Endobrevin/VAMP8 mediates exocytotic release of hexosaminidase from rat basophilic leukaemia cells

Lysosomal inhibition sensitizes TMEM16A-expressing cancer cells to chemotherapy

Cisplatin is the first line therapy for patients with head and neck cancer. However, resistance to cisplatin remains a major concern. High expression of the calcium-activated chloride channel TMEM16A in tumors portends poor survival in these patients, possibly because of drug resistance. Here, we show that TMEM16A drives the sequestration of cisplatin into lysosomes. Subsequently, cisplatin is expelled via the delivery of lysosomes to the cell surface. We show that TMEM16A enhances this process, thereby promoting cisplatin resistance. We also show that lysosomal inhibition synergizes with cisplatin to induce tumor cell death. Our data uncovers a new fundamental feature of both lysosomal physiology and cancer cell biology that can potentially impact the treatment of patients with head and neck cancer.

Squamous cell carcinoma of the head and neck (SCCHN) is a devastating disease that continues to have low cure rates despite the recent advances in therapies. Cisplatin is the most used chemotherapy agent, and treatment failure is largely driven by resistance to this drug. Amplification of chromosomal band 11q13 occurs in ∼30% of SCCHN tumors. This region harbors the ANO1 gene that encodes the TMEM16A ion channel, which is responsible for calcium-activated chloride transport in epithelial tissues. TMEM16A overexpression is associated with cisplatin resistance, and high TMEM16A levels correlate with decreased survival. However, the mechanistic underpinning of this effect remains unknown. Lysosomal biogenesis and exocytosis have been implicated in cancer because of their roles in the clearance of damaged organelles and exocytosis of chemotherapeutic drugs and toxins. Here, we show that TMEM16A overexpression promotes lysosomal biogenesis and exocytosis, which is consistent with the expulsion of intracellular cisplatin. Using a combination of genetic and pharmacologic approaches, we find that TMEM16A promotes lysosomal flux in a manner that requires reactive oxygen species, TRPML1, and the activation of the β-catenin–melanocyte-inducing transcription factor pathway. The lysosomal inhibitor hydroxychloroquine (HCQ) synergizes with cisplatin in killing SCCHN cells in vitro. Using a murine model of SCCHN, we show that HCQ and cisplatin retard the growth of cisplatin-resistant patient-derived xenografts in vivo. We propose that TMEM16A enables cell survival by the up-regulation of lysosomal sequestration and exocytosis of the cytotoxic drugs. These results uncover a model of treatment for resistance in cancer, its reversal, and a role for TMEM16A.

The high affinity IgE receptor: a signaling update

Here we update receptor proximal and distant signaling events of the mast cell high affinity IgE receptor (FcεRI) launching immediate type I hypersensitivity and an inflammatory cytokine-chemokine cascade. Different physiologic antigen concentrations, their affinity, and valency for the IgE ligand produce distinct intracellular signaling events with different outcomes. Investigating mast cell degranulation has revealed a complex molecular machinery that relays proximal signaling to cytoskeletal reorganization, granule transport and membrane fusion. Several new phosphorylation- and calcium-responsive effectors have been described. FcεRI signaling also promotes de novo gene transcription. Recent progress has identified enhancers at genes that are upregulated in mast cells after stimulation through FcεRI using next generation sequencing methods. Enhancers at genes that respond to antigenic stimulation in human mast cells revealed Ca²⁺-dependency. Stimulation-responsive super enhancers in mouse mast cells have also been identified. Mast cell lineage-determining transcription factor GATA2 primes these enhancers to respond to antigenic stimulation.

Cytoskeletal Transport, Reorganization, and Fusion Regulation in Mast Cell-Stimulus Secretion Coupling

Mast cells are well known for their role in allergies and many chronic inflammatory diseases. They release upon stimulation, e.g., via the IgE receptor, numerous bioactive compounds from cytoplasmic secretory granules. The regulation of granule secretion and its interaction with the cytoskeleton and transport mechanisms has only recently begun to be understood. These studies have provided new insight into the interaction between the secretory machinery and cytoskeletal elements in the regulation of the degranulation process. They suggest a tight coupling of these two systems, implying a series of specific signaling effectors and adaptor molecules. Here we review recent knowledge describing the signaling events regulating cytoskeletal reorganization and secretory granule transport machinery in conjunction with the membrane fusion machinery that occur during mast cell degranulation. The new insight into MC biology offers novel strategies to treat human allergic and inflammatory diseases targeting the late steps that affect harmful release from granular stores leaving regulatory cytokine secretion intact.

Synergistic actions of v-SNARE transmembrane domains and membrane-curvature modifying lipids in neurotransmitter release

Vesicle fusion is mediated by assembly of SNARE proteins between opposing membranes. While previous work suggested an active role of SNARE transmembrane domains (TMDs) in promoting membrane merger (Dhara et al., 2016), the underlying mechanism remained elusive. Here, we show that naturally-occurring v-SNARE TMD variants differentially regulate fusion pore dynamics in mouse chromaffin cells, indicating TMD flexibility as a mechanistic determinant that facilitates transmitter release from differentially-sized vesicles. Membrane curvature-promoting phospholipids like lysophosphatidylcholine or oleic acid profoundly alter pore expansion and fully rescue the decelerated fusion kinetics of TMD-rigidifying VAMP2 mutants. Thus, v-SNARE TMDs and phospholipids cooperate in supporting membrane curvature at the fusion pore neck. Oppositely, slowing of pore kinetics by the SNARE-regulator complexin-2 withstands the curvature-driven speeding of fusion, indicating that pore evolution is tightly coupled to progressive SNARE complex formation. Collectively, TMD-mediated support of membrane curvature and SNARE force-generated membrane bending promote fusion pore formation and expansion.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

Superiority of SpiroZin2 Versus FluoZin-3 for monitoring vesicular Zn2+ allows tracking of lysosomal Zn2+ pools

Small-molecule fluorescent probes are powerful and ubiquitous tools for measuring the concentration and distribution of analytes in living cells. However, accurate characterization of these analytes requires rigorous evaluation of cell-to-cell heterogeneity in fluorescence intensities and intracellular distribution of probes. In this study, we perform a parallel and systematic comparison of two small-molecule fluorescent vesicular Zn²⁺ probes, FluoZin-3 AM and SpiroZin2, to evaluate each probe for measurement of vesicular Zn²⁺ pools. Our results reveal that SpiroZin2 is a specific lysosomal vesicular Zn²⁺ probe and affords uniform measurement of resting Zn²⁺ levels at the single cell level with proper calibration. In contrast, FluoZin-3 AM produces highly variable fluorescence intensities and non-specifically localizes in the cytosol and multiple vesicular compartments. We further applied SpiroZin2 to lactating mouse mammary epithelial cells and detected a transient increase of lysosomal free Zn²⁺ at 24-hour after lactation hormone treatment, which implies that lysosomes play a role in the regulation of Zn²⁺ homeostasis during lactation. This study demonstrates the need for critical characterization of small-molecule fluorescent probes to define the concentration and localization of analytes in different cell populations, and reveals SpiroZin2 to be capable of reporting diverse perturbations to lysosomal Zn²⁺.

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.

Phosphorylation of residues inside the SNARE complex suppresses secretory vesicle fusion

Membrane fusion is essential for eukaryotic life, requiring SNARE proteins to zipper up in an α‐helical bundle to pull two membranes together. Here, we show that vesicle fusion can be suppressed by phosphorylation of core conserved residues inside the SNARE domain. We took a proteomics approach using a PKCB knockout mast cell model and found that the key mast cell secretory protein VAMP8 becomes phosphorylated by PKC at multiple residues in the SNARE domain. Our data suggest that VAMP8 phosphorylation reduces vesicle fusion in vitro and suppresses secretion in living cells, allowing vesicles to dock but preventing fusion with the plasma membrane. Markedly, we show that the phosphorylation motif is absent in all eukaryotic neuronal VAMPs, but present in all other VAMPs. Thus, phosphorylation of SNARE domains is a general mechanism to restrict how much cells secrete, opening the door for new therapeutic strategies for suppression of secretion.

v-SNARE transmembrane domains function as catalysts for vesicle fusion

Vesicle fusion is mediated by an assembly of SNARE proteins between opposing membranes, but it is unknown whether transmembrane domains (TMDs) of SNARE proteins serve mechanistic functions that go beyond passive anchoring of the force-generating SNAREpin to the fusing membranes. Here, we show that conformational flexibility of synaptobrevin-2 TMD is essential for efficient Ca(2+)-triggered exocytosis and actively promotes membrane fusion as well as fusion pore expansion. Specifically, the introduction of helix-stabilizing leucine residues within the TMD region spanning the vesicle's outer leaflet strongly impairs exocytosis and decelerates fusion pore dilation. In contrast, increasing the number of helix-destabilizing, ß-branched valine or isoleucine residues within the TMD restores normal secretion but accelerates fusion pore expansion beyond the rate found for the wildtype protein. These observations provide evidence that the synaptobrevin-2 TMD catalyzes the fusion process by its structural flexibility, actively setting the pace of fusion pore expansion.

Phosphorylation of syntaxin-3 at Thr 14 negatively regulates exocytosis in RBL-2H3 mast cells

Recent studies have revealed that soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins interact with each other, forming a SNARE complex that induces exocytosis in mast cells. Previously, we reported that syntaxin-3, a SNARE protein, regulates mast cell exocytosis and is constantly phosphorylated. In this study, we tried to identify the amino acid residue that is phosphorylated in mast cells, and to elucidate the regulatory mechanism of exocytosis by phosphorylation in syntaxin-3. We found that Thr 14 of syntaxin-3 was a phosphorylation site in mast cells. In addition, the overexpression of a constitutively dephosphorylated syntaxin-3 (T14A) mutant enhanced mast cell exocytosis. We also showed that the phosphomimetic mutation of syntaxin-3 at Thr 14 (T14E) induced structural changes in syntaxin-3, and this mutation inhibited binding of syntaxin-3 to Munc18-2. These results suggest that phosphorylated syntaxin-3 at Thr 14 negatively regulates mast cell exocytosis by impairing the interaction between syntaxin-3 and Munc18-2.

Differential Effects of Munc18s on Multiple Degranulation-Relevant Trans-SNARE Complexes

Mast cell exocytosis, which includes compound degranulation and vesicle-associated piecemeal degranulation, requires multiple Q- and R- SNAREs. It is not clear how these SNAREs pair to form functional trans-SNARE complexes and how these trans-SNARE complexes are selectively regulated for fusion. Here we undertake a comprehensive examination of the capacity of two Q-SNARE subcomplexes (syntaxin3/SNAP-23 and syntaxin4/SNAP-23) to form fusogenic trans-SNARE complexes with each of the four granule-borne R-SNAREs (VAMP2, 3, 7, 8). We report the identification of at least six distinct trans-SNARE complexes under enhanced tethering conditions: i) VAMP2/syntaxin3/SNAP-23, ii) VAMP2/syntaxin4/SNAP-23, iii) VAMP3/syntaxin3/SNAP-23, iv) VAMP3/syntaxin4/SNAP-23, v) VAMP8/syntaxin3/SNAP-23, and vi) VAMP8/syntaxin4/SNAP-23. We show for the first time that Munc18a operates synergistically with SNAP-23-based non-neuronal SNARE complexes (i to iv) in lipid mixing, in contrast to Munc18b and c, which exhibit no positive effect on any SNARE combination tested. Pre-incubation with Munc18a renders the SNARE-dependent fusion reactions insensitive to the otherwise inhibitory R-SNARE cytoplasmic domains, suggesting a protective role of Munc18a for its cognate SNAREs. Our findings substantiate the recently discovered but unexpected requirement for Munc18a in mast cell exocytosis, and implicate post-translational modifications in Munc18b/c activation.

Effects of PIP2 on membrane fusion between mast cell SNARE liposomes mediated by synaptotagmin 2

Recent studies have revealed that SNARE proteins are involved in exocytotic release in mast cells. Previously, we reported that mast cell SNARE proteins induce membrane fusion between liposomes. Moreover, we found that synaptotagmin 2, a candidate Ca2+ sensor for mast cell exocytosis, enhanced SNARE-mediated membrane fusion via Ca2+ and phosphatidylserine. Phosphatidylinositol 4,5-bisphosphate (PIP2) is an acidic phospholipid like phosphatidylserine. In the present study, we investigated whether PIP2 is involved in the enhancement effect of synaptotagmin 2 on SNARE-mediated membrane fusion. PIP2 did not show any significant effect on SNARE-mediated membrane fusion by itself. In the presence of Ca2+, synaptotagmin 2 enhanced SNARE-mediated membrane fusion between liposomes containing PIP2. However, even in the presence of Ca2+, when we used 100% PC liposomes, synaptotagmin 2 did not show any significant effect on SNARE-mediated membrane fusion. These results indicated that PIP2 is involved in the enhancement effect of synaptotagmin 2 on membrane fusion between liposomes containing mast cell SNARE proteins.

Role of SNARE proteins in tumourigenesis and their potential as targets for novel anti-cancer therapeutics

The function of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) in cellular trafficking, membrane fusion and vesicle release in synaptic nerve terminals is well characterised. Recent studies suggest that SNAREs are also important in the control of tumourigenesis through the regulation of multiple signalling and transportation pathways. The majority of published studies investigated the effects of knockdown/knockout or overexpression of particular SNAREs on the normal function of cells as well as their dysfunction in tumourigenesis promotion. SNAREs are involved in the regulation of cancer cell invasion, chemo-resistance, the transportation of autocrine and paracrine factors, autophagy, apoptosis and the phosphorylation of kinases essential for cancer cell biogenesis. This evidence highlights SNAREs as potential targets for novel cancer therapy. This is the first review to summarise the expression and role of SNAREs in cancer biology at the cellular level, their interaction with non-SNARE proteins and modulation of cellular signalling cascades. Finally, a strategy is proposed for developing novel anti-cancer therapeutics using targeted delivery of a SNARE-inactivating protease into malignant cells.

Escherichia coli exposure inhibits exocytic SNARE-mediated membrane fusion in mast cells

Mast cells orchestrate the allergic response through the release of proinflammatory mediators, which is driven by the fusion of cytoplasmic secretory granules with the plasma membrane. During this process, SNARE proteins including Syntaxin4, SNAP23 and VAMP8 play a key role. Following stimulation, the kinase IKKβ interacts with and phosphorylates the t-SNARE SNAP23. Phosphorylated SNAP23 then associates with Syntaxin4 and the v-SNARE VAMP8 to form a ternary SNARE complex, which drives membrane fusion and mediator release. Interestingly, mast cell degranulation is impaired following exposure to bacteria such as Escherichia coli. However, the molecular mechanism(s) by which this occurs is unknown. Here, we show that E. coli exposure rapidly and additively inhibits degranulation in the RBL-2H3 rat mast cell line. Following co-culture with E. coli, the interaction between IKKβ and SNAP23 is disrupted, resulting in the hypophosphorylation of SNAP23. Subsequent formation of the ternary SNARE complex between SNAP23, Syntaxin4 and VAMP8 is strongly reduced. Collectively, these results demonstrate that E. coli exposure inhibits the formation of VAMP8-containing exocytic SNARE complexes and thus the release of VAMP8-dependent granules by interfering with SNAP23 phosphorylation.

Polyphenols differentially inhibit degranulation of distinct subsets of vesicles in mast cells by specific interaction with granule-type-dependent SNARE complexes

Anti-allergic effects of dietary polyphenols were extensively studied in numerous allergic disease models, but the molecular mechanisms of anti-allergic effects by polyphenols remain poorly understood. In the present study, we show that the release of granular cargo molecules, contained in distinct subsets of granules of mast cells, is specifically mediated by two sets of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, and that various polyphenols differentially inhibit the formation of those SNARE complexes. Expression analysis of RBL-2H3 cells for 11 SNARE genes and a lipid mixing assay of 24 possible combinations of reconstituted SNAREs indicated that the only two active SNARE complexes involved in mast cell degranulation are Syn (syntaxin) 4/SNAP (23 kDa synaptosome-associated protein)-23/VAMP (vesicle-associated membrane protein) 2 and Syn4/SNAP-23/VAMP8. Various polyphenols selectively or commonly interfered with ternary complex formation of these two SNARE complexes, thereby stopping membrane fusion between granules and plasma membrane. This led to the differential effect of polyphenols on degranulation of three distinct subsets of granules. These results suggest the possibility that formation of a variety of SNARE complexes in numerous cell types is controlled by polyphenols which, in turn, might regulate corresponding membrane trafficking.

Syntaxin 11 is required for NK and CD8⁺ T-cell cytotoxicity and neutrophil degranulation

Syntaxin 11 (STX11) controls vesicular trafficking and is a key player in exocytosis. Since Stx11 mutations are causally associated with a familial hemophagocytic lymphohistio-cytosis, we wanted to clarify whether STX11 is functionally important for key immune cell populations. This was studied in primary cells obtained from newly generated Stx11(-/-) mice. Our data revealed that STX11 is not only widely expressed in different immune cells, but also induced upon LPS or IFN-γ treatment. However, Stx11 deficiency does not affect macrophage phagocytic function and cytokine secretion, mast cell activation, or antigen presentation by DCs. Instead, STX11 selectively controls lymphocyte cytotoxicity in NK and activated CD8(+) T cells and degranulation in neutrophils. Stx11(-/-) NK cells and CTLs show impaired degranulation, despite a comparable activation, maturation and expression of the complex-forming partners MUNC18-2 and VTI1B. In addition, Stx11(-/-) CTLs and NK cells produce abnormal levels of IFN-γ. Since functional reconstitution rescues the defective phenotype of Stx11(-/-) CTLs, we suggest a direct, specific and key role of STX11 in controlling lymphocyte cytotoxicity, cytokine production and secretion. Finally, we show that these mice are a very useful tool for dissecting the role of STX11 in vesicular trafficking and secretion.

SNARE complex-mediated degranulation in mast cells

Mast cell function and dysregulation is important in the development and progression of allergic and autoimmune disease. Identifying novel proteins involved in mast cell function and disease progression is the first step in the design of new therapeutic strategies. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of proteins demonstrated to mediate the transport and fusion of secretory vesicles to the membrane in mast cells, leading to the subsequent release of the vesicle cargo through an exocytotic mechanism. The functional role[s] of specific SNARE family member complexes in mast cell degranulation has not been fully elucidated. Here, we review recent and historical data on the expression, formation and localization of various SNARE proteins and their complexes in murine and human mast cells. We summarize the functional data identifying the key SNARE family members that appear to participate in mast cell degranulation. Furthermore, we discuss the utilization of RNA interference (RNAi) methods to validate SNARE function and the use of siRNA as a therapeutic approach to the treatment of inflammatory disease. These studies provide an overview of the specific SNARE proteins and complexes that serve as novel targets for the development of new therapies to treat allergic and autoimmune disease.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Dual role of VAMP8 in regulating insulin exocytosis and islet β cell growth

Optimal insulin secretion required to maintain glucose homeostasis is the summation of total pancreatic islet β cell mass and intrinsic secretory capacity of individual β cells, which are regulated by distinct mechanisms that could be amplified by glucagon-like-peptide-1 (GLP-1). Because of these actions of GLP-1 on islet β cells, GLP-1 has been deployed to treat diabetes. We employed SNARE protein VAMP8-null mice to demonstrate that VAMP8 mediates insulin granule recruitment to the plasma membrane, which partly accounts for GLP-1 potentiation of glucose-stimulated insulin secretion. VAMP8-null mice also exhibited increased islet β cell mass from increased β cell mitosis, with β cell proliferative activity greatly amplified by GLP-1. Thus, despite the β cell exocytotic defect, VAMP8-null mice have an increased total insulin secretory capacity, which improved glucose homeostasis. We conclude that these VAMP8-mediated events partly underlie the therapeutic actions of GLP-1 on insulin secretion and β cell growth.

The SNARE Machinery in Mast Cell Secretion

Mast cells are known as inflammatory cells which exert their functions in allergic and anaphylactic reactions by secretion of numerous inflammatory mediators. During an allergic response, the high-affinity IgE receptor, FcεRI, becomes cross-linked by receptor-bound IgE and antigen resulting in immediate release of pre-synthesized mediators – stored in granules – as well as in de novo synthesis of various mediators like cytokines and chemokines. Soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNARE) proteins were found to play a central role in regulating membrane fusion events during exocytosis. In addition, several accessory regulators like Munc13, Munc18, Rab GTPases, secretory carrier membrane proteins, complexins, or synaptotagmins were found to be involved in membrane fusion. In this review we summarize our current knowledge about the SNARE machinery and its mechanism of action in mast cell secretion.

The impact of bacterial infection on mast cell degranulation

In developed countries, the prevalence of allergy is on the rise. Although the causes are unknown, it seems that (1) the disappearance of microbiota may play a role in the increase of allergies and (2) exposure to bacterial infections during childhood decreases the incidence of allergies. Although several cell types are involved in the development of allergy, mast cells play a major role in orchestrating inflammation. Upon activation, mast cell secretory granules fuse with the plasma membrane, resulting in the release of a number of inflammatory mediators. In addition to allergy, mast cells contribute to the innate immune response against a variety of bacteria. This is accomplished through the secretion of cytokines and other soluble mediators. Interestingly, there is growing evidence that mast cells exposed to bacteria down-regulate degranulation in response to IgE/Allergen stimulation. This inhibitory effect seems to require direct contact between bacteria and mast cells, but the intracellular mechanism by which bacterial contact suppresses allergic responses is unknown. Here, we review different aspects of mast cell physiology and discuss hypotheses as to how bacteria may influence mast cell degranulation.

Omalizumab inhibits acceleration of FcεRI-mediated responsiveness of immature human mast cells by immunoglobulin E

BACKGROUND

A large body of evidence has demonstrated that treatment with omalizumab is clinically effective for the management of moderate to severe allergic asthma, emphasizing the importance of IgE in the pathogenesis of allergic asthma. We hypothesized that IgE accelerates FcεRI-mediated responsiveness of "immature" human mast cells (MCs) and that omalizumab downregulates the acceleration.

OBJECTIVES

To examine when MC progenitors acquired the ability to degranulate following FcεRI aggregation, whether IgE accelerates the responsiveness of immature MCs following FcεRI aggregation, and whether omalizumab regulates such an acceleration.

METHODS

Gene expression was examined using a microarray and quantitative reverse transcription polymerase chain reaction. Protein expression was investigated using FACS. Histamine release was examined using an EIA.

RESULTS

The time-course analysis of the mRNA expression of MC-related genes, including FcεRI, in Kit(+) sorted cells during the differentiation and histamine experiments revealed that the expression level of FcεRI in 5 week (w)-cultured MCs was not sufficient to induce degranulation following FcεRI aggregation but that 5 w-cultured MCs were fully responsive to calcium ionophore. By addition of IgE in culture medium FcεRI expression level and FcεRI-mediated histamine release of 5 w-cultured MCs were significantly increased compared with those without addition of IgE, whereas the expression level of tryptase and number of MCs was not affected. Omalizumab significantly inhibited IgE-dependent enhancement of FcεRI expression level and FcεRI-mediated histamine release.

CONCLUSIONS

High levels of IgE in the microenvironment in vivo may upregulate the responsiveness of immature MCs to allergens. Omalizumab may inhibit the IgE-mediated responsiveness of not only mature MCs, but also immature MCs.

Evidence for lysosomal exocytosis and release of aggrecan-degrading hydrolases from hypertrophic chondrocytes, in vitro and in vivo

The abundant proteoglycan, aggrecan, is resorbed from growth plate cartilage during endochondral bone ossification, yet mice with genetically-ablated aggrecan-degrading activity have no defects in bone formation. To account for this apparent anomaly, we propose that lysosomal hydrolases degrade extracellular, hyaluronan-bound aggrecan aggregates in growth plate cartilage, and that lysosomal hydrolases are released from hypertrophic chondrocytes into growth plate cartilage via Ca²⁺-dependent lysosomal exocytosis. In this study we confirm that hypertrophic chondrocytes release hydrolases via lysosomal exocytosis in vitro and we show in vivo evidence for lysosomal exocytosis in hypertrophic chondrocytes during skeletal development. We show that lysosome-associated membrane protein 1 (LAMP1) is detected at the cell surface following in vitro treatment of epiphyseal chondrocytes with the calcium ionophore, ionomycin. Furthermore, we show that in addition to the lysosomal exocytosis markers, cathepsin D and β-hexosaminidase, ionomycin induces release of aggrecan- and hyaluronan-degrading activity from cultured epiphyseal chondrocytes. We identify VAMP-8 and VAMP7 as v-SNARE proteins with potential roles in lysosomal exocytosis in hypertrophic chondrocytes, based on their colocalisation with LAMP1 at the cell surface in secondary ossification centers in mouse tibiae. We propose that resorbing growth plate cartilage involves release of destructive hydrolases from hypertrophic chondrocytes, via lysosomal exocytosis.

Effects of synaptotagmin 2 on membrane fusion between liposomes that contain SNAREs involved in exocytosis in mast cells

Mast cells play a pivotal role in allergic responses. Antigen stimulation causes elevation of the intracellular Ca(2+) concentration, which triggers the exocytotic release of inflammatory mediators such as histamine. Recent research, including our own, has revealed that SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins such as syntaxin-3, -4, SNAP-23, and VAMP-8 are involved in exocytosis. Although exocytosis in mast cells is Ca(2+) dependent, the target molecule that interacts with Ca(2+) is not clear. Synaptotagmin is a Ca(2+) sensor and regulates exocytosis in neuronal cells. However, the role of synaptotagmin 2, a member of the synaptotagmin family, in exocytosis in mast cells remains controversial. In this study, we investigated the role of synaptotagmin 2 by a liposome-based fusion assay. SNARE proteins (SNAP-23, syntaxin-3, VAMP-8) and synaptotagmin 2 were expressed in Escherichia coli and purified as GST-tagged or His-tagged fusion proteins. These SNARE proteins were incorporated by a detergent dialysis method. Membrane fusion between liposomes was monitored by fluorescence resonance energy transfer between fluorescent-labeled phospholipids. In the presence of Ca(2+), low synaptotagmin 2 concentration inhibited membrane fusion between SNARE-containing liposomes, while high synaptotagmin 2 concentration enhanced membrane fusion. This enhancement required phosphatidylserine as a membrane component. These results suggest that synaptotagmin 2 regulates membrane fusion of SNARE-containing liposomes involved in exocytosis in mast cells, and that this regulation is dependent on synaptotagmin 2 concentration, Ca(2+), and phosphatidylserine.

Biosurfactant mannosyl-erythritol lipid inhibits secretion of inflammatory mediators from RBL-2H3 cells

BACKGROUND

Biosurfactant mannosyl-erythritol lipids (MELs) are glycolipids produced by microbes that have various biological activities. It has been reported that MELs exhibit excellent surface-activity and also various bioactivities, such as induction of cell differentiation and apoptosis. However, little is known about their action related to drug discovery or drug seeds.

METHODS

We investigated the effects of MELs on the secretion of inflammatory mediators from mast cells that play a central role in allergic responses. Mast cells secrete three kinds of inflammatory mediators and we quantified these secreted mediators by photometer or ELISA. The action mechanisms of MELs were studied by Ca(2+)-sensitive fluorescence dye and Western blotting of phosphorylated proteins.

RESULTS

MELs inhibited exocytotic release by antigen stimulation in a dose-dependent manner. We also found that MELs inhibited antigen-induced secretion of leukotriene C(4) and cytokine TNF-α (tumor necrosis factor-α). The inhibitory action of MELs on mediator secretion was mediated by inhibition of Ca(2+) increase, phosphorylation of MAP kinases and SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) that serve as a molecular machinery for exocytotic membrane fusion.

CONCLUSIONS

MELs have anti-inflammatory action inhibiting the secretion of inflammatory mediators from mast cells.

GENERAL SIGNIFICANCE

MELs affects two of major intracellular signaling pathways including Ca(2+) increase and MAP kinases. MELs also inhibited the phosphorylation of SNARE proteins that is crucial for not only exocytosis but also intracellular vesicular trafficking.

Vesicle-associated membrane protein 8 (VAMP8) is a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) selectively required for sequential granule-to-granule fusion

Compound exocytosis is found in many cell types and is the major form of regulated secretion in acinar and mast cells. Its key characteristic is the homotypic fusion of secretory granules. These then secrete their combined output through a single fusion pore to the outside. The control of compound exocytosis remains poorly understood. Although soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) such as syntaxin 2, SNAP23 (synaptosome-associated protein of 23 kDa), and SNAP25 have been suggested to play a role, none has been proven. Vesicle-associated membrane protein 8 (VAMP8) is a SNARE first associated with endocytic processes but more recently has been suggested as an R-SNARE in regulated exocytosis. Secretion in acinar cells is reduced when VAMP8 function is inhibited and is less in VAMP8 knock-out mice. Based on electron microscopy experiments, it was suggested that VAMP8 may be involved in compound exocytosis. Here we have tested the hypothesis that VAMP8 controls homotypic granule-to-granule fusion during sequential compound exocytosis. We use a new assay to distinguish primary fusion events (fusion with the cell membrane) from secondary fusion events (granule-granule fusion). Our data show the pancreatic acinar cells from VAMP8 knock-out animals have a specific reduction in secondary granule fusion but that primary granule fusion is unaffected. Furthermore, immunoprecipitation experiments show syntaxin 2 association with VAMP2, whereas syntaxin 3 associates with VAMP8. Taken together our data indicate that granule-to-granule fusion is regulated by VAMP8 containing SNARE complexes distinct from those that regulate primary granule fusion.

Small-interfering RNA-mediated identification and regulation of the ternary SNARE complex mediating RBL-2H3 mast cell degranulation

Dysregulation of mast cell function contributes to allergic and autoimmune disease that affects more than 70 million persons in the United States alone. Identifying novel mast cell targets that mediate disease or disease progression is required for the development of innovative therapeutics for the treatment of allergy/asthma and autoimmune disease. RNA interference technologies offer hope both as basic research tools for target identification and as potential, novel, specific therapeutics. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of evolutionarily conserved proteins that have been postulated to mediate the transport and fusion of inflammatory mediator-laden vesicles to the membrane in mast cells leading to their subsequent exocytosis. The functional role(s) of specific SNARE family member complexes in mast cell degranulation has not been fully elucidated. Here, we characterize the functional importance of SNARE complexes in FcεRI receptor-mediated degranulation of RBL-2H3 cells utilizing RNA interference. We demonstrate that ternary SNARE complexes of synaptosomal-associated protein-23, Syntaxin 4 and vesicle-associated membrane protein-7 (VAMP-7) or VAMP-8 are directly involved in mast cell degranulation. Additionally, we evaluate the siRNAs directed against these molecules as potential therapeutic agents for disease intervention. These studies have identified specific SNARE proteins and complexes that serve as novel targets for the development of siRNA therapies to treat allergic and autoimmune disease.

The mechanisms of exocytosis in mast cells

Upon activation through high affinity IgE receptors (FcεRI), mast cells (MCs) can release up to 100% of their content of preformed mediators stored in cytoplasmic secretory granules by compound exocytosis. This causes Type I immediate hypersensitivity reactions and, in the case of inappropriate activation by allergens, the symptoms of allergy. Recent work has uncovered a central role of SNARE (Soluble N-ethylmaleimide-Sensitive Factor (NSF) Attachment Protein (SNAP) Receptors) proteins in regulating the numerous membrane fusion events during exocytosis. This has defined a series of new molecular actors in MC exocytosis that participate in the regulation of membrane fusion and the connection of the fusion machinery with early signaling events. The purpose of this chapter is to describe these proteins and provide a brief overview on their mechanism of action.

VAMP3 is associated with endothelial weibel-palade bodies and participates in their Ca(2+)-dependent exocytosis

Weibel-Palade bodies (WPBs) are secretory organelles of endothelial cells that store the thrombogenic glycoprotein von Willebrand factor (vWF). Endothelial activation, e.g. by histamine and thrombin, triggers the Ca(2+)-dependent exocytosis of WPB that releases vWF into the vasculature and thereby initiates platelet capture and thrombus formation. Towards understanding the molecular mechanisms underlying this regulated WPB exocytosis, we here identify components of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) machinery associated with WPB. We show that vesicle-associated membrane protein (VAMP) 3 and VAMP8 are present on WPB and that VAMP3, but not VAMP8 forms a stable complex with syntaxin 4 and SNAP23, two plasma membrane-associated SNAREs in endothelial cells. By introducing mutant SNARE proteins into permeabilized endothelial cells we also show that soluble VAMP3 but not VAMP8 mutants comprising the cytoplasmic domain interfere with efficient vWF secretion. This indicates that endothelial cells specifically select VAMP 3 over VAMP8 to cooperate with syntaxin 4 and SNAP23 in the Ca(2+)-triggered fusion of WPB with the plasma membrane. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Stimulus-secretion coupling by high-affinity IgE receptor: new developments

Stimulation of mast cells through their high affinity IgE receptor (FcεRI) leads to the secretion of pre- and neoformed soluble mediators through vesicular carriers. This process is highly regulated in order to adapt the secretion of these potentially dangerous factors to the physiological needs. This regulation requires numerous essential effectors that are necessary to transmit the initial signal of FcεRI aggregation and couple it to the sophisticated secretory machinery of membrane fusion. Studies in recent years have led to the discovery of a series of new effector molecules that link FcεRI to secretion. We describe here some of the new developments that have allowed to obtain a clearer picture of stimulus/secretion coupling in mast cells.

Protein trafficking in immune cells

The majority of cells of the immune system are specialized secretory cells, whose function depends on regulated exocytosis. The latter is mediated by vesicular transport involving the sorting of specialized cargo into the secretory granules (SGs), thereby generating the transport vesicles; their transport along the microtubules and eventually their signal-dependent fusion with the plasma membrane. Each of these steps is tightly controlled by mechanisms, which involve the participation of specific sorting signals on the cargo proteins and their recognition by cognate adaptor proteins, posttranslational modifications of the cargo proteins and multiple GTPases and SNARE proteins. In some of the cells (i.e. mast cells, T killer cells) an intimate connection exists between the secretory system and the endocytic one, whereby the SGs are lysosome related organelles (LROs) also referred to as secretory lysosomes. Herein, we discuss these mechanisms in health and disease states.

The exocytosis of lytic granules is impaired in Vti1b- or Vamp8-deficient CTL leading to a reduced cytotoxic activity following antigen-specific activation

The exocytosis of cytotoxic proteins stored in lytic granules of activated CTL is a key event during killing of target cells. Membrane fusion events that are mediated by soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) proteins are crucial, as demonstrated by patients with familial hemophagocytic lymphohistocytosis type 4 who have mutations in the SNARE protein syntaxin-11 that result in an impaired degranulation of cytotoxic cells. We found an increased mRNA expression of the SNARE protein genes Vti1b and Vamp8 during Ag-specific activation of CTL from TCR-transgenic OT-I mice. Therefore, we investigated the cytolytic activity of CTL from TCR-transgenic Vti1b and Vamp8 knockout mice. At 3 d as well as at 4 d of Ag-specific stimulation, the degranulation of CTL was significantly reduced in Vti1b and Vamp8 knockout mice, as determined by cell surface expression of the degranulation marker CD107a. After 3 d of Ag-specific stimulation, the cytolytic activity of Vti1b- and Vamp8-deficient CTL was reduced to approximately 50% compared with heterozygous controls. However, 4 d after stimulation, the cytotoxic activity of Vti1b- as well as Vamp8-deficient CTL was not impaired anymore. The capacity of Vti1b- and Vamp8-deficient dendritic cells to process Ags and to stimulate the proliferation of CTL was not reduced, arguing against an indirect effect on the activation of CTL. These findings suggest a role of the SNARE proteins vti1b and vesicle-associated membrane protein 8 in the degranulation of CTL. However, a deficiency can apparently be compensated and affects only transiently the cytotoxic activity of CTL during their development to armed effector cells.

A role for VAMP8/endobrevin in surface deployment of the water channel aquaporin 2

Vesicle-associated-membrane protein 8 (VAMP8) is highly expressed in the kidney, but the exact physiological and molecular functions executed by this v-SNARE protein in nephrons remain elusive. Here, we show that the depletion of VAMP8 in mice resulted in hydronephrosis. Furthermore, the level of the vasopressin-responsive water channel aquaporin 2 (AQP2) was increased by three- to fivefold in VAMP8-null mice. Forskolin and [desamino-Cys(1), D-Arg(8)]-vasopressin (DDAVP)-induced AQP2 exocytosis was impaired in VAMP8-null collecting duct cells. VAMP8 was revealed to colocalize with AQP2 on intracellular vesicles and to interact with the plasma membrane t-SNARE proteins syntaxin4 and syntaxin3, suggesting that VAMP8 mediates the regulated fusion of AQP2-positive vesicles with the plasma membrane.

Multiple roles of the vesicular-SNARE TI-VAMP in post-Golgi and endosomal trafficking

SNARE (Soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are the core machinery of membrane fusion. Vesicular SNAREs (v-SNAREs) interact with their target SNAREs (t-SNAREs) to form SNARE complexes which mediate membrane fusion. Here we review the basic properties and functions of the v-SNARE TI-VAMP/VAMP7 (Tetanus neurotoxin insensitive-vesicle associated membrane protein). TI-VAMP interacts with its t-SNARE partners, particularly plasmalemmal syntaxins, to mediate membrane fusion and with several regulatory proteins especially via its amino-terminal regulatory Longin domain. Partners include AP-3, Hrb/(Human immunodeficiency virus Rev binding) protein, and Varp (Vps9 domain and ankyrin repeats containing protein) and regulate TI-VAMP's function and targeting. TI-VAMP is involved both in secretory and endocytic pathways which mediate neurite outgrowth and synaptic transmission, plasma membrane remodeling and lysosomal secretion.

Role of VAMP8/endobrevin in constitutive exocytotic pathway in HeLa cells

To evaluate the role of VAMP8/endobrevin in constitutive exocytosis, we have examined the exocytotic pathways of VAMP8 and human growth hormone, both GFP-tagged, by total internal reflection fluorescence microscopy (TIRF-M). Human GH-GFP and VAMP8-GFP were similarly expressed in small round vesicles and elongated tubular vesicles in HeLa cells, and were mostly exocytosed at the peripheral area of the cells. VAMP8-GFP gave 2 types of exocytotic images: a burst type and a non-burst type. The burst type showed a sharp transient increase in the peak fluorescence intensity and a much slower decrease in the average intensity in the active windows, where exocytosis took place, as observed in the "full-fusion" type of exocytosis. The non-burst type showed a relatively long-lasting fusion to the plasma membrane with little transfer of VAMP8-GFP to the plasma membrane, as observed in the so-called "kiss-and-run" type of exocytosis. Endogenous VAMP8 and hGH-GFP were colocalized on the same vesicles at least in part. However, the constitutive exocytosis of hGH-GFP and CLuc, a secreted luciferase from Cypridina noctiluca, was normal, even when siRNAs for VAMP8 and VAMP3 robustly decreased their proteins. These results suggest that VAMP8 is not essential for constitutive exocytosis, although it can be involved in the exocytosis.

Transfection with liver-type glutaminase cDNA alters gene expression and reduces survival, migration and proliferation of T98G glioma cells

Liver-type glutaminase (LGA) is a glutaminase isoform that has been implicated in transcription modulation. LGA mRNA is absent from postoperative samples of primary gliomas and is low in cultured astrocytes. In this study, stable transfection of T98G cells with a vector carrying human LGA sequence increased the expression of LGA mRNA and protein, and the ability of the cells to degrade glutamine (Gln), as manifested by a three-fold reduction of their steady-state Gln content and a 2.5-fold increase of their glutamate (Glu) content. The transfected cells (TLGA cells) showed a 40% decrease of cell survival as assessed by colony formation, well correlated with significant reduction of mitochondrial activity as demonstrated with MTT test. Also, a 45% reduction of cell migration and a 47% decrease of proliferation index (Ki67 immunostaining) were found as compared with sham-transfected cells. Microarray analysis, which included over 47,000 transcripts, revealed a significantly altered expression of 85 genes in TLGA, but not in sham-transfected or control cells (P < 0.005). Microarray data were confirmed with real-time PCR analysis for eight genes potentially relevant to malignancy: S100A16, CAPN2, FNDC3B, DYNC1LI1, TIMP4, MGMT, ADM, and TIMP1. Of these changes, decreased expression of S100A16 and MGMT can be best reconciled with the current views on the role of their protein products in glioma malignancy. Malignancy-reducing effect of newly inserted LGA mRNA in glioblastoma cells can be reconciled with a hypothesis that absence of such a modulatory mechanism in glia-derived tumors deprived of LGA mRNA may facilitate some aspects of their progression.

Stable isotope labeling by amino acids in cell culture and differential plasma membrane proteome quantitation identify new substrates for the MARCH9 transmembrane E3 ligase

The regulation of cell surface receptor expression is essential for immune cell differentiation and function. At the plasma membrane ubiquitination is an important post-translational mechanism for regulating expression of a wide range of surface proteins. MARCH9, a member of the RING-CH family of transmembrane E3 ubiquitin ligases, down-regulates CD4, major histocompatibility complex-I (MHC), and ICAM-1 in lymphoid cells. To identify novel MARCH9 substrates, we used high throughput flow cytometry and quantitative mass spectrometry by stable isotope labeling by amino acids in cell culture (SILAC) to determine the differential expression of plasma membrane proteins in a MARCH9-expressing B cell line. This combined approach identified 13 potential new MARCH9 targets. All of the SILAC-identified targets for which antibodies were available were subsequently confirmed by flow cytometry, validating the proteomics results. A close correlation (r(2) = 0.93) between -fold down-regulation as determined by SILAC and flow cytometry was found, with no false positive hits detected. The potential new MARCH9 substrates cover a wide range of functions and include receptor-type protein-tyrosine phosphatases (e.g. PTPRJ/CD148) as well as Fc gamma receptor IIB (CD32B), HLA-DQ, signaling lymphocytic activation molecule (CD150), and polio virus receptor (CD155). The identification of plasma membrane targets by SILAC with confirmation by flow cytometry represents a novel and powerful approach to analyze changes in the plasma membrane proteome.

Thymic alterations in mice deficient for the SNARE protein VAMP8/endobrevin

SNARE (soluble-N-ethylmaleimide-sensitive factor attachment receptor) proteins mediate the recognition and fusion of transport vesicles in eukaryotic cells. The SNARE protein VAMP8 (also called endobrevin) is involved in the fusion of late endosomes and in some pathways of regulated exocytosis. In a subset of mice deficient for the SNARE protein VAMP8, a severe alteration of the thymus and in T lymphocyte development was observed and characterized. The size of the thymus and the number of thymocytes were dramatically reduced compared with those in heterozygous littermates. Further, the compartmentalization into cortex and medulla and the organization of the thymus epithelium were disturbed. The numbers of all thymocyte subpopulations were reduced, with the CD4 and CD8 double-positive thymocytes being most severely affected. The proportion of proliferating thymocytes was reduced, and the staining of apoptotic cells in situ and ex vivo indicated an increased number of apoptotic cells. Isolated thymocytes of Vamp8^−/− mice were more susceptible to various apoptotic stimuli including glucocorticoids, FAS receptor, and CD3/CD28-mediated signaling in vitro, even before an increased number of apoptotic cells was detectable in situ. However, bone marrow of phenotypically affected Vamp8^−/− mice was readily able to repopulate immunodeficient hosts suggesting that the SNARE protein VAMP8 has a specific function in the thymic stroma affecting the proliferation and apoptosis of T lymphocytes during maturation in the thymus.

The regulated exocytosis of enlargeosomes is mediated by a SNARE machinery that includes VAMP4

The mechanisms governing the fast, regulated exocytosis of enlargeosomes have been unknown, except for the participation of annexin-2 in a pre-fusion step. We investigated whether any SNAREs are involved. In PC12-27 cells, which are enlargeosome-rich, the expressed SNAREs exhibited various distributions (trans-Golgi network, scattered puncta, plasma membrane); however, only VAMP4 was colocalized in discrete puncta with the enlargeosome marker desmoyokin. The exocytosis of the organelle, revealed by capacitance increases and by surface appearance of desmoyokin, was largely inhibited by microinjection of anti-VAMP4, anti-syntaxin-6 and anti-SNAP23 antibodies, by incubation with botulinum toxin E, and by transfection of VAMP4 and syntaxin-6 siRNAs. Microinjection of the antibodies anti-VAMP7, anti-VAMP8 and anti-syntaxin-4, and transfection with the VAMP8 siRNA were ineffective. Inhibition of enlargeosome exocytosis by VAMP4 siRNA also occurred in a cell type that was competent for neurosecretion, SH-SY5Y. Moreover, in cells expressing a VAMP4-GFP construct, enlargeosome exocytosis and surface appearance of fluorescence occurred concomitantly, and many ensuing surface patches were co-labelled by GFP and desmoyokin. VAMP4, an R-SNARE that has never been shown to participate in regulated exocytoses, therefore appears to be harboured in the membrane of enlargeosomes and to be a member of the machinery mediating their regulated exocytosis. Syntaxin-6 and SNAP23 appear also to be needed for the process to occur; however, the mechanism of their participation, whether direct or indirect, remains undefined.