SNAP-29 is a promiscuous syntaxin-binding SNARE

Transcriptome Discovery of Genes in the Three Phases of Autophagy That Are Upregulated During Atrial Fibrillation

Background: Autophagy may contribute to the maintenance of atrial fibrillation (AF), but no previous study has concurrently surveyed all 3 phases of autophagy, namely autophagosome formation, lysosome formation, and autophagosome-lysosome fusion. Here we aimed to identify disorders involving various phases of autophagy during AF. Methods and Results: We used bioinformatic techniques to analyze publicly available DNA microarray datasets from the left atrium (LA) and right atrium (RA) of 7 patients with AF and 6 patients with normal sinus rhythm who underwent valvular surgeries. We compared gene expression levels in the LA (AF-LA) and RA of patients with AF with those in the LA and RA of patients with normal sinus rhythm. Several differentially expressed genes in the AF-LA sample were significantly associated with the Gene Ontogeny term 'Autophagy', indicating that the expression of autophagic genes was specifically altered in this dataset. In particular, the expression of genes known or suspected to be involved in autophagosome formation (autophagy related 5 [ATG5], autophagy related 10 [ATG10], autophagy related 12 [ATG12], and light chain 3B [LC3B]), lysosome formation (lysosomal associated membrane protein 1 [LAMP1] and lysosomal associated membrane protein 2 [LAMP2]), and autophagosome-lysosome fusion (synaptosome associated protein 29 [SNAP29], SNAP associated protein [SNAPIN], and syntaxin 17 [STX17]) was significantly upregulated in the LA-AF dataset. Conclusions: Autophagy is activated excessively in, and may perpetuate, AF.

SNARE-binding protein synaptosomal-associated protein of 29 kDa (SNAP29) regulates the intracellular sequestration of glucose transporter 4 (GLUT4) vesicles in adipocytes

AIMS/INTRODUCTION

Insulin stimulates translocation of glucose transporter 4 (GLUT4) from the perinuclear location to the plasma membrane. In the unstimulated state, intracellular vesicles containing GLUT4 are sequestered into specialized storage vesicles that have come to be known as the insulin-responsive compartment (IRC). The IRC is a functional compartment in the perinuclear region that is a target of the insulin signaling cascade, although its precise nature is unclear. Here, we report a novel molecular mechanism facilitating formation of the IRC.

MATERIALS AND METHODS

We determined synaptosomal-associated protein of 29 kDa (SNAP29) by mass spectrometry to be an EH domain-containing protein 1 (EHD1)-binding protein. Then, its expression was confirmed by western blotting. Subcellular localization of SNAP29 was determined by immunofluorescent microscopy. Interactions between SNAP29 and syntaxins were determined by immunoprecipitation. We measured glucose uptake and GLUT4 translocation in 3T3-L1 adipocyte expressing SNAP29 or silencing SNAP29.

RESULTS

We found SNAP29 to be localized in the perinuclear region and to show partial co-localization with GLUT4 under basal conditions. We also found that SNAP29 binds to syntaxin6, a Qc-SNARE, in adipocytes. In SNAP29-expressing cells, vesicles containing GLUT4 were observed to aggregate around the perinuclear region. In contrast, when SNAP29 was silenced, perinuclear GLUT4 vesicles were dispersed throughout the cytosol. Insulin-stimulated glucose uptake was inhibited in both SNAP29-expressing and SNAP29-silenced cells.

CONCLUSIONS

These data suggest that SNAP29 sequesters and anchors GLUT4-containing vesicles in the perinuclear region, and might have a role in the biogenesis of the perinuclear IRC.

Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation

The Golgi is the central organelle of the secretory pathway and it houses the majority of the glycosylation machinery, which includes glycosylation enzymes and sugar transporters. Correct compartmentalization of the glycosylation machinery is achieved by retrograde vesicular trafficking as the secretory cargo moves forward by cisternal maturation. The vesicular trafficking machinery which includes vesicular coats, small GTPases, tethers and SNAREs, play a major role in coordinating the Golgi trafficking thereby achieving Golgi homeostasis. Glycosylation is a template-independent process, so its fidelity heavily relies on appropriate localization of the glycosylation machinery and Golgi homeostasis. Mutations in the glycosylation enzymes, sugar transporters, Golgi ion channels and several vesicle tethering factors cause congenital disorders of glycosylation (CDG) which encompass a group of multisystem disorders with varying severities. Here, we focus on the Golgi vesicle tethering and fusion machinery, namely, multisubunit tethering complexes and SNAREs and their role in Golgi trafficking and glycosylation. This review is a comprehensive summary of all the identified CDG causing mutations of the Golgi trafficking machinery in humans.

New insights regarding SNARE proteins in autophagosome-lysosome fusion

Macroautophagy/autophagy refers to the engulfment of cellular contents selected for lysosomal degradation. The final step in autophagy is the fusion of autophagosome with the lysosome, which is mediated by SNARE proteins. Of the SNAREs, autophagosome-localized Q-SNAREs, such as STX17 and SNAP29, and lysosome-localized R-SNAREs, such as VAMP8 or VAMP7, have been reported to be involved. Recent studies also reveal participation of the R-SNARE, YKT6, in autophagosome-lysosome fusion. These SNAREs, with the help of other regulatory factors, act coordinately to spatiotemporally control the fusion process. Besides regulating autophagosome-lysosome fusion, some SNAREs, such as STX17, also function in other autophagic processes, including autophagosome formation and mitophagy. A better understanding of the functions of SNAREs will shed light on the molecular mechanisms of autophagosome-lysosome fusion as well as on the mechanisms by which autophagy is globally regulated.Abbreviations: ATG: autophagy related; DNM1L: dynamin 1 like; ER: endoplasmic reticulum; GABARAP: GABA type A receptor-associated protein; GABARAPL1: GABA type A receptor associated protein like 1; IRGM: immunity related GTPase M; LAMP2: lysosomal associated membrane protein 2; MAP1LC3B/LC3: microtubule associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; PIK3R4: phosphoinositide-3-kinase regulatory subunit 4; PLEKHM1: pleckstrin homology and RUN domain containing M1; PRKN: PRKN RBR E3 ubiquitin protein ligase; RAB2A: RAB2A, member RAS oncogene family; RAB33B: RAB33B, member RAS oncogene family; RAB7A: RAB7A, member RAS oncogene family; RB1CC1: RB1 inducible coiled-coil 1; RTN3: reticulon 3; RUBCNL: rubicon like autophagy enhancer; SNARE: soluble N-ethylmaleimide-sensitive factor attachment protein receptor; SNAP29: synaptosomal associated protein 29; STX17: syntaxin 17; ULK1: unc-51 like autophagy activating kinase 1; VAMP7: vesicle associated membrane protein 7; VAMP8: vesicle associated membrane protein 8; YKT6: YKT6 v-SNARE homolog.

Generation and Characterization of a CRISPR/Cas9-Mediated SNAP29 Knockout in Human Fibroblasts

Loss-of-function mutations in the synaptosomal-associated protein 29 (SNAP29) lead to the rare autosomal recessive neurocutaneous cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma (CEDNIK) syndrome. SNAP29 is a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein. So far, it has been shown to be involved in membrane fusion, epidermal differentiation, formation of primary cilia, and autophagy. Recently, we reported the successful generation of two mouse models for the human CEDNIK syndrome. The aim of this investigation was the generation of a CRISPR/Cas9-mediated SNAP29 knockout (KO) in an immortalized human cell line to further investigate the role of SNAP29 in cellular homeostasis and signaling in humans independently of animal models. Comparison of different methods of delivery for CRISPR/Cas9 plasmids into the cell revealed that lentiviral transduction is more efficient than transfection methods. Here, we reported to the best of our knowledge the first successful generation of a CRISPR/Cas9-mediated SNAP29 KO in immortalized human MRC5Vi fibroblasts (c.169_196delinsTTCGT) via lentiviral transduction.

Activity of the SNARE Protein SNAP29 at the Endoplasmic Reticulum and Golgi Apparatus

Snap29 is a conserved regulator of membrane fusion essential to complete autophagy and to support other cellular processes, including cell division. In humans, inactivating SNAP29 mutations causes CEDNIK syndrome, a rare multi-systemic disorder characterized by congenital neuro-cutaneous alterations. The fibroblasts of CEDNIK patients show alterations of the Golgi apparatus (GA). However, whether and how Snap29 acts at the GA is unclear. Here we investigate SNAP29 function at the GA and endoplasmic reticulum (ER). As part of the elongated structures in proximity to these membrane compartments, a pool of SNAP29 forms a complex with Syntaxin18, or with Syntaxin5, which we find is required to engage SEC22B-loaded vesicles. Consistent with this, in HeLa cells, in neuroepithelial stem cells, and in vivo, decreased SNAP29 activity alters GA architecture and reduces ER to GA trafficking. Our data reveal a new regulatory function of Snap29 in promoting secretory trafficking.

SNAREs and developmental disorders

Members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family mediate membrane fusion processes associated with vesicular trafficking and autophagy. SNAREs mediate core membrane fusion processes essential for all cells, but some SNAREs serve cell/tissue type-specific exocytic/endocytic functions, and are therefore critical for various aspects of embryonic development. Mutations or variants of their encoding genes could give rise to developmental disorders, such as those affecting the nervous system and immune system in humans. Mutations to components in the canonical synaptic vesicle fusion SNARE complex (VAMP2, STX1A/B, and SNAP25) and a key regulator of SNARE complex formation MUNC18-1, produce variant phenotypes of autism, intellectual disability, movement disorders, and epilepsy. STX11 and MUNC18-2 mutations underlie 2 subtypes of familial hemophagocytic lymphohistiocytosis. STX3 mutations contribute to variant microvillus inclusion disease. Chromosomal microdeletions involving STX16 play a role in pseudohypoparathyroidism type IB associated with abnormal imprinting of the GNAS complex locus. In this short review, I discuss these and other SNARE gene mutations and variants that are known to be associated with a variety developmental disorders, with a focus on their underlying cellular and molecular pathological basis deciphered through disease modeling. Possible pathogenic potentials of other SNAREs whose variants could be disease predisposing are also speculated upon.

New Perspectives on SNARE Function in the Yeast Minimal Endomembrane System

Saccharomyces cerevisiae is one of the best model organisms for the study of endocytic membrane trafficking. While studies in mammalian cells have characterized the temporal and morphological features of the endocytic pathway, studies in budding yeast have led the way in the analysis of the endosomal trafficking machinery components and their functions. Eukaryotic endomembrane systems were thought to be highly conserved from yeast to mammals, with the fusion of plasma membrane-derived vesicles to the early or recycling endosome being a common feature. Upon endosome maturation, cargos are then sorted for reuse or degraded via the endo-lysosomal (endo-vacuolar in yeast) pathway. However, recent studies have shown that budding yeast has a minimal endomembrane system that is fundamentally different from that of mammalian cells, with plasma membrane-derived vesicles fusing directly to a trans-Golgi compartment which acts as an early endosome. Thus, the Golgi, rather than the endosome, acts as the primary acceptor of endocytic vesicles, sorting cargo to pre-vacuolar endosomes for degradation. The field must now integrate these new findings into a broader understanding of the endomembrane system across eukaryotes. This article synthesizes what we know about the machinery mediating endocytic membrane fusion with this new model for yeast endomembrane function.

Autophagy and endocytosis – interconnections and interdependencies

Autophagy and endocytosis are membrane-vesicle-based cellular pathways for degradation and recycling of intracellular and extracellular components, respectively. These pathways have a common endpoint at the lysosome, where their cargo is degraded. In addition, the two pathways intersect at different stages during vesicle formation, fusion and trafficking, and share parts of the molecular machinery. Accumulating evidence shows that autophagy is dependent upon endocytosis and vice versa. The emerging joint network of autophagy and endocytosis is of vital importance for cellular metabolism and signaling, and thus also highly relevant in disease settings. In this Review, we will discuss examples of how the autophagy machinery impacts on endocytosis and cell signaling, and highlight how endocytosis regulates the different steps in autophagy in mammalian cells. Finally, we will focus on the interplay of these pathways in the quality control of their common endpoint, the lysosome.

In the line-up: deleted genes associated with DiGeorge/22q11.2 deletion syndrome: are they all suspects?

BACKGROUND

22q11.2 deletion syndrome (22q11DS), a copy number variation (CNV) disorder, occurs in approximately 1:4000 live births due to a heterozygous microdeletion at position 11.2 (proximal) on the q arm of human chromosome 22 (hChr22) (McDonald-McGinn and Sullivan, Medicine 90:1-18, 2011). This disorder was known as DiGeorge syndrome, Velo-cardio-facial syndrome (VCFS) or conotruncal anomaly face syndrome (CTAF) based upon diagnostic cardiovascular, pharyngeal, and craniofacial anomalies (McDonald-McGinn and Sullivan, Medicine 90:1-18, 2011; Burn et al., J Med Genet 30:822-4, 1993) before this phenotypic spectrum was associated with 22q11.2 CNVs. Subsequently, 22q11.2 deletion emerged as a major genomic lesion associated with vulnerability for several clinically defined behavioral deficits common to a number of neurodevelopmental disorders (Fernandez et al., Principles of Developmental Genetics, 2015; Robin and Shprintzen, J Pediatr 147:90-6, 2005; Schneider et al., Am J Psychiatry 171:627-39, 2014).

RESULTS

The mechanistic relationships between heterozygously deleted 22q11.2 genes and 22q11DS phenotypes are still unknown. We assembled a comprehensive "line-up" of the 36 protein coding loci in the 1.5 Mb minimal critical deleted region on hChr22q11.2, plus 20 protein coding loci in the distal 1.5 Mb that defines the 3 Mb typical 22q11DS deletion. We categorized candidates based upon apparent primary cell biological functions. We analyzed 41 of these genes that encode known proteins to determine whether haploinsufficiency of any single 22q11.2 gene-a one gene to one phenotype correspondence due to heterozygous deletion restricted to that locus-versus complex multigenic interactions can account for single or multiple 22q11DS phenotypes.

CONCLUSIONS

Our 22q11.2 functional genomic assessment does not support current theories of single gene haploinsufficiency for one or all 22q11DS phenotypes. Shared molecular functions, convergence on fundamental cell biological processes, and related consequences of individual 22q11.2 genes point to a matrix of multigenic interactions due to diminished 22q11.2 gene dosage. These interactions target fundamental cellular mechanisms essential for development, maturation, or homeostasis at subsets of 22q11DS phenotypic sites.

Postsynaptic SNARE Proteins: Role in Synaptic Transmission and Plasticity

Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins mediate membrane fusion events in eukaryotic cells. Traditionally recognized as major players in regulating presynaptic neurotransmitter release, accumulative evidence over recent years has identified several SNARE proteins implicated in important postsynaptic processes such as neurotransmitter receptor trafficking and synaptic plasticity. Here we analyze the emerging data revealing this novel functional dimension for SNAREs with a focus on the molecular specialization of vesicular recycling and fusion in dendrites compared to those at axon terminals and its impact in synaptic transmission and plasticity.

Finding the Middle Ground for Autophagic Fusion Requirements

Autophagosome/amphisome-lysosome fusion is a highly regulated process at the protein, lipid, and biochemical level. Each primary component of fusion, such as the core SNAREs, HOPS complex, or physical positioning by microtubule-associated dynein motors, are regulated at multiple points to ensure optimum conditions for autophagic flux to proceed. With the complexity of the membrane fusion system, it is not difficult to imagine how autophagic flux defect-related disorders, such as Huntington's disease, non-familial Alzheimer's disease, and Vici syndrome develop. Each membrane fusion step is regulated at the protein, lipid, and ion level. This review aims to discuss the recent developments toward understanding the regulation of autophagosome, amphisome, and lysosome fusion requirements for successful autophagic flux.

A possible postsynaptic role for SNAP-25 in hippocampal synapses

The SNARE protein SNAP-25 is well documented as regulator of presynaptic vesicle exocytosis. Increasing evidence suggests roles for SNARE proteins in postsynaptic trafficking of glutamate receptors as a basic mechanism in synaptic plasticity. Despite these indications, detailed quantitative subsynaptic localization studies of SNAP-25 have never been performed. Here, we provide novel electron microscopic data of SNAP-25 localization in postsynaptic spines. In addition to its expected presynaptic localization, we show that the protein is also present in the postsynaptic density (PSD), the postsynaptic lateral membrane and on small vesicles in the postsynaptic cytoplasm. We further investigated possible changes in synaptic SNAP-25 protein expression after hippocampal long-term potentiation (LTP). Quantitative analysis of immunogold-labeled electron microscopy sections did not show statistically significant changes of SNAP-25 gold particle densities 1 h after LTP induction, indicating that local trafficking of SNAP-25 does not play a role in the early phases of LTP. However, the strong expression of SNAP-25 in postsynaptic plasma membranes suggests a function of the protein in postsynaptic vesicle exocytosis and a possible role in hippocampal synaptic plasticity.

O-GlcNAc-modified SNAP29 inhibits autophagy-mediated degradation via the disturbed SNAP29-STX17-VAMP8 complex and exacerbates myocardial injury in type I diabetic rats

The O-linked β-N-acetylglucosamine (O-GlcNAc) modification and autophagy are associated with diabetic myocardial injury, however, the molecular mechanisms between the two processes remain to be fully elucidated. The purpose of the present study was to elucidate the molecular regulation of autophagy by O-GlcNAc-modified synaptosomal-associated protein 29 (SNAP29) in diabetic myocardial injury. A rat model of type I diabetes was established via intraperitoneal injection of streptozotocin (STZ; 55 mg/kg). Significant increases in the O-GlcNAc modification and accumulation of the autophagy markers microtubule-associated protein 1 light chain 3α II/I and P62, which suggest that autophagic flux is inhibited, were observed in rats 8 weeks following STZ induction. Subsequently, the selective O-GlcNAcase inhibitor, thiamet G, increased the level of O-GlcNAc modification, which further disrupted autophagic flux; deteriorated cardiac diastolic function, as indicated by an increased left ventricular filling peak velocity/atrial contraction flow peak velocity ratio shown by echocardiography; and exacerbated myocardial abnormalities, as characterized by cardiomyocyte disorganization and fat and interstitial fibrosis accumulation. By contrast, 6-diazo-5-oxo-L-norleucine, an inhibitor of glucosamine fructose-6-phosphate aminotransferase isomerizing 1, acted as an O-GlcNAc antagonist and reduced the level of O-GlcNAc modification, which maintained autophagic flux and improved cardiac diastolic function. In vitro, high glucose (25 mM) was used to stimulate primary neonatal rat cardiomyocytes (NRCMs). Consistent with the myocardium of diabetic rats, it was also shown in the NRCMs that O-GlcNAc modification of SNAP29 negatively regulated autophagic flux. The application of the short hairpin RNA interference lysosome-associated membrane protein (LAMP2) and the autophagy inhibitor 3-methyladenine demonstrated that high glucose inhibited autophagy-mediated degradation rather than affected the initial stage of autophagy. Finally, co-immunoprecipitation was used to determine the role of the O-GlcNAc-modified substrate protein SNAP29, which acted as an SNAP29-syntaxin-17 (STX17)-vesicle-associated membrane protein 8 (VAMP8) complex during disease progression. The present study is the first, to the best of our knowledge, to demonstrate that SNAP29 is an O-GlcNAc substrate and that an increase in O-GlcNAc-modified SNAP29 inhibits SNAP29-STX17-VAMP8 complex formation, thereby inhibiting the degradation of autophagy and exacerbating myocardial injury in type I diabetic rats.

Exposure of Rat Neural Stem Cells to Ethanol Affects Cell Numbers and Alters Expression of 28 Proteins

Developing brain cells express many proteins but little is known of how their protein composition responds to chronic exposure to alcohol and/or how such changes might relate to alcohol toxicity. We used cultures derived from embryonic rat brain (previously shown to contain mostly neural stem cells; rat NSC, rNSC), exposed them to ethanol (25-100 mM) for up to 96 h and studied how they reacted. Ethanol (50 and 100 mM) reduced cell numbers indicating either compromised cell proliferation, cytotoxicity or both. Increased lipid peroxidation was consistent with the presence of oxidative stress accompanying alcohol-induced cytotoxicity. Proteomics revealed 28 proteins as altered by ethanol (50 mM for 96 h). Some were constituents of cytoskeleton, others were involved in transcription/translation, signal transduction and oxidative stress. Nucleophosmin (NPM1) and dead-end protein homolog 1 (DND1) were further studied by immunological techniques in cultured neurons and astrocytes (derived from brain tissue at embryonic ages E15 and E20, respectively). In the case of DND1 (but not NPM1) ethanol induced similar pattern of changes in both types of cells. Given the critical role of the protein NPM1 in cell proliferation and differentiation, its reduced expression in the ethanol-exposed rNSC could, in part, explain the lower cells numbers. We conclude that chronic ethanol profoundly alters protein composition of rNSC to the extent that their functioning-including proliferation and survival-would be seriously compromised. Translated to humans, such changes could point the way towards mechanisms underlying the fetal alcohol spectrum disorder and/or alcoholism later in life.

How to use a multipurpose SNARE: The emerging role of Snap29 in cellular health

Despite extensive study, regulation of membrane trafficking is incompletely understood. In particular, the specific role of SNARE (Soluble NSF Attachment REceptor) proteins for distinct trafficking steps and their mechanism of action, beyond the core function in membrane fusion, are still elusive. Snap29 is a SNARE protein related to Snap25 that gathered a lot of attention in recent years. Here, we review the study of Snap29 and its emerging involvement in autophagy, a self eating process that is key to cell adaptation to changing environments, and in other trafficking pathways. We also discuss Snap29 role in synaptic transmission and in cell division, which might extend the repertoire of SNARE-mediated functions. Finally, we present evidence connecting Snap29 to human disease, highlighting the importance of Snap29 function in tissue development and homeostasis.

Distinct Localization of SNAP47 Protein in GABAergic and Glutamatergic Neurons in the Mouse and the Rat Hippocampus

Synaptosomal-associated protein of 47 kDa (SNAP47) isoform is an atypical member of the SNAP family, which does not contribute directly to exocytosis and synaptic vesicle (SV) recycling. Initial characterization of SNAP47 revealed a widespread expression in nervous tissue, but little is known about its cellular and subcellular localization in hippocampal neurons. Therefore, in the present study we applied multiple-immunofluorescence labeling, immuno-electron microscopy and in situ hybridization (ISH) and analyzed the localization of SNAP47 in pre- and postsynaptic compartments of glutamatergic and GABAergic neurons in the mouse and rat hippocampus. While the immunofluorescence signal for SNAP47 showed a widespread distribution in both mouse and rat, the labeling pattern was complementary in the two species: in the mouse the immunolabeling was higher over the CA3 stratum radiatum, oriens and cell body layer. In contrast, in the rat the labeling was stronger over the CA1 neuropil and in the CA3 stratum lucidum. Furthermore, in the mouse high somatic labeling for SNAP47 was observed in GABAergic interneurons (INs). On the contrary, in the rat, while most INs were positive, they blended in with the high neuropil labeling. ISH confirmed the high expression of SNAP47 RNA in INs in the mouse. Co-staining for SNAP47 and pre- and postsynaptic markers in the rat revealed a strong co-localization postsynaptically with PSD95 in dendritic spines of pyramidal cells and, to a lesser extent, presynaptically, with ZnT3 and vesicular glutamate transporter 1 (VGLUT1) in glutamatergic terminals such as mossy fiber (MF) boutons. Ultrastructural analysis confirmed the pre- and postsynaptic localization at glutamatergic synapses. Furthermore, in the mouse hippocampus SNAP47 was found to be localized at low levels to dendritic shafts and axon terminals of putative INs forming symmetric synapses, indicating that this protein could be trafficked to both post- and presynaptic sites in both major cell types. These results reveal divergent localization of SNAP47 protein in mouse and rat hippocampus indicating species- and cell type-specific differences. SNAP47 is likely to be involved in unique fusion machinery which is distinct from the one involved in presynaptic neurotransmitter release. Nonetheless, our data suggest that SNAP47 may be involved not only postsynaptic, but also in presynaptic function.

Dynamics of the mouse brain cortical synaptic proteome during postnatal brain development

Development of the brain involves the formation and maturation of numerous synapses. This process requires prominent changes of the synaptic proteome and potentially involves thousands of different proteins at every synapse. To date the proteome analysis of synapse development has been studied sparsely. Here, we analyzed the cortical synaptic membrane proteome of juvenile postnatal days 9 (P9), P15, P21, P27, adolescent (P35) and different adult ages P70, P140 and P280 of C57Bl6/J mice. Using a quantitative proteomics workflow we quantified 1560 proteins of which 696 showed statistically significant differences over time. Synaptic proteins generally showed increased levels during maturation, whereas proteins involved in protein synthesis generally decreased in abundance. In several cases, proteins from a single functional molecular entity, e.g., subunits of the NMDA receptor, showed differences in their temporal regulation, which may reflect specific synaptic development features of connectivity, strength and plasticity. SNARE proteins, Snap 29/47 and Stx 7/8/12, showed higher expression in immature animals. Finally, we evaluated the function of Cxadr that showed high expression levels at P9 and a fast decline in expression during neuronal development. Knock down of the expression of Cxadr in cultured primary mouse neurons revealed a significant decrease in synapse density.

An essential step of kinetochore formation controlled by the SNARE protein Snap29

The kinetochore is an essential structure that mediates accurate chromosome segregation in mitosis and meiosis. While many of the kinetochore components have been identified, the mechanisms of kinetochore assembly remain elusive. Here, we identify a novel role for Snap29, an unconventional SNARE, in promoting kinetochore assembly during mitosis in Drosophila and human cells. Snap29 localizes to the outer kinetochore and prevents chromosome mis‐segregation and the formation of cells with fragmented nuclei. Snap29 promotes accurate chromosome segregation by mediating the recruitment of Knl1 at the kinetochore and ensuring stable microtubule attachments. Correct Knl1 localization to kinetochore requires human or Drosophila Snap29, and is prevented by a Snap29 point mutant that blocks Snap29 release from SNARE fusion complexes. Such mutant causes ectopic Knl1 recruitment to trafficking compartments. We propose that part of the outer kinetochore is functionally similar to membrane fusion interfaces.

Direct targeting of membrane fusion by SNARE mimicry: Convergent evolution of Legionella effectors

Legionella pneumophila, the Gram-negative pathogen causing Legionnaires' disease, infects host cells by hijacking endocytic pathways and forming a Legionella-containing vacuole (LCV) in which the bacteria replicate. To promote LCV expansion and prevent lysosomal targeting, effector proteins are translocated into the host cell where they alter membrane traffic. Here we show that three of these effectors [LegC2 (Legionella eukaryotic-like gene C2)/YlfB (yeast lethal factor B), LegC3, and LegC7/YlfA] functionally mimic glutamine (Q)-SNARE proteins. In infected cells, the three proteins selectively form complexes with the endosomal arginine (R)-SNARE vesicle-associated membrane protein 4 (VAMP4). When reconstituted in proteoliposomes, these proteins avidly fuse with liposomes containing VAMP4, resulting in a stable complex with properties resembling canonical SNARE complexes. Intriguingly, however, the LegC/SNARE hybrid complex cannot be disassembled by N-ethylmaleimide-sensitive factor. We conclude that LegCs use SNARE mimicry to divert VAMP4-containing vesicles for fusion with the LCV, thus promoting its expansion. In addition, the LegC/VAMP4 complex avoids the host's disassembly machinery, thus effectively trapping VAMP4 in an inactive state.

Molecular Functions of Glycoconjugates in Autophagy

Glycoconjugates, glycans, carbohydrates, and sugars: these terms encompass a class of biomolecules that are diverse in both form and function ranging from free oligosaccharides, glycoproteins, and proteoglycans, to glycolipids that make up a complex glycan code that impacts normal physiology and disease. Recent data suggest that one mechanism by which glycoconjugates impact physiology is through the regulation of the process of autophagy. Autophagy is a degradative pathway necessary for differentiation, organism development, and the maintenance of cell and tissue homeostasis. In this review, we will highlight what is known about the regulation of autophagy by glycoconjugates focusing on signaling mechanisms from the extracellular surface and the regulatory roles of intracellular glycans. Glycan signaling from the extracellular matrix converges on "master" regulators of autophagy including AMPK and mTORC1, thus impacting their localization, activity, and/or expression. Within the intracellular milieu, gangliosides are constituents of the autophagosome membrane, a subset of proteins composing the autophagic machinery are regulated by glycosylation, and oligosaccharide exposure in the cytosol triggers an autophagic response. The examples discussed provide some mechanistic insights into glycan regulation of autophagy and reveal areas for future investigation.

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.

Establishment of Two Mouse Models for CEDNIK Syndrome Reveals the Pivotal Role of SNAP29 in Epidermal Differentiation

Loss-of-function mutations in the synaptosomal-associated protein 29 (SNAP29) gene cause the cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma syndrome. In this study, we created total (Snap29(-/-)) as well as keratinocyte-specific (Snap29(fl/fl)/K14-Cre) Snap29 knockout mice. Both mutant mice exhibited a congenital distinct ichthyotic phenotype resulting in neonatal lethality. Mutant mice revealed acanthosis and hyperkeratosis as well as abnormal keratinocyte differentiation and increased proliferation. In addition, the epidermal barrier was severely impaired. These results indicate an essential role of SNAP29 in epidermal differentiation and barrier formation. Markedly decreased deposition of lamellar body contents in mutant mice epidermis and the observation of malformed lamellar bodies indicate severe impairments in lamellar body function due to the Snap29 knockout. We also found increased microtubule associated protein-1 light chain 3, isoform B-II levels, unchanged p62/SQSTM1 protein amounts, and strong induction of the endoplasmic reticulum stress marker C/EBP homologous protein in mutant mice. This emphasizes a role of SNAP29 in autophagy and endoplasmic reticulum stress. Our murine models serve as powerful tools for investigating keratinocyte differentiation processes and provide insights into the essential contribution of SNAP29 to epidermal differentiation.

The Exocyst Subunit Sec6 Interacts with Assembled Exocytic SNARE Complexes

In eukaryotic cells, membrane-bound vesicles carry cargo between intracellular compartments, to and from the cell surface, and into the extracellular environment. Many conserved families of proteins are required for properly localized vesicle fusion, including the multisubunit tethering complexes and the SNARE complexes. These protein complexes work together to promote proper vesicle fusion in intracellular trafficking pathways. However, the mechanism by which the exocyst, the exocytosis-specific multisubunit tethering complex, interacts with the exocytic SNAREs to mediate vesicle targeting and fusion is currently unknown. We have demonstrated previously that the Saccharomyces cerevisiae exocyst subunit Sec6 directly bound the plasma membrane SNARE protein Sec9 in vitro and that Sec6 inhibited the assembly of the binary Sso1-Sec9 SNARE complex. Therefore, we hypothesized that the interaction between Sec6 and Sec9 prevented the assembly of premature SNARE complexes at sites of exocytosis. To map the determinants of this interaction, we used cross-linking and mass spectrometry analyses to identify residues required for binding. Mutation of residues identified by this approach resulted in a growth defect when introduced into yeast. Contrary to our previous hypothesis, we discovered that Sec6 does not change the rate of SNARE assembly but, rather, binds both the binary Sec9-Sso1 and ternary Sec9-Sso1-Snc2 SNARE complexes. Together, these results suggest a new model in which Sec6 promotes SNARE complex assembly, similar to the role proposed for other tether subunit-SNARE interactions.

SNAP23 Regulates Endothelial Exocytosis of von Willebrand Factor

Endothelial exocytosis regulates vascular thrombosis and inflammation. The trafficking and release of endothelial vesicles is mediated by SNARE (Soluble NSF Attachment protein REceptors) molecules, but the exact identity of endothelial SNAREs has been unclear. Three SNARE molecules form a ternary complex, including isoforms of the syntaxin (STX), vesicle-associated membrane protein (VAMP), and synaptosomal-associated protein (SNAP) families. We now identify SNAP23 as the predominant endothelial SNAP isoform that mediates endothelial exocytosis of von Willebrand Factor (VWF). SNAP23 was localized to the plasma membrane. Knockdown of SNAP23 decreased endothelial exocytosis, suggesting it is important for endothelial exocytosis. SNAP23 interacted with the endothelial exocytic machinery, and formed complexes with other known endothelial SNARE molecules. Taken together, these data suggest that SNAP23 is a key component of the endothelial SNARE machinery that mediates endothelial exocytosis.

O-GlcNAc-modification of SNAP-29 regulates autophagosome maturation

The mechanism by which nutrient status regulates the fusion of autophagosomes with endosomes/lysosomes is poorly understood. Here, we report that O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) mediates O-GlcNAcylation of the SNARE protein SNAP-29 and regulates autophagy in a nutrient-dependent manner. In mammalian cells, OGT knockdown, or mutating the O-GlcNAc sites in SNAP-29, promotes the formation of a SNAP-29-containing SNARE complex, increases fusion between autophagosomes and endosomes/lysosomes, and promotes autophagic flux. In Caenorhabditis elegans, depletion of ogt-1 has a similar effect on autophagy; moreover, expression of an O-GlcNAc-defective SNAP-29 mutant facilitates autophagic degradation of protein aggregates. O-GlcNAcylated SNAP-29 levels are reduced during starvation in mammalian cells and in C. elegans. Our study reveals a mechanism by which O-GlcNAc-modification integrates nutrient status with autophagosome maturation.

The role of non-canonical SNAREs in synaptic vesicle recycling

An increasing number of studies suggest that distinct pools of synaptic vesicles drive specific forms of neurotransmission. Interspersed with these functional studies are analyses of the synaptic vesicle proteome which have consistently detected the presence of so-called “non-canonical” SNAREs that typically function in fusion and trafficking of other subcellular structures within the neuron. The recent identification of certain non-canonical vesicular SNAREs driving spontaneous (e.g., VAMP7 and vti1a) or evoked asynchronous (e.g., VAMP4) release integrates and corroborates existing data from functional and proteomic studies and implies that at least some complement of non-canonical SNAREs resident on synaptic vesicles function in neurotransmission. Here, we discuss the specific roles in neurotransmission of proteins homologous to each member of the classical neuronal SNARE complex consisting of synaptobrevin2, syntaxin-1 and SNAP-25.

Drosophila SNAP-29 is an essential SNARE that binds multiple proteins involved in membrane traffic

Each membrane fusion event along the secretory and endocytic pathways requires a specific set of SNAREs to assemble into a 4-helical coiled-coil, the so-called trans-SNARE complex. Although most SNAREs contribute one helix to the trans-SNARE complex, members of the SNAP-25 family contribute two helixes. We report the characterization of the Drosophila homologue of SNAP-29 (dSNAP-29), which is expressed throughout development. Unlike the other SNAP-25 like proteins in fruit fly (i.e., dSNAP-25 and dSNAP-24), which form SDS-resistant SNARE complexes with their cognate SNAREs, dSNAP-29 does not participate in any SDS-resistant complexes, despite its interaction with dsyntaxin1 and dsyntaxin16 in vitro. Immunofluorescence studies indicated that dSNAP-29 is distributed in various tissues, locating in small intracellular puncta and on the plasma membrane, where it associates with EH domain-containing proteins implicated in the endocytic pathway. Overexpression and RNAi studies suggested that dSNAP-29 mediates an essential process in Drosophila development.

Multiple functions of the SNARE protein Snap29 in autophagy, endocytic, and exocytic trafficking during epithelial formation in Drosophila

How autophagic degradation is linked to endosomal trafficking routes is little known. Here we screened a collection of uncharacterized Drosophila mutants affecting membrane transport to identify new genes that also have a role in autophagy. We isolated a loss of function mutant in Snap29 (Synaptosomal-associated protein 29 kDa), the gene encoding the Drosophila homolog of the human protein SNAP29 and have characterized its function in vivo. Snap29 contains 2 soluble NSF attachment protein receptor (SNARE) domains and a asparagine-proline-phenylalanine (NPF motif) at its N terminus and rescue experiments indicate that both SNARE domains are required for function, whereas the NPF motif is in part dispensable. We find that Snap29 interacts with SNARE proteins, localizes to multiple trafficking organelles, and is required for protein trafficking and for proper Golgi apparatus morphology. Developing tissue lacking Snap29 displays distinctive epithelial architecture defects and accumulates large amounts of autophagosomes, highlighting a major role of Snap29 in autophagy and secretion. Mutants for autophagy genes do not display epithelial architecture or secretion defects, suggesting that the these alterations of the Snap29 mutant are unlikely to be caused by the impairment of autophagy. In contrast, we find evidence of elevated levels of hop-Stat92E (hopscotch-signal transducer and activator of transcription protein at 92E) ligand, receptor, and associated signaling, which might underlie the epithelial defects. In summary, our findings support a role of Snap29 at key steps of membrane trafficking, and predict that signaling defects may contribute to the pathogenesis of cerebral dysgenesis, neuropathy, ichthyosis, and palmoplantar keratoderma (CEDNIK), a human congenital syndrome due to loss of Snap29.

Role of the SNARE protein SNAP23 on cAMP-stimulated renin release in mouse juxtaglomerular cells

Renin, the rate-limiting enzyme in the formation of angiotensin II, is synthesized and stored in granules in juxtaglomerular (JG) cells. Therefore, the controlled mechanism involved in renin release is essential for the regulation of blood pressure. Exocytosis of renin-containing granules is likely involved in renin release; a process stimulated by cAMP. We found that the "soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptor" (SNARE) protein VAMP2 mediates cAMP-stimulated renin release and exocytosis in JG cells. To mediate exocytosis, VAMP2 must interact with a synaptosome-associated protein (SNAP). In the renal cortex, the isoform SNAP23 is abundantly expressed. We hypothesized that SNAP23 mediates cAMP-stimulated renin release from primary cultures of mouse JG cells. We found that SNAP23 protein is expressed and colocalized with renin-containing granules in primary cultures of mouse JG cell lysates. Thus, we then tested the involvement of SNAP23 in cAMP-stimulated renin release by transducing JG cells with a dominant-negative SNAP23 construct. In control JG cells transduced with a scrambled sequence, increasing cAMP stimulated renin release from 1.3 ± 0.3 to 5.3 ± 1.2% of renin content. In cells transduced with dominant-negative SNAP23, cAMP increased renin from 1.0 ± 0.1 to 3.0 ± 0.6% of renin content, a 50% blockade. Botulinum toxin E, which cleaves and inactivates SNAP23, reduced cAMP-stimulated renin release by 42 ± 17%. Finally, adenovirus-mediated silencing of SNAP23 significantly blocked cAMP-stimulated renin release by 50 ± 13%. We concluded that the SNARE protein SNAP23 mediates cAMP-stimulated renin release. These data show that renin release is a SNARE-dependent process.

Hemizygous mutations in SNAP29 unmask autosomal recessive conditions and contribute to atypical findings in patients with 22q11.2DS

Background

22q11.2 deletion syndrome (22q11.2DS) is the most common microdeletion disorder, affecting an estimated 1 : 2000–4000 live births. Patients with 22q11.2DS have a broad spectrum of phenotypic abnormalities which generally includes congenital cardiac abnormalities, palatal anomalies, and immunodeficiency. Additional findings, such as skeletal anomalies and autoimmune disorders, can confer significant morbidity in a subset of patients. 22q11.2DS is a contiguous gene DS and over 40 genes are deleted in patients; thus deletion of several genes within this region contributes to the clinical features. Mutations outside or on the remaining 22q11.2 allele are also known to modify the phenotype.

Methods

We utilised whole exome, targeted exome and/or Sanger sequencing to examine the genome of 17 patients with 22q11.2 deletions and phenotypic features found in <10% of affected individuals.

Results and conclusions

In four unrelated patients, we identified three novel mutations in SNAP29, the gene implicated in the autosomal recessive condition cerebral dysgenesis, neuropathy, ichthyosis and keratoderma (CEDNIK). SNAP29 maps to 22q11.2 and encodes a soluble SNARE protein that is predicted to mediate vesicle fusion at the endoplasmic reticulum or Golgi membranes. This work confirms that the phenotypic variability observed in a subset of patients with 22q11.2DS is due to mutations on the non-deleted chromosome, which leads to unmasking of autosomal recessive conditions such as CEDNIK, Kousseff, and a potentially autosomal recessive form of Opitz G/BBB syndrome. Furthermore, our work implicates SNAP29 as a major modifier of variable expressivity in 22q11.2 DS patients.

A novel function for SNAP29 (synaptosomal-associated protein of 29 kDa) in mast cell phagocytosis

Mast cells play a critical role in the innate immune response to bacterial infection. They internalize and kill a variety of bacteria and process antigen for presentation to T cells via MHC molecules. Although mast cell phagocytosis appears to play a significant role during bacterial infection, little is known about the proteins involved in its regulation. In this study, we demonstrate that the SNARE protein SNAP29 is involved in mast cell phagocytosis. SNAP29 is localized in the endocytic pathway and is transiently recruited to Escherichia coli (E. coli)-containing phagosomes. Interestingly, overexpression of SNAP29 significantly increases the internalization and killing of E. coli, while it does not affect mast cell exocytosis of inflammatory mediators. To our knowledge, these data are the first to demonstrate a novel function of SNAP29 in mast cell phagocytosis and have implications in protection against bacterial infection.

Essential roles of snap-29 in C. elegans

SNARE domain proteins are key molecules mediating intracellular fusion events. SNAP25 family proteins are unique target-SNAREs possessing two SNARE domains. Here we report the genetic, molecular, and cell biological characterization of C. elegans SNAP-29. We found that snap-29 is an essential gene required throughout the life-cycle. Depletion of snap-29 by RNAi in adults results in sterility associated with endomitotic oocytes and pre-meiotic maturation of the oocytes. Many of the embryos that are produced are multinucleated, indicating a defect in embryonic cytokinesis. A profound defect in secretion by oocytes and early embryos in animals lacking SNAP-29 appears to be the underlying defect connecting these phenotypes. Further analysis revealed defects in basolateral and apical secretion by intestinal epithelial cells in animals lacking SNAP-29, indicating a broad requirement for this protein in the secretory pathway. A SNAP-29-GFP fusion protein was enriched on recycling endosomes, and loss of SNAP-29 disrupted recycling endosome morphology. Taken together these results suggest a requirement for SNAP-29 in the fusion of post-Golgi vesicles with the recycling endosome for cargo to reach the cell surface.

Differential expression of synaptic proteins after chronic restraint stress in rat prefrontal cortex and hippocampus

Prolonged stress has been associated with altered synaptic plasticity but little is known about the molecular components and mechanisms involved in the stress response. In this study, we examined the effect of chronic restraint stress (CRS) on the expression of genes associated with synaptic vesicle exocytosis in rat prefrontal cortex and hippocampus. Rats were stressed daily using a 21day restraint stress paradigm, with durations of half an hour or 6h. RNA and protein were extracted from the same tissue sample and used for real-time quantitative polymerase chain reaction (real-time qPCR) and immunoblotting, respectively. Focusing on the SNARE complex, we investigated the expression of the SNARE core components syntaxin 1A, SNAP-25, and VAMP2 at both transcriptional and protein levels. In addition, the expression of 10 SNARE regulatory proteins was investigated at the transcriptional level. Overall, the prefrontal cortex was more sensitive to CRS compared to the hippocampus. In prefrontal cortex, CRS induced increased mRNA levels of VAMP2, VAMP1, syntaxin 1A, snapin, synaptotagmins I and III, and synapsins I and II, whereas SNAP-25 was down-regulated after CRS. Immunoblotting demonstrated equivalent changes in protein levels of VAMP2, syntaxin 1A, and SNAP-25. In hippocampus, we found increased mRNA levels of VAMP2 and SNAP-29 and a decrease in VAMP1 levels. Immunoblotting revealed decreased VAMP2 protein levels despite increased mRNA levels. Changes in the expression of synaptic proteins may accompany or contribute to the morphological, functional, and behavioral changes observed in experimental models of stress and may have relevance to the pathophysiology of stress-related disorders.

A targeted siRNA screen to identify SNAREs required for constitutive secretion in mammalian cells

The role of SNAREs in mammalian constitutive secretion remains poorly defined. To address this, we have developed a novel flow cytometry-based assay for measuring constitutive secretion and have performed a targeted SNARE and Sec1/Munc18 (SM) protein-specific siRNA screen (38 SNAREs, 4 SNARE-like proteins and 7 SM proteins). We have identified the endoplasmic reticulum (ER)/Golgi SNAREs syntaxin 5, syntaxin 17, syntaxin 18, GS27, SLT1, Sec20, Sec22b, Ykt6 and the SM protein Sly1, along with the post-Golgi SNAREs SNAP-29 and syntaxin 19, as being required for constitutive secretion. Depletion of SNAP-29 or syntaxin 19 causes a decrease in the number of fusion events at the cell surface and in SNAP-29-depleted cells causes an increase in the number of docked vesicles at the plasma membrane as determined by total internal reflection fluorescence (TIRF) microscopy. Analysis of syntaxin 19-interacting partners by mass spectrometry indicates that syntaxin 19 can form SNARE complexes with SNAP-23, SNAP-25, SNAP-29, VAMP3 and VAMP8, supporting its role in Golgi to plasma membrane transport or fusion. Surprisingly, we have failed to detect any requirement for a post-Golgi-specific R-SNARE in this process.

Loss of SNAP29 impairs endocytic recycling and cell motility

Intracellular membrane trafficking depends on the ordered formation and consumption of transport intermediates and requires that membranes fuse with each other in a tightly regulated and highly specific manner. Membrane anchored SNAREs assemble into SNARE complexes that bring membranes together to promote fusion. SNAP29 is a ubiquitous synaptosomal-associated SNARE protein. It interacts with several syntaxins and with the EH domain containing protein EHD1. Loss of functional SNAP29 results in CEDNIK syndrome (Cerebral Dysgenesis, Neuropathy, Ichthyosis and Keratoderma). Using fibroblast cell lines derived from CEDNIK patients, we show that SNAP29 mediates endocytic recycling of transferrin and β1-integrin. Impaired β1-integrin recycling affected cell motility, as reflected by changes in cell spreading and wound healing. No major changes were detected in exocytosis of VSVG protein from the Golgi apparatus, although the Golgi system acquired a dispersed morphology in SNAP29 deficient cells. Our results emphasize the importance of SNAP29 mediated membrane fusion in endocytic recycling and consequently, in cell motility.

The SNARE protein SNAP-29 interacts with the GTPase Rab3A: Implications for membrane trafficking in myelinating glia

During myelin formation, vast amounts of specialized membrane proteins and lipids are trafficked toward the growing sheath in cell surface-directed transport vesicles. Soluble N-ethylmaleimide-sensitive factor (NSF) attachment proteins (SNAPs) are important components of molecular complexes required for membrane fusion. We have analyzed the expression profile and molecular interactions of SNAP-29 in the nervous system. In addition to its known enrichment in neuronal synapses, SNAP-29 is abundant in oligodendrocytes during myelination and in noncompact myelin of the peripheral nervous system. By yeast two-hybrid screen and coimmunoprecipitation, we found that the GTPases Rab3A, Rab24, and septin 4 bind to the N-terminal domain of SNAP-29. The interaction with Rab24 or septin 4 was GTP independent. In contrast, interaction between SNAP-29 and Rab3A was GTP dependent, and colocalization was extensive both in synapses and in myelinating glia. In HEK293 cells, cytoplasmic SNAP-29 pools were redistributed upon coexpression with Rab3A, and surface-directed trafficking of myelin proteolipid protein was enhanced by overexpression of SNAP-29 and Rab3A. Interestingly, the abundance of SNAP-29 in sciatic nerves was increased during remyelination and in a rat model of Charcot-Marie-Tooth disease, two pathological situations with increased myelin membrane biogenesis. We suggest that Rab3A may regulate SNAP-29-mediated membrane fusion during myelination.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

SNAREs in neurons – beyond synaptic vesicle exocytosis (Review)

The paradigm for soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) function in mammalian cells has been built on advancements in our understanding of structural and biochemical aspects of synaptic vesicle exocytosis, involving specifically synaptobrevin, syntaxin 1 and SNAP25. Interestingly, a good number of SNAREs which are not directly involved in neurotransmitter exocytosis, are either brain-enriched or have distinct neuron-specific functions. Syntaxins 12/13 regulates glutamate receptor recycling via its interaction with neuron-enriched endosomal protein of 21 kDa (NEEP21). TI-VAMP/VAMP7 is essential for neuronal morphogenesis and mediates the vesicular transport processes underlying neurite outgrowth. Ykt6 is highly enriched in the cerebral cortex and hippocampus and is targeted to a novel compartment in neurons. Syntaxin 16 has a moderate expression level in many tissues, but is rather enriched in the brain. Here, we review and discuss the neuron-specific physiology and possible pathology of these and other (such as SNAP-29 and Vti1a-beta) members of the SNARE family.

Differential abilities of SNAP-25 homologs to support neuronal function

The SNAP receptor (SNARE) complex, consisting of synaptosome-associated protein of 25 kDa (SNAP-25), synaptobrevin-2, and syntaxin-1, is involved in synaptic vesicles exocytosis. In addition, SNAP-25 has been implicated in constitutive exocytosis processes required for neurite outgrowth. However, at least three isoforms of SNAP-25 have been reported from neurons: SNAP-23, which is also present in non-neuronal cells, and the two alternative splice variants SNAP-25a and SNAP-25b. Here, we studied the differential ability of these isoforms to support the functions previously broadly ascribed to "SNAP-25." We studied the rescue of snap-25 null neurons in culture with different SNAP-25 homologs. We find that deletion of SNAP-25 leads to strongly reduced neuron survival, and, in the few surviving cells, impaired arborization, reduced spontaneous release, and complete arrest of evoked release. Lentiviral expression of SNAP-25a, SNAP-25b, or SNAP-23 rescued neuronal survival, arborization, amplitude, and frequency of spontaneous events. Also evoked release was rescued by all isoforms, but synchronous release required SNAP-25a/b in both glutamatergic and GABAergic neurons. SNAP-23 supported asynchronous release only, reminiscent of synaptotagmin-1 null neurons. SNAP-25b was superior to SNAP-25a in vesicle priming, resembling the shift to larger releasable vesicle pools that accompanies synaptic maturation. These data demonstrate a differential ability of SNAP-25b, SNAP-25a, and SNAP-23 to support neuronal function.

SNAP-23 is not essential for constitutive exocytosis in HeLa cells

We applied the small interfering RNA (siRNA) technique and over-expression of a dominant-negative mutant to evaluate the role of SNAP-23, a non-neuronal isoform of SNAP-25, in constitutive exocytosis from HeLa cells. Although the protein level of SNAP-23 was reduced to less than 10% of the control value by siRNA directed against SNAP-23, exocytosis of SEAP (secreted alkaline phosphatase) was normal. Double knockdown of SNAP-23 and syntaxin-4 also failed to inhibit the secretion. Furthermore, over-expression of deltaC8-SNAP-23, a dominant-negative mutant of SNAP-23, did not abrogate SEAP secretion. These results suggest that SNAP-23 is not essential for constitutive exocytosis of SEAP.

Munc18-2 regulates exocytotic membrane fusion positively interacting with syntaxin-3 in RBL-2H3 cells

Recent studies have revealed that SNARE proteins are involved in exocytotic granular content release in mast cells as well as in neurotransmitter release in neural cells. However, the proteins that regulate the structure and activity of SNARE proteins in mast cells are not well understood. Munc18 is one such regulatory protein that plays a crucial role in neurotransmitter release. In this study, we investigated the role of Munc18 and its mechanism for regulating exocytotic release (degranulation) in rat basophilic leukemia cells (RBL-2H3). We obtained RBL-2H3 cells that express a low level of Munc18-2 and found that degranulation was remarkably inhibited in knockdown cells without any change in the expression level of syntaxins or Ca(2+) mobilization. We also observed the behavior of secretory granules in a single cell, and found no significant changes in their number and distribution in Munc18-2 knockdown cells. Using chimera proteins fused with fluorescent proteins, we demonstrated that Munc18-2 interacted with syntaxin-3, but not with syntaxin-4, in vivo. Interestingly, this interaction occurred not only on plasma membrane but also on secretory granules, suggesting that Munc18-2 may regulate granule-granule fusion as well as granule-plasma membrane fusion. These observations suggest that Munc18-2 together with syntaxin-3 regulate degranulation positively during the process of membrane fusion between secretory granules and plasma membrane, rather than during processes that regulate the number or behavior of secretory granules.

A Multigene Family Encoding R-SNAREs in the Ciliate Paramecium tetraurelia

SNARE proteins (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) mediate membrane interactions and are conventionally divided into Q-SNAREs and R-SNAREs according to the possession of a glutamine or arginine residue at the core of their SNARE domain. Here, we describe a set of R-SNAREs from the ciliate Paramecium tetraurelia consisting of seven families encoded by 12 genes that are expressed simultaneously. The complexity of the endomembrane system in Paramecium can explain this high number of genes. All P. tetraurelia synaptobrevins (PtSybs) possess a SNARE domain and show homology to the Longin family of R-SNAREs such as Ykt6, Sec22 and tetanus toxin-insensitive VAMP (TI-VAMP). We localized four exemplary PtSyb subfamilies with GFP constructs and antibodies on the light and electron microscopic level. PtSyb1-1, PtSyb1-2 and PtSyb3-1 were found in the endoplasmic reticulum, whereas PtSyb2 is localized exclusively in the contractile vacuole complex. PtSyb6 was found cytosolic but also resides in regularly arranged structures at the cell cortex (parasomal sacs), the cytoproct and oral apparatus, probably representing endocytotic compartments. With gene silencing, we showed that the R-SNARE of the contractile vacuole complex, PtSyb2, functions to maintain structural integrity as well as functionality of the osmoregulatory system but also affects cell division.

A mutation in SNAP29, coding for a SNARE protein involved in intracellular trafficking, causes a novel neurocutaneous syndrome characterized by cerebral dysgenesis, neuropathy, ichthyosis, and palmoplantar keratoderma

Neurocutaneous syndromes represent a vast, largely heterogeneous group of disorders characterized by neurological and dermatological manifestations, reflecting the common embryonic origin of epidermal and neural tissues. In the present report, we describe a novel neurocutaneous syndrome characterized by cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma (CEDNIK syndrome). Using homozygosity mapping in two large families, we localized the disease gene to 22q11.2 and identified, in all patients, a 1-bp deletion in SNAP29, which codes for a SNARE protein involved in vesicle fusion. SNAP29 expression was decreased in the skin of the patients, resulting in abnormal maturation of lamellar granules and, as a consequence, in mislocation of epidermal lipids and proteases. These data underscore the importance of vesicle trafficking regulatory mechanisms for proper neuroectodermal differentiation.

SNAP-29-mediated modulation of synaptic transmission in cultured hippocampal neurons

Identifying the molecules that regulate both the recycling of synaptic vesicles and the SNARE components required for fusion is critical for elucidating the molecular mechanisms underlying synaptic plasticity. SNAP-29 was initially isolated as a syntaxin-binding and ubiquitously expressed protein. Previous studies have suggested that SNAP-29 inhibits SNARE complex disassembly, thereby reducing synaptic transmission in cultured superior cervical ganglion neurons in an activity-dependent manner. However, the role of SNAP-29 in regulating synaptic vesicle recycling and short-term plasticity in the central nervous system remains unclear. In the present study, we examined the effect of SNAP-29 on synaptic transmission in cultured hippocampal neurons by dual patch clamp whole-cell recording, FM dye imaging, and immunocytochemistry. Our results demonstrated that exogenous expression of SNAP-29 in presynaptic neurons significantly decreased the efficiency of synaptic transmission after repetitive firing within a few minutes under low and moderate frequency stimulations (0.1 and 1 Hz). In contrast, SNAP-29 did not affect the density of synapses and basal synaptic transmission. Whereas neurotransmitter release was unaffected during intensive stimulation, recovery after synaptic depression was impaired by SNAP-29. Furthermore, knockdown of SNAP-29 expression in neurons by small interfering RNA increased the efficiency of synaptic transmission during repetitive firing. These findings suggest that SNAP-29 acts as a negative modulator for neurotransmitter release, probably by slowing recycling of the SNARE-based fusion machinery and synaptic vesicle turnover.