ fluxes. The Ca2+ fluxes are common to the reported pathways that have been utilized by other toxins in killing nucleated cells. These reactions terminate in very high levels of cell killing, and based on these considerations, we suggest additional strategies to further enhance mAb-based targeting of cancer with complement.

A common signaling pathway leading to degranulation in mast cells and its regulation by CCR1-ligand

BACKGROUND

Signal transduction pathways mediated by various receptors expressed on mast cells are thought to be complex, and inhibitory signals that turn off activating signals are not known.

METHODS

Upstream signaling cascades mediated by several known receptors in bone marrow-derived mast cells that lead to degranulation and mediator release were studied by immunoblotting and immunoprecipitation. Small interfering RNAs and knockout mice were used to confirm findings.

RESULTS

All ligands tested including IgE/Ag, SCF, HSP70, CCL3, and its valiant eMIP induced phosphorylation of linker for activation of T cells (LAT), which triggered their receptor-mediated downstream signaling cascades that controlled degranulation and mediator release. Phosphorylation of lymphocyte-specific protein kinase (Lck) was induced by each ligand, which commonly played an indispensable role in LAT phosphorylation. In contrast, phosphorylation of spleen tyrosine kinase was additionally induced in cells stimulated only with IgE/Ag and SCF, which is also associated with LAT phosphorylation in part. Degranulation and mediator release induced by IgE/Ag, SCF, or HSP70 were enhanced by nanomolar doses of CCR1 ligands CCL3 and eMIP via enhanced LAT phosphorylation. On the other hand, micromolar doses of CCR1 ligand inhibited degranulation and mediator release from mast cells stimulated with IgE/Ag, SCF, or HSP70 by de-phosphorylation of phosphorylated Lck with Src homology region 2 domain-containing phosphatase-1.

CONCLUSIONS

Linker for activation of T cells plays a central role in signal transduction pathways in mast cells stimulated with any ligand tested. Dose-dependent alternate costimulation and inhibition of CCR1 ligands in IgE/Ag-, SCF-, or HSP70-stimulated mast cells occur at the level of Lck-LAT phosphorylation.

Rhes Tunnels: A Radical New Way of Communication in the Brain's Striatum?

Ras homolog enriched in the striatum (Rhes) is a striatal enriched protein that promotes the formation of thin membranous tubes resembling tunneling nanotubes (TNT)-"Rhes tunnels"-that connect neighboring cell and transport cargoes: vesicles and proteins between the neuronal cells. Here the literature on TNT-like structures is reviewed, and the implications of Rhes-mediated TNT, the mechanisms of its formation, and its potential in novel cell-to-cell communication in regulating striatal biology and disease are emphasized. Thought-provoking ideas regarding how Rhes-mediated TNT, if it exists, in vivo, would radically change the way neurons communicate in the brain are discussed.

Cytonemes, Their Formation, Regulation, and Roles in Signaling and Communication in Tumorigenesis

Increasing evidence during the past two decades shows that cells interconnect and communicate through cytonemes. These cytoskeleton-driven extensions of specialized membrane territories are involved in cell–cell signaling in development, patterning, and differentiation, but also in the maintenance of adult tissue homeostasis, tissue regeneration, and cancer. Brain tumor cells in glioblastoma extend ultralong membrane protrusions (named tumor microtubes, TMs), which contribute to invasion, proliferation, radioresistance, and tumor progression. Here we review the mechanisms underlying cytoneme formation, regulation, and their roles in cell signaling and communication in epithelial cells and other cell types. Furthermore, we discuss the recent discovery of glial cytonemes in the Drosophila glial cells that alter Wingless (Wg)/Frizzled (Fz) signaling between glia and neurons. Research on cytoneme formation, maintenance, and cell signaling mechanisms will help to better understand not only physiological developmental processes and tissue homeostasis but also cancer progression.

Mast Cell Cytonemes as a Defense Mechanism against Coxiella burnetii

Mast cells (MCs) are found in tissues that are in close contact with external environment, such as skin, lungs, or intestinal mucosa but also in the placenta during pregnancy. If their role in mediating allergic conditions is established, several studies now highlight their importance during infection with extracellular pathogens. This study showed a new and effective antimicrobial mechanism of MCs against Coxiella burnetii, an intracellular bacterium whose infection during pregnancy is associated with abortion, preterm labor, and stillbirth. The data reveal that in response to C. burnetii, MCs release extracellular actin filaments that contain antimicrobial agents and are capable to trap and kill bacteria. We show that this mechanism is dependent on the cooperation of two membrane receptors, CD36 and Toll-like receptor 4, and may occur in the placenta during pregnancy by using ex vivo placental MCs. Overall, this study reports an unexpected role for MCs during infection with intracellular bacteria and suggests that MC response to C. burnetii infection is a protective defense mechanism during pregnancy.

Mast cells (MCs) are critical mediators of inflammation; however, their microbicidal activity against invading pathogens remains largely unknown. Here, we describe a nonpreviously reported antibacterial mechanism used by MCs against Coxiella burnetii, the agent of Q fever. We show that C. burnetii interaction with MCs does not result in bacterial uptake but rather induces the formation of extracellular actin filaments named cytonemes. MC cytonemes express cathelicidin and neutrophil elastase and mediate the capture and destruction of entrapped bacteria. We provide evidence that MC cytoneme formation and microbicidal activity are dependent on the cooperation of the scavenger receptor CD36 and Toll-like receptor 4. Taken together, our results suggest that MCs use an extracellular sophisticated mechanism of defense to eliminate intracellular pathogens, such as C. burnetii, before their entry into host cells.

Mast cell-neural interactions contribute to pain and itch

Mast cells are best recognized for their role in allergy and anaphylaxis, but increasing evidence supports their role in neurogenic inflammation leading to pain and itch. Mast cells act as a "power house" by releasing algogenic and pruritogenic mediators, which initiate a reciprocal communication with specific nociceptors on sensory nerve fibers. Consequently, nerve fibers release inflammatory and vasoactive neuropeptides, which in turn activate mast cells in a feedback mechanism, thus promoting a vicious cycle of mast cell and nociceptor activation leading to neurogenic inflammation and pain/pruritus. Mechanisms underlying mast cell differentiation, activation, and intercellular interactions with inflammatory, vascular, and neural systems are deeply influenced by their microenvironment, imparting enormous heterogeneity and complexity in understanding their contribution to pain and pruritus. Neurogenic inflammation is central to both pain and pruritus, but specific mediators released by mast cells to promote this process may vary depending upon their location, stimuli, underlying pathology, gender, and species. Therefore, in this review, we present the contribution of mast cells in pathological conditions, including distressing pruritus exacerbated by psychologic stress and experienced by the majority of patients with psoriasis and atopic dermatitis and in different pain syndromes due to mastocytosis, sickle cell disease, and cancer.

Emerging role of contact-mediated cell communication in tissue development and diseases

Cells of multicellular organisms are in continuous conversation with the neighbouring cells. The sender cells signal the receiver cells to influence their behaviour in transport, metabolism, motility, division, and growth. How cells communicate with each other can be categorized by biochemical signalling processes, which can be characterised by the distance between the sender cell and the receiver cell. Existing classifications describe autocrine signals as those where the sender cell is identical to the receiver cell. Complementary to this scenario, paracrine signalling describes signalling between a sender cell and a different receiver cell. Finally, juxtacrine signalling describes the exchange of information between adjacent cells by direct cell contact, whereas endocrine signalling describes the exchange of information, e.g., by hormones between distant cells or even organs through the bloodstream. In the last two decades, however, an unexpected communication mechanism has been identified which uses cell protrusions to exchange chemical signals by direct contact over long distances. These signalling protrusions can deliver signals in both ways, from sender to receiver and vice versa. We are starting to understand the morphology and function of these signalling protrusions in many tissues and this accumulation of findings forces us to revise our view of contact-dependent cell communication. In this review, we will focus on the two main categories of signalling protrusions, cytonemes and tunnelling nanotubes. These signalling protrusions emerge as essential structural components of a vibrant communication network in the development and tissue homeostasis of any multicellular organism.

The roles of TNFAIP2 in cancers and infectious diseases

TNFα‐induced protein 2 (TNFAIP2) is a primary response gene of TNFα. TNFAIP2 is highly expressed in immune cells and the urinary bladder. The expression of TNFAIP2 is regulated by multiple transcription factors and signalling pathways, including NF‐κB, KLF5 and retinoic acid. Physiologically, TNFAIP2 appears to be a multiple functional mediator not only for inflammation, angiogenesis and tunneling nanotube (TNT) formation but also as a regulator of cell proliferation and migration. The expression of TNFAIP2 is frequently abnormal in human cancers and in infectious diseases. Due to its significant functions in cell proliferation, angiogenesis, migration and invasion, TNFAIP2 could be a potential diagnostic biomarker and therapeutic target for cancer.

LST1: A multifunctional gene encoded in the MHC class III region

The LST1 gene is located in the MHC class III cluster between the MHC class I and II regions. While most genes in this cluster have been sufficiently characterised, a definitive function and expression pattern for LST1 still remains elusive. In the present review we describe its promotor, gene organisation, splice variants and expression in human tissues, cell lines and cancer. We focus on LST1 expression in inflammation and discuss known correlations with autoimmune diseases and cancer. Current data on LST1 polymorphisms and their known associations with pathologies are also discussed in detail. We summarize the potential functions that have been described for the full-length LST1 protein including its function as a transmembrane adaptor protein with inhibitory signal transduction and its role as a membrane scaffold facilitating the formation of tunnelling nanotubes. We also discuss further potential functions by compiling all known LST1-interacting proteins. Furthermore, we address knowledge gaps and conflictive issues regarding disease association, non-hematopoietic expression and the discrepancy between RNA and protein expression data.

Homotypic endothelial nanotubes induced by wheat germ agglutinin and thrombin

Endothelial barrier formation is maintained by intercellular communication through junctional proteins. The mechanisms involved in maintaining endothelial communication subsequent to barrier disruption remain unclear. It is known that low numbers of endothelial cells can be interconnected by homotypic actin-driven tunneling nanotubes (TNTs) which could be important for intercellular transfer of information in vascular physiology. Here we sought insight into the triggers for TNT formation. Wheat germ agglutinin, a C-type lectin and known label for TNTs, unexpectedly caused striking induction of TNTs. A succinylated derivative was by contrast inactive, suggesting mediation by a sialylated protein. Through siRNA-mediated knockdown we identified that this protein was likely to be CD31, an important sialylated membrane protein normally at endothelial cell junctions. We subsequently considered thrombin as a physiological inducer of endothelial TNTs because it reduces junctional contact. Thrombin reduced junctional contact, redistributed CD31 and induced TNTs, but its effect on TNTs was CD31-independent. Thrombin-induced TNTs nevertheless required PKCα, a known mediator of thrombin-dependent junctional remodelling, suggesting a necessity for junctional proteins in TNT formation. Indeed, TNT-inducing effects of wheat germ agglutinin and thrombin were both correlated with cortical actin rearrangement and similarly Ca²⁺-dependent, suggesting common underlying mechanisms. Once formed, Ca²⁺ signalling along TNTs was observed.

Basic Fibroblast Growth Factor 2 Is a Determinant of CD4 T Cell-Airway Smooth Muscle Cell Communication through Membrane Conduits

Activated CD4 T cells connect to airway smooth muscle cells (ASMCs) in vitro via lymphocyte-derived membrane conduits (LMCs) structurally similar to membrane nanotubes with unknown intercellular signals triggering their formation. We examined the structure and function of CD4 T cell-derived LMCs, and we established a role for ASMC-derived basic fibroblast growth factor 2 (FGF2b) and FGF receptor (FGFR)1 in LMC formation. Blocking FGF2b's synthesis and FGFR1 function reduced LMC formation. Mitochondrial flux from ASMCs to T cells was partially FGF2b and FGFR1 dependent. LMC formation by CD4 T cells and mitochondrial transfer from ASMCs was increased in the presence of asthmatic ASMCs that expressed more mRNA for FGF2b compared with normal ASMCs. These observations identify ASMC-derived FGF2b as a factor needed for LMC formation by CD4 T cells, affecting intercellular communication.

Neuroendocrinology of mast cells: Challenges and controversies

Mast cells (MC) are hemotopoietically derived tissue immune cells that are ubiquitous in the body, including neuroendocrine organs such as the hypothalamus, pineal, pituitary, ovaries, pancreas and uterus where their action is not well understood. Mast cells have historically been associated with allergies because of their rich content of histamine and tryptase, but more recently with regulation of immunity and inflammation due to their synthesis and release of numerous cytokines and chemokines. Mast cells are located perivascularly and express numerous receptors for diverse ligands such as allergens, pathogens, neurotransmitters, neuropeptides and hormones including acetylcholine, calcitonin gene-related peptide (CGRP), corticosteroids, corticotropin-releasing hormone (CRH), β-endorphin, epinephrine, 17β-oestradiol, gonadotrophins, hemokinin-A (HKA), leptin, melatonin, neurotensin (NT), parathyroid hormone (PTH), substance P (SP) and vasoactive intestinal peptide (VIP). Moreover, MC can synthesize and release most of their neurohormonal triggers, including adrenocorticotropin hormone (ACTH), CRH, endorphins, HKA, leptin, melatonin, NT, SP and VIP. Animal experiments have shown that diencephalic MC increase in number during courting in doves, while stimulation of brain and nasal MC leads to activation of the hypothalamic-pituitary-adrenal (HPA) axis. Recent evidence indicates that MC reactivity exhibits diurnal variations, and it is interesting that melatonin appears to regulate MC secretion. However, the way MC change their phenotype or secrete specific molecules selectively at different pathophysiological settings still remains unknown. Mast cells developed over 500 million years ago and may have served as the original prototype neuroimmunoendocrine cell and then evolved into a master regulator of such interactions, especially as most of the known diseases involve neuroinflammation that worsens with stress.

Linked in: immunologic membrane nanotube networks

Membrane nanotubes, also termed tunneling nanotubes, are F-actin-based structures that can form direct cytoplasmic connections and support rapid communication between distant cells. These nanoscale conduits have been observed in diverse cell types, including immune, neuronal, stromal, cancer, and stem cells. Until recently, little was known about the mechanisms involved in membrane nanotube development in myeloid origin APCs or how membrane nanotube networks support their ability to bridge innate and adaptive immunity. New research has provided insight into the modes of induction and regulation of the immune process of "reticulation" or the development of multicellular membrane nanotube networks in dendritic cells. Preprogramming by acute type 1 inflammatory mediators at their immature stage licenses mature type 1-polarized dendritic cells to reticulate upon subsequent interaction with CD40 ligand-expressing CD4(+) Th cells. Dendritic cell reticulation can support direct antigen transfer for amplification of specific T cell responses and can be positively or negatively regulated by signals from distinct Th cell subsets. Membrane nanotubes not only enhance the ability of immature dendritic cells to sense pathogens and rapidly mobilize nearby antigen-presenting cells in the peripheral tissues but also likely support communication of pathogen-related information from mature migratory dendritic cells to resident dendritic cells in lymph nodes. Therefore, the reticulation process facilitates a coordinated multicellular response for the efficient initiation of cell-mediated adaptive immune responses. Herein, we discuss studies focused on the molecular mechanisms of membrane nanotube formation, structure, and function in the context of immunity and how pathogens, such as HIV-1, may use dendritic cell reticulation to circumvent host defenses.

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.

Identification of a family of fatty-acid-speciated sonic hedgehog proteins, whose members display differential biological properties

Hedgehog (HH) proteins are proteolytically processed into a biologically active form that is covalently modified by cholesterol and palmitate. However, most studies of HH biogenesis have characterized protein from cells in which HH is overexpressed. We purified Sonic Hedgehog (SHH) from cells expressing physiologically relevant levels and showed that it was more potent than SHH isolated from overexpressing cells. Furthermore, the SHH in our preparations was modified with a diverse spectrum of fatty acids on its amino termini, and this spectrum of fatty acids varied dramatically depending on the growth conditions of the cells. The fatty acid composition of SHH affected its trafficking to lipid rafts as well as its potency. Our results suggest that HH proteins exist as a family of diverse lipid-speciated proteins that might be altered in different physiological and pathological contexts in order to regulate distinct properties of HH proteins.

Cytonemes and the dispersion of morphogens

Filopodia are cellular protrusions that have been implicated in many types of mechanosensory activities. Morphogens are signaling proteins that regulate the patterned development of embryos and tissues. Both have long histories that date to the beginnings of cell and developmental biology in the early 20th century, but recent findings tie specialized filopodia called cytonemes to morphogen movement and morphogen signaling. This review explores the conceptual and experimental background for a model of paracrine signaling in which the exchange of morphogens between cells is directed to sites where cytonemes directly link cells that produce morphogens to cells that receive and respond to them.

In vitro effects of cocaine on tunneling nanotube formation and extracellular vesicle release in glioblastoma cell cultures

The effects of cocaine (150 nM, 300 nM, and 150 μM) on human glioblastoma cell cultures were studied on tunneling nanotube formation (1-h cocaine treatment) and extracellular vesicle release (1-, 3-, and 8-h cocaine treatment). Cocaine significantly increased the number of tunneling nanotubes only at the lowest concentration used. The release of extracellular vesicles (mainly exosomes) into the medium was stimulated by cocaine at each concentration used with a maximum effect at the highest concentration tested (150 μM). Moreover, cocaine (150 nM) significantly increased the number of vesicles with 61-80 nm diameter while at concentrations of 300 nM and 150 μM, and the smaller vesicles (30-40 nm diameter) were significantly increased with a reduction of the larger vesicles (41-60 nm diameter). A time dependence in the release of extracellular vesicles was observed. In view of the proposed role of these novel intercellular communication modes in the glial-neuronal plasticity, it seems possible that they can participate in the processes leading to cocaine addiction. The molecular target/s involved in these cocaine effects could be specific molecular components of plasma membrane lipid rafts and/or cocaine-induced modifications in cytoplasmic lipid composition.

Communicating by touch--neurons are not alone

Long-distance cell-cell communication is essential for organ development and function. Whereas neurons communicate at long distances by transferring signals at sites of direct contact (i.e., at synapses), it has been presumed that the only way other cell types signal is by dispersing signals through extracellular fluid--indirectly. Recent evidence from Drosophila suggests that non-neuronal cells also exchange signaling proteins at sites of direct contact, even when long distances separate the cells. We review here contact-mediated signaling in neurons and discuss how this signaling mechanism is shared by other cell types.

Actin cytoskeleton reorganization correlates with polarization of secretory vesicle and cell morphology in the degranulation of mast cell subtypes in human colon tissues

Mast cells play a central role in the intestinal immune response. To investigate the relationship between degranulation, cell polarization and the reorganization of actin cytoskeleton of mast cells, we used fluorescence or gold labeling methods to identify different mast cell subtypes in human colon. The reorganization of filamentous actin was visualized and then the polarization of secretory vesicles, as well as cell surfaces, was analyzed by fluorescence microscopy and electron microscopy. Our results first showed a diversity of filamentous actin assembly or disassembly within the contacting cell membrane of different mast cell subtypes. The polarization and degranulation of secretory vesicles was not only accompanied with the assembly and disassembly of filamentous actin at the cell periphery, but also with changes of cell surface polarization. Our study provides an insight into the local membranous structures and suggested correlations of cytoskeleton arrangement with the polarization of secretory vesicles and cell surface configuration during mast cell degranulation.

Cytonemes as specialized signaling filopodia

Development creates a vast array of forms and patterns with elegant economy, using a small vocabulary of pattern-generating proteins such as BMPs, FGFs and Hh in similar ways in many different contexts. Despite much theoretical and experimental work, the signaling mechanisms that disperse these morphogen signaling proteins remain controversial. Here, we review the conceptual background and evidence that establishes a fundamental and essential role for cytonemes as specialized filopodia that transport signaling proteins between signaling cells. This evidence suggests that cytoneme-mediated signaling is a dispersal mechanism that delivers signaling proteins directly at sites of cell-cell contact.

Perivascular mast cells dynamically probe cutaneous blood vessels to capture immunoglobulin E

Mast cells are tissue-resident immune cells that play a central role in allergic disease. These contributions are largely dependent on the acquisition of antigen-specific immunoglobulin E (IgE). Despite this requirement, studies of mast cell and IgE interactions have overlooked the mechanism by which mast cells acquire IgE from the blood. To address this gap, we developed reporter IgE molecules and employed imaging techniques to study mast cell function in situ. Our data demonstrate that skin mast cells exhibit selective uptake of IgE based on perivascular positioning. Furthermore, perivascular mast cells acquire IgE by extending cell processes across the vessel wall to capture luminal IgE. These data demonstrate how tissue mast cells acquire IgE and reveal a strategy by which extravascular cells monitor blood contents to capture molecules central to cellular function.

LST1 promotes the assembly of a molecular machinery responsible for tunneling nanotube formation

Carefully orchestrated intercellular communication is an essential prerequisite for the development of multicellular organisms. In recent years, tunneling nanotubes (TNT) have emerged as a novel and widespread mechanism of cell-cell communication. However, the molecular basis of their formation is still poorly understood. In the present study we report that the transmembrane MHC class III protein leukocyte specific transcript 1 (LST1) induces the formation of functional nanotubes and is required for endogenous nanotube generation. Mechanistically, we found that LST1 induces nanotube formation by recruiting the small GTPase RalA to the plasma membrane and promoting its interaction with the exocyst complex. Furthermore, we determined that LST1 recruits the actin-crosslinking protein filamin to the plasma membrane and interacts with M-Sec, myosin and myoferlin. These results allow us to suggest a molecular model for nanotube generation. In this proposal LST1 functions as a membrane scaffold mediating the assembly of a multimolecular complex, which controls the formation of functional nanotubes.

Tunneling nanotubes enable intercellular transfer of MHC class I molecules

Carefully orchestrated intercellular communication is an essential prerequisite for an effective immune response. In recent years tunneling nanotubes (TNT) have emerged as a novel mechanism of cell-cell communication. These long membrane protrusions can establish cytoplasmic continuity between distant cells and enable the exchange of cellular components. In the present study we addressed the question whether these structures can facilitate the intercellular transfer of MHC class I molecules. We found a transmembrane HLA-A2-EGFP but not a soluble HLA-G1s-EGFP fusion protein to be effectively transferred between HeLa cells. Inhibition of actin polymerization significantly reduced the HLA-A2 transfer rate, indicating that transfer is dependent on tunneling nanotubes, whose de novo formation requires actin polymerization. Furthermore, overexpression of the nanotube-inducing protein LST1 promoted transfer of HLA-A2. Moreover, LST1 protein expression is enhanced in antigen presenting cells. Our results indicate that tunneling nanotubes can mediate transfer of MHC class I molecules between distant cells.

RhoD activated by fibroblast growth factor induces cytoneme-like cellular protrusions through mDia3C

The small GTPase RhoD, activated by fibroblast growth factor (FGF) signaling, forms actin-based, cytoneme-like, thin and long cellular protrusions through activating mDia3C. These protrusions transmit FGF receptors toward the cell body. They are likely to be responsible for intercellular communication between FGF-producing cells and target cells.

The small GTPase RhoD regulates actin cytoskeleton to collapse actin stress fibers and focal adhesions, resulting in suppression of cell migration and cytokinesis. It also induces alignment of early endosomes along actin filaments and reduces their motility. We show here that a constitutively activated RhoD generated two types of actin-containing thin peripheral cellular protrusions distinct from Cdc42-induced filopodia. One was longer, almost straight, immotile, and sensitive to fixation, whereas the other was shorter, undulating, motile, and resistant to fixation. Moreover, cells expressing wild-type RhoD extended protrusions toward fibroblast growth factor (FGF) 2/4/8–coated beads. Stimulation of wild-type RhoD-expressing cells with these FGFs also caused formation of cellular protrusions. Nodules moved through the RhoD-induced longer protrusions, mainly toward the cell body. Exogenously expressed FGF receptor was associated with these moving nodules containing endosome-like vesicles. These results suggest that the protrusions are responsible for intercellular communication mediated by FGF and its receptor. Accordingly, the protrusions are morphologically and functionally equivalent to cytonemes. RhoD was activated by FGF2/4/8. Knockdown of RhoD interfered with FGF-induced protrusion formation. Activated RhoD specifically bound to mDia3C and facilitated actin polymerization together with mDia3C. mDia3C was localized to the tips or stems of the protrusions. In addition, constitutively activated mDia3C formed protrusions without RhoD or FGF stimulation. Knockdown of mDia3 obstructed RhoD-induced protrusion formation. These results imply that RhoD activated by FGF signaling forms cytoneme-like protrusions through activation of mDia3C, which induces actin filament formation.

Novel identified receptors on mast cells

Mast cells (MC) are major participants in the allergic reaction. In addition they possess immunomodulatory roles in the innate and adaptive immune reactions. Their functions are modulated through a number of activating and inhibitory receptors expressed on their surface. This review deals with some of the most recently described receptors, their expression patterns, ligand(s), signal transduction mechanisms, possible cross-talk with other receptors and, last but not least, regulatory functions that the MC can perform based on their receptor expression in health or in disease. Where the receptor role on MC is still not clear, evidences from other hematopoietic cells expressing them is provided as a possible insight for their function on MC. Suggested strategies to modulate these receptors’ activity for the purpose of therapeutic intervention are also discussed.

Cytoskeleton in mast cell signaling

Mast cell activation mediated by the high affinity receptor for IgE (FcεRI) is a key event in allergic response and inflammation. Other receptors on mast cells, as c-Kit for stem cell factor and G protein-coupled receptors (GPCRs) synergistically enhance the FcεRI-mediated release of inflammatory mediators. Activation of various signaling pathways in mast cells results in changes in cell morphology, adhesion to substrate, exocytosis, and migration. Reorganization of cytoskeleton is pivotal in all these processes. Cytoskeletal proteins also play an important role in initial stages of FcεRI and other surface receptors induced triggering. Highly dynamic microtubules formed by αβ-tubulin dimers as well as microfilaments build up from polymerized actin are affected in activated cells by kinases/phosphatases, Rho GTPases and changes in concentration of cytosolic Ca(2+). Also important are nucleation proteins; the γ-tubulin complexes in case of microtubules or Arp 2/3 complex with its nucleation promoting factors and formins in case of microfilaments. The dynamic nature of microtubules and microfilaments in activated cells depends on many associated/regulatory proteins. Changes in rigidity of activated mast cells reflect changes in intermediate filaments build up from vimentin. This review offers a critical appraisal of current knowledge on the role of cytoskeleton in mast cells signaling.

Mast cells as cellular sensors in inflammation and immunity

Mast cells are localized in tissues. Intense research on these cells over the years has demonstrated their role as effector cells in the maintenance of tissue integrity following injury produced by infectious agents, toxins, metabolic states, etc. After stimulation they release a sophisticated array of inflammatory mediators, cytokines, and growth factors to orchestrate an inflammatory response. These mediators can directly initiate tissue responses on resident cells, but they have also been shown to regulate other infiltrating immune cell functions. Research in recent years has revealed that the outcome of mast cell actions is not always detrimental for the host but can also limit disease development. In addition, mast cell functions highly depend on the physiological context in the organism. Depending on the genetic background, strength of the injurious event, the particular microenvironment, mast cells direct responses ranging from pro- to anti-inflammatory. It appears that they have evolved as cellular sensors to discern their environment in order to initiate an appropriate physiological response either aimed to favor inflammation for repair or at the contrary limit the inflammatory process to prevent further damage. Like every sophisticated machinery, its dysregulation leads to pathology. Given the broad distribution of mast cells in tissues this also explains their implication in many inflammatory diseases.

Targeting chemokine receptors in allergic disease

The directed migration of cells in response to chemical cues is known as chemoattraction, and plays a key role in the temporal and spatial positioning of cells in lower- and higher-order life forms. Key molecules in this process are the chemotactic cytokines, or chemokines, which, in humans, constitute a family of approx. 40 molecules. Chemokines exert their effects by binding to specific GPCRs (G-protein-coupled receptors) which are present on a wide variety of mature cells and their progenitors, notably leucocytes. The inappropriate or excessive generation of chemokines is a key component of the inflammatory response observed in several clinically important diseases, notably allergic diseases such as asthma. Consequently, much time and effort has been directed towards understanding which chemokine receptors and ligands are important in the allergic response with a view to therapeutic intervention. Such strategies can take several forms, although, as the superfamily of GPCRs has historically proved amenable to blockade by small molecules, the development of specific antagonists has been has been a major focus of several groups. In the present review, I detail the roles of chemokines and their receptors in allergic disease and also highlight current progress in the development of relevant chemokine receptor antagonists.

Lipid rafts in mast cell biology

Mast cells have long been recognized to have a direct and critical role in allergic and inflammatory reactions. In allergic diseases, these cells exert both local and systemic responses, including allergic rhinitis and anaphylaxis. Mast cell mediators are also related to many chronic inflammatory conditions. Besides the roles in pathological conditions, the biological functions of mast cells include roles in innate immunity, involvement in host defense mechanisms against parasites, immunomodulation of the immune system, tissue repair, and angiogenesis. Despite their growing significance in physiological and pathological conditions, much still remains to be learned about mast cell biology. This paper presents evidence that lipid rafts or raft components modulate many of the biological processes in mast cells, such as degranulation and endocytosis, play a role in mast cell development and recruitment, and contribute to the overall preservation of mast cell structure and organization.

Malaria parasites form filamentous cell-to-cell connections during reproduction in the mosquito midgut

Physical contact is important for the interaction between animal cells, but it can represent a major challenge for protists like malaria parasites. Recently, novel filamentous cell-cell contacts have been identified in different types of eukaryotic cells and termed nanotubes due to their morphological appearance. Nanotubes represent small dynamic membranous extensions that consist of F-actin and are considered an ancient feature evolved by eukaryotic cells to establish contact for communication. We here describe similar tubular structures in the malaria pathogen Plasmodium falciparum, which emerge from the surfaces of the forming gametes upon gametocyte activation in the mosquito midgut. The filaments can exhibit a length of > 100 μm and contain the F-actin isoform actin 2. They actively form within a few minutes after gametocyte activation and persist until the zygote transforms into the ookinete. The filaments originate from the parasite plasma membrane, are close ended and express adhesion proteins on their surfaces that are typically found in gametes, like Pfs230, Pfs48/45 or Pfs25, but not the zygote surface protein Pfs28. We show that these tubular structures represent long-distance cell-to-cell connections between sexual stage parasites and demonstrate that they meet the characteristics of nanotubes. We propose that malaria parasites utilize these adhesive "nanotubes" in order to facilitate intercellular contact between gametes during reproduction in the mosquito midgut.