Syntaxin 5 regulates endoplasmic reticulum to Golgi transport

Trafficking of Connexin36 (Cx36) in the early secretory pathway

Gap junctions formed by the major neuronal connexin Cx36 function as electrical synapses in the nervous system and provide unique functions such as synchronizing activities or network oscillations. Although the physiological significance of electrical synapses for neuronal networks is well established, little is known about the pathways that regulate the transport of its main component: Cx36. Here we have used HEK293T cells as an expression system in combination with siRNA and BioID screens to study the transition of Cx36 from the ER to the cis Golgi. Our data indicate that the C-terminal tip of Cx36 is a key factor in this process, mediating binding interactions with two distinct components in the early secretory pathway: the COPII complex and the Golgi stacking protein Grasp55. The C-terminal amino acid valine serves as an ER export signal to recruit COPII cargo receptors Sec24A/B/C at ER exit sites, whereas the PDZ binding motif "SAYV" mediates an interaction with Grasp55. These two interactions have opposing effects in their respective compartments. While Sec24 subunits carry Cx36 out of the ER, Grasp55 stabilizes Cx36 in the Golgi as shown in over expression experiments. These early regulatory steps of Cx36 are expected to be essential for the formation, function, regulation and plasticity of electrical synapses in the developing and mature nervous system.

Natural Adeno-Associated Virus Serotypes and Engineered Adeno-Associated Virus Capsid Variants: Tropism Differences and Mechanistic Insights

Today, adeno-associated virus (AAV)-based vectors are arguably the most promising in vivo gene delivery vehicles for durable therapeutic gene expression. Advances in molecular engineering, high-throughput screening platforms, and computational techniques have resulted in a toolbox of capsid variants with enhanced performance over parental serotypes. Despite their considerable promise and emerging clinical success, there are still obstacles hindering their broader use, including limited transduction capabilities, tissue/cell type-specific tropism and penetration into tissues through anatomical barriers, off-target tissue biodistribution, intracellular degradation, immune recognition, and a lack of translatability from preclinical models to clinical settings. Here, we first describe the transduction mechanisms of natural AAV serotypes and explore the current understanding of the systemic and cellular hurdles to efficient transduction. We then outline progress in developing designer AAV capsid variants, highlighting the seminal discoveries of variants which can transduce the central nervous system upon systemic administration, and, to a lesser extent, discuss the targeting of the peripheral nervous system, eye, ear, lung, liver, heart, and skeletal muscle, emphasizing their tissue and cell specificity and translational promise. In particular, we dive deeper into the molecular mechanisms behind their enhanced properties, with a focus on their engagement with host cell receptors previously inaccessible to natural AAV serotypes. Finally, we summarize the main findings of our review and discuss future directions.

Intrinsically disordered region-mediated condensation of IFN-inducible SCOTIN/SHISA-5 inhibits ER-to-Golgi vesicle transport

Newly synthesized proteins in the endoplasmic reticulum (ER) are sorted by coat protein complex II (COPII) at the ER exit site en route to the Golgi. Under cellular stresses, COPII proteins become targets of regulation to control the transport. Here, we show that the COPII outer coat proteins Sec31 and Sec13 are selectively sequestered into the biomolecular condensate of SCOTIN/SHISA-5, which interferes with COPII vesicle formation and inhibits ER-to-Golgi transport. SCOTIN is an ER transmembrane protein with a cytosolic intrinsically disordered region (IDR), which is required and essential for the formation of condensates. Upon IFN-γ stimulation, which is a cellular condition that induces SCOTIN expression and condensation, ER-to-Golgi transport was inhibited in a SCOTIN-dependent manner. Furthermore, cancer-associated mutations of SCOTIN perturb its ability to form condensates and control transport. Together, we propose that SCOTIN impedes the ER-to-Golgi transport through its ability to form biomolecular condensates at the ER membrane.

An imaging-based RNA interference screen for modulators of the Rab6-mediated Golgi-to-ER pathway in mammalian cells

In mammalian cells, membrane traffic pathways play a critical role in connecting the various compartments of the endomembrane system. Each of these pathways is highly regulated, requiring specific machinery to ensure their fidelity. In the early secretory pathway, transport between the endoplasmic reticulum (ER) and Golgi apparatus is largely regulated via cytoplasmic coat protein complexes that play a role in identifying cargo and forming the transport carriers. The secretory pathway is counterbalanced by the retrograde pathway, which is essential for the recycling of molecules from the Golgi back to the ER. It is believed that there are at least two mechanisms to achieve this - one using the cytoplasmic COPI coat complex, and another, poorly characterised pathway, regulated by the small GTPase Rab6. In this work, we describe a systematic RNA interference screen targeting proteins associated with membrane fusion, in order to identify the machinery responsible for the fusion of Golgi-derived Rab6 carriers at the ER. We not only assess the delivery of Rab6 to the ER, but also one of its cargo molecules, the Shiga-like toxin B-chain. These screens reveal that three proteins, VAMP4, STX5, and SCFD1/SLY1, are all important for the fusion of Rab6 carriers at the ER. Live cell imaging experiments also show that the depletion of SCFD1/SLY1 prevents the membrane fusion event, suggesting that this molecule is an essential regulator of this pathway.

Identification of two pathways mediating protein targeting from ER to lipid droplets

Pathways localizing proteins to their sites of action are essential for eukaryotic cell organization and function. Although mechanisms of protein targeting to many organelles have been defined, how proteins, such as metabolic enzymes, target from the endoplasmic reticulum (ER) to cellular lipid droplets (LDs) is poorly understood. Here we identify two distinct pathways for ER-to-LD protein targeting: early targeting at LD formation sites during formation, and late targeting to mature LDs after their formation. Using systematic, unbiased approaches in Drosophila cells, we identified specific membrane-fusion machinery, including regulators, a tether and SNARE proteins, that are required for the late targeting pathway. Components of this fusion machinery localize to LD–ER interfaces and organize at ER exit sites. We identified multiple cargoes for early and late ER-to-LD targeting pathways. Our findings provide a model for how proteins target to LDs from the ER either during LD formation or by protein-catalysed formation of membrane bridges.

Song et al. identify two protein-targeting pathways from the endoplasmic reticulum to (1) early lipid droplets (LDs) and (2) mature lipid droplets. They define key factors mediating the second, late pathway and its many cargoes.

Golgi stress induces upregulation of the ER-Golgi SNARE Syntaxin-5, altered βAPP processing, and Caspase-3-dependent apoptosis in NG108-15 cells

The involvement of secretory pathways and Golgi dysfunction in neuronal cells during Alzheimer's disease progression is poorly understood. Our previous overexpression and knockdown studies revealed that the intracellular protein level of Syntaxin-5, an endoplasmic reticulum-Golgi soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE), modulates beta-amyloid precursor protein processing in neuronal cells. We recently showed that changes in endogenous Syntaxin-5 protein expression occur under stress induction. Syntaxin-5 was upregulated by endoplasmic reticulum stress but was degraded by Caspase-3 during apoptosis in neuronal cells. In addition, we showed that sustained endoplasmic reticulum stress promotes Caspase-3-dependent apoptosis during the later phase of the endoplasmic reticulum stress response in NG108-15 cells. In this study, to elucidate the consequences of secretory pathway dysfunction in beta-amyloid precursor protein processing that lead to neuronal cell death, we examined the effect of various stresses on endoplasmic reticulum-Golgi SNARE expression and beta-amyloid precursor protein processing. By using compounds to disrupt Golgi function, we show that Golgi stress promotes upregulation of the endoplasmic reticulum-Golgi SNARE Syntaxin-5, and prolonged stress causes Caspase-3-dependent apoptosis. Golgi stress induced intracellular beta-amyloid precursor protein accumulation and a concomitant decrease in total amyloid-beta production. We also examined the protective effect of the chemical chaperone 4-phenylbutylate on changes in amyloid-beta production and the activation of Caspase-3 induced by endoplasmic reticulum and Golgi stress. The compound alleviated the increase in the amyloid-beta 1-42/amyloid-beta 1-40 ratio induced by endoplasmic reticulum and Golgi stress. Furthermore, 4-phenylbutylate could rescue Caspase-3-dependent apoptosis induced by prolonged organelle stress. These results suggest that organelle stress originating from the endoplasmic reticulum and Golgi has a substantial impact on the amyloidogenic processing of beta-amyloid precursor protein and Caspase-3-dependent apoptosis, leading to neuronal cell death.

Unconventional Protein Secretion in Brain Tumors Biology: Enlightening the Mechanisms for Tumor Survival and Progression

Non-canonical secretion pathways, collectively known as unconventional protein secretion (UPS), are alternative secretory mechanisms usually associated with stress-inducing conditions. UPS allows proteins that lack a signal peptide to be secreted, avoiding the conventional endoplasmic reticulum-Golgi complex secretory pathway. Molecules that generally rely on the canonical pathway to be secreted may also use the Golgi bypass, one of the unconventional routes, to reach the extracellular space. UPS studies have been increasingly growing in the literature, including its implication in the biology of several diseases. Intercellular communication between brain tumor cells and the tumor microenvironment is orchestrated by various molecules, including canonical and non-canonical secreted proteins that modulate tumor growth, proliferation, and invasion. Adult brain tumors such as gliomas, which are aggressive and fatal cancers with a dismal prognosis, could exploit UPS mechanisms to communicate with their microenvironment. Herein, we provide functional insights into the UPS machinery in the context of tumor biology, with a particular focus on the secreted proteins by alternative routes as key regulators in the maintenance of brain tumors.

Maintaining Golgi Homeostasis: A Balancing Act of Two Proteolytic Pathways

The Golgi apparatus is a central hub for cellular protein trafficking and signaling. Golgi structure and function is tightly coupled and undergoes dynamic changes in health and disease. A crucial requirement for maintaining Golgi homeostasis is the ability of the Golgi to target aberrant, misfolded, or otherwise unwanted proteins to degradation. Recent studies have revealed that the Golgi apparatus may degrade such proteins through autophagy, retrograde trafficking to the ER for ER-associated degradation (ERAD), and locally, through Golgi apparatus-related degradation (GARD). Here, we review recent discoveries in these mechanisms, highlighting the role of the Golgi in maintaining cellular homeostasis.

Congenital disorder of glycosylation caused by starting site-specific variant in syntaxin-5

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein syntaxin-5 (Stx5) is essential for Golgi transport. In humans, the STX5 mRNA encodes two protein isoforms, Stx5 Long (Stx5L) from the first starting methionine and Stx5 Short (Stx5S) from an alternative starting methionine at position 55. In this study, we identify a human disorder caused by a single missense substitution in the second starting methionine (p.M55V), resulting in complete loss of the short isoform. Patients suffer from an early fatal multisystem disease, including severe liver disease, skeletal abnormalities and abnormal glycosylation. Primary human dermal fibroblasts isolated from these patients show defective glycosylation, altered Golgi morphology as measured by electron microscopy, mislocalization of glycosyltransferases, and compromised ER-Golgi trafficking. Measurements of cognate binding SNAREs, based on biotin-synchronizable forms of Stx5 (the RUSH system) and Förster resonance energy transfer (FRET), revealed that the short isoform of Stx5 is essential for intra-Golgi transport. Alternative starting codons of Stx5 are thus linked to human disease, demonstrating that the site of translation initiation is an important new layer of regulating protein trafficking.

ER residential chaperone GRP78 unconventionally relocalizes to the cell surface via endosomal transport

Despite new advances on the functions of ER chaperones at the cell surface, the translocation mechanisms whereby these chaperones can escape from the ER to the cell surface are just emerging. Previously we reported that in many cancer types, upon ER stress, IRE1α binds to and triggers SRC activation resulting in KDEL receptor dispersion from the Golgi and suppression of retrograde transport. In this study, using a combination of molecular, biochemical, and imaging approaches, we discovered that in colon and lung cancer, upon ER stress, ER chaperones, such as GRP78 bypass the Golgi and unconventionally traffic to the cell surface via endosomal transport mediated by Rab GTPases (Rab4, 11 and 15). Such unconventional transport is driven by membrane fusion between ER-derived vesicles and endosomes requiring the v-SNARE BET1 and t-SNARE Syntaxin 13. Furthermore, GRP78 loading into ER-derived vesicles requires the co-chaperone DNAJC3 that is regulated by ER-stress induced PERK-AKT-mTOR signaling.

The Golgi Apparatus May Be a Potential Therapeutic Target for Apoptosis-Related Neurological Diseases

Increasing evidence shows that, in addition to the classical function of protein processing and transport, the Golgi apparatus (GA) is also involved in apoptosis, one of the most common forms of cell death. The structure and the function of the GA is damaged during apoptosis. However, the specific effect of the GA on the apoptosis process is unclear; it may be involved in initiating or promoting apoptosis, or it may inhibit apoptosis. Golgi-related apoptosis is associated with a variety of neurological diseases including glioma, Alzheimer’s disease (AD), Parkinson’s disease (PD), and ischemic stroke. This review summarizes the changes and the possible mechanisms of Golgi structure and function during apoptosis. In addition, we also explore the possible mechanisms by which the GA regulates apoptosis and summarize the potential relationship between the Golgi and certain neurological diseases from the perspective of apoptosis. Elucidation of the interaction between the GA and apoptosis broadens our understanding of the pathological mechanisms of neurological diseases and provides new research directions for the treatment of these diseases. Therefore, we propose that the GA may be a potential therapeutic target for apoptosis-related neurological diseases.

Mutations in GET4 disrupt the transmembrane domain recognition complex pathway

The transmembrane domain recognition complex (TRC) targets cytoplasmic C-terminal tail-anchored (TA) proteins to their respective membranes in the endoplasmic reticulum (ER), Golgi, and mitochondria. It is composed of three proteins, GET4, BAG6, and GET5. We identified an individual with compound heterozygous missense variants (p.Arg122His, p.Ile279Met) in GET4 that reduced all three TRC proteins by 70% to 90% in his fibroblasts, suggesting a possible defect in TA protein targeting. He presented with global developmental delay, intellectual disabilities, seizures, facial dysmorphism, and delayed bone age. We found the TA protein, syntaxin 5, is poorly targeted to Golgi membranes compared to normal controls. Since GET4 regulates ER to Golgi transport, we hypothesized that such transport would be disrupted in his fibroblasts, and discovered that retrograde (but not anterograde) transport was significantly reduced. Despite reduction in the three TRC proteins, their mRNA levels were unchanged, suggesting increased degradation in patient fibroblasts. Treating fibroblasts with the FDA-approved proteasome inhibitor, bortezomib (10 nM), restored syntaxin 5 localization and nearly normalized the levels of all three TRC proteins. Our study identifies the first individual with GET4 mutations.

Sugary Logistics Gone Wrong: Membrane Trafficking and Congenital Disorders of Glycosylation

Glycosylation is an important post-translational modification for both intracellular and secreted proteins. For glycosylation to occur, cargo must be transported after synthesis through the different compartments of the Golgi apparatus where distinct monosaccharides are sequentially bound and trimmed, resulting in increasingly complex branched glycan structures. Of utmost importance for this process is the intraorganellar environment of the Golgi. Each Golgi compartment has a distinct pH, which is maintained by the vacuolar H+-ATPase (V-ATPase). Moreover, tethering factors such as Golgins and the conserved oligomeric Golgi (COG) complex, in concert with coatomer (COPI) and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion, efficiently deliver glycosylation enzymes to the right Golgi compartment. Together, these factors maintain intra-Golgi trafficking of proteins involved in glycosylation and thereby enable proper glycosylation. However, pathogenic mutations in these factors can cause defective glycosylation and lead to diseases with a wide variety of symptoms such as liver dysfunction and skin and bone disorders. Collectively, this group of disorders is known as congenital disorders of glycosylation (CDG). Recent technological advances have enabled the robust identification of novel CDGs related to membrane trafficking components. In this review, we highlight differences and similarities between membrane trafficking-related CDGs.

Stx5-Mediated ER-Golgi Transport in Mammals and Yeast

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) syntaxin 5 (Stx5) in mammals and its ortholog Sed5p in Saccharomyces cerevisiae mediate anterograde and retrograde endoplasmic reticulum (ER)-Golgi trafficking. Stx5 and Sed5p are structurally highly conserved and are both regulated by interactions with other ER-Golgi SNARE proteins, the Sec1/Munc18-like protein Scfd1/Sly1p and the membrane tethering complexes COG, p115, and GM130. Despite these similarities, yeast Sed5p and mammalian Stx5 are differently recruited to COPII-coated vesicles, and Stx5 interacts with the microtubular cytoskeleton, whereas Sed5p does not. In this review, we argue that these different Stx5 interactions contribute to structural differences in ER-Golgi transport between mammalian and yeast cells. Insight into the function of Stx5 is important given its essential role in the secretory pathway of eukaryotic cells and its involvement in infections and neurodegenerative diseases.

Differential cell surface recruitment of the superoxide-producing NADPH oxidases Nox1, Nox2 and Nox5: The role of the small GTPase Sar1

Transmembrane glycoproteins, synthesized at the endoplasmic reticulum (ER), generally reach the Golgi apparatus in COPII-coated vesicles en route to the cell surface. Here, we show that the bona fide nonglycoprotein Nox5, a transmembrane superoxide-producing NADPH oxidase, is transported to the cell surface in a manner resistant to co-expression of Sar1 (H79G), a GTP-fixed mutant of the small GTPase Sar1, which blocks COPII vesicle fission from the ER. In contrast, Sar1 (H79G) effectively inhibits ER-to-Golgi transport of glycoproteins including the Nox5-related oxidase Nox2. The trafficking of Nox2, but not that of Nox5, is highly sensitive to over-expression of syntaxin 5 (Stx5), a t-SNARE required for COPII ER-to-Golgi transport. Thus, Nox2 and Nox5 mainly traffic via the Sar1/Stx5-dependent and -independent pathways, respectively. Both participate in Nox1 trafficking, as Nox1 advances to the cell surface in two differentially N-glycosylated forms, one complex and one high mannose, in a Sar1/Stx5-dependent and -independent manner, respectively. Nox2 and Nox5 also can use both pathways: a glycosylation-defective mutant Nox2 is weakly recruited to the plasma membrane in a less Sar1-dependent manner; N-glycosylated Nox5 mutants reach the cell surface in part as the complex form Sar1-dependently, albeit mainly as the high-mannose form in a Sar1-independent manner.

A Mechanism Coupling Systemic Energy Sensing to Adipokine Secretion

Adipocytes sense systemic nutrient status and systemically communicate this information by releasing adipokines. The mechanisms that couple nutritional state to adipokine release are unknown. Here, we investigated how Unpaired 2 (Upd2), a structural and functional ortholog of the primary human adipokine leptin, is released from Drosophila fat cells. We find that Golgi reassembly stacking protein (GRASP), an unconventional secretion pathway component, is required for Upd2 secretion. In nutrient-rich fat cells, GRASP clusters in close proximity to the apical side of lipid droplets (LDs). During nutrient deprivation, glucagon-mediated increase in calcium (Ca²⁺) levels, via calmodulin kinase II (CaMKII) phosphorylation, inhibits proximal GRASP localization to LDs. Using a heterologous cell system, we show that human leptin secretion is also regulated by Ca²⁺ and CaMKII. In summary, we describe a mechanism by which increased cytosolic Ca²⁺ negatively regulates adipokine secretion and have uncovered an evolutionarily conserved molecular link between intracellular Ca²⁺ levels and energy homeostasis.

VAMP3/Syb and YKT6 are required for the fusion of constitutive secretory carriers with the plasma membrane

The cellular machinery required for the fusion of constitutive secretory vesicles with the plasma membrane in metazoans remains poorly defined. To address this problem we have developed a powerful, quantitative assay for measuring secretion and used it in combination with combinatorial gene depletion studies in Drosophila cells. This has allowed us to identify at least three SNARE complexes mediating Golgi to PM transport (STX1, SNAP24/29 and Syb; STX1, SNAP24/29 and YKT6; STX4, SNAP24 and Syb). RNAi mediated depletion of YKT6 and VAMP3 in mammalian cells also blocks constitutive secretion suggesting that YKT6 has an evolutionarily conserved role in this process. The unexpected role of YKT6 in plasma membrane fusion may in part explain why RNAi and gene disruption studies have failed to produce the expected phenotypes in higher eukaryotes.

Sec24C/D-isoform-specific sorting of the preassembled ER-Golgi Q-SNARE complex

SNAREs are incorporated into COPII vesicles by direct interaction with Sec24. In mammals, Sec24 isoforms recruit either Sec22b or the Q-SNARE complex comprising Syntaxin5, GS27, and Bet1. Analysis of immunoisolated COPII vesicles and intracellular localization of Sec24 isoforms indicates that all ER-Golgi SNAREs are present on the same vesicles.

Secretory proteins are exported from the endoplasmic reticulum in COPII vesicles. SNARE proteins—core machinery for membrane fusion—are incorporated into COPII vesicles by direct interaction with Sec24. Here we report a novel mechanism for sorting of the ER–Golgi Q-SNAREs into COPII vesicles. Different mammalian Sec24 isoforms recruit either the R-SNARE Sec22b or the Q-SNAREs Syntaxin5, GS27, and Bet1. Syntaxin5 is the only Q-SNARE that directly interacts with Sec24C, requiring its “open” conformation. Mutation within the IxM cargo-binding site of Sec24C led to a drastic reduction in sorting of all three Q-SNAREs into COPII vesicles, implying their ER export as a preassembled complex. Analysis of immunoisolated COPII vesicles and intracellular localization of Sec24 isoforms indicate that all ER–Golgi SNAREs are present on the same vesicle. Combined with existing data, our findings yield a general concept of how Sec24 isoforms can recruit fusogenic SNARE subunits to keep them functionally apart and thus prime mammalian COPII vesicles for homotypic fusion.

The Q-soluble N-Ethylmaleimide-sensitive Factor Attachment Protein Receptor (Q-SNARE) SNAP-47 Regulates Trafficking of Selected Vesicle-associated Membrane Proteins (VAMPs)

SNAREs constitute the core machinery of intracellular membrane fusion, but vesicular SNAREs localize to specific compartments via largely unknown mechanisms. Here, we identified an interaction between VAMP7 and SNAP-47 using a proteomics approach. We found that SNAP-47 mainly localized to cytoplasm, the endoplasmic reticulum (ER), and ERGIC and could also shuttle between the cytoplasm and the nucleus. SNAP-47 preferentially interacted with the trans-Golgi network VAMP4 and post-Golgi VAMP7 and -8. SNAP-47 also interacted with ER and Golgi syntaxin 5 and with syntaxin 1 in the absence of Munc18a, when syntaxin 1 is retained in the ER. A C-terminally truncated SNAP-47 was impaired in interaction with VAMPs and affected their subcellular distribution. SNAP-47 silencing further shifted the subcellular localization of VAMP4 from the Golgi apparatus to the ER. WT and mutant SNAP-47 overexpression impaired VAMP7 exocytic activity. We conclude that SNAP-47 plays a role in the proper localization and function of a subset of VAMPs likely via regulation of their transport through the early secretory pathway.

Estrogen and androgen regulate actin-remodeling and endocytosis-related genes during rat spermiation

Spermiation, the sperm release process, is imperative to male fertility and reproduction. Morphologically, it is characterized by removal of atypical adherens junctions called ectoplasmic specializations, and formation of transient endocytic devices called tubulobulbar complexes requiring cytoskeleton remodeling and recruitment of proteins needed for endocytosis. Earlier, estrogen administration to adult male rats was seen to cause spermiation failure due to disruption of tubulobulbar complexes. This was accompanied by reduction in intratesticular testosterone levels and increase in intratesticular estrogen along with deregulation of genes involved in cytoskeleton remodeling (Arpc1b, Evl and Capg) and endocytosis (Picalm, Eea1 and Stx5a). In the present study, we aim to understand the role of estrogen and androgen in regulating these genes independently using seminiferous tubule culture system treated with estrogen, androgen or agonists and antagonists of estrogen receptors. We find that transcripts of Arpc1b, Evl and Picalm are responsive to estrogen while those of Picalm, Eea1 and Stx5a are responsive to androgen. We also find that the estrogen regulation of Arpc1b and Evl is mediated through estrogen receptor β and that of Picalm occurs through estrogen receptors α and β. Localization of these proteins at or in the vicinity of tubulobulbar complexes reveals that ARPC1B, EVL, PICALM, EEA1 and STX5A seem to be involved in spermiation. Thus, estrogen and androgen regulate specific genes in seminiferous tubules that could play a role in spermiation.

Contribution of the long form of syntaxin 5 to the organization of the endoplasmic reticulum

The SNARE protein syntaxin 5 exists as long (42 kDa) and short (35 kDa) isoforms. The short form is principally localized in the Golgi complex, whereas the long form resides not only in the Golgi but also in the endoplasmic reticulum (ER). Although the Golgi-localized short form has been extensively investigated, little is known about the long form. In the present study, we demonstrate that the long form of syntaxin 5 functions to shape the ER. We found that overexpression of the long form of syntaxin 5 induces rearrangement and co-alignment of the ER membrane with microtubules, the pattern of which is quite similar to that observed in cells overexpressing CLIMP-63, a linker between the ER membrane and microtubules. The ability of syntaxin 5 to induce ER-microtubule rearrangement is not related to its SNARE function, but correlates with its binding affinities for CLIMP-63, and CLIMP-63 is essential for the induction of this rearrangement. Microtubule co-sedimentation assays demonstrated that the long form of syntaxin 5 has a substantial microtubule-binding activity. These results suggest that the long form of syntaxin 5 contributes to the regulation of ER structure by interacting with both CLIMP-63 and microtubules. Indeed, depletion of syntaxin 5 caused the spreading of the ER to the cell periphery, similar to the phenotype observed in cells treated with the microtubule-depolymerizing reagent nocodazole. Our results disclose a previously undescribed function of the long form of syntaxin 5 that is not related to its function as a SNARE.

SNARE motif: a common motif used by pathogens to manipulate membrane fusion

To penetrate host cells through their membranes, pathogens use a variety of molecular components in which the presence of heptad repeat motifs seems to be a prevailing element. Heptad repeats are characterized by a pattern of seven, generally hydrophobic, residues. In order to initiate membrane fusion, viruses use glycoproteins-containing heptad repeats. These proteins are structurally and functionally similar to the SNARE proteins known to be involved in eukaryotic membrane fusion. SNAREs also display a heptad repeat motif called the "SNARE motif". As bacterial genomes are being sequenced, microorganisms also appear to be carrying membrane proteins resembling eukaryotic SNAREs. This category of SNARE-like proteins might share similar functions and could be used by microorganisms to either promote or block membrane fusion. Such a recurrence across pathogenic organisms suggests that this architectural motif was evolutionarily selected because it most effectively ensures the survival of pathogens within the eukaryotic environment.

Syntaxin 1A regulates surface expression of beta-cell ATP-sensitive potassium channels

The pancreatic ATP-sensitive potassium (K(ATP)) channel consisting of four inwardly rectifying potassium channel 6.2 (Kir6.2) and four sulfonylurea receptor SUR1 subunits plays a key role in insulin secretion by linking glucose metabolism to membrane excitability. Syntaxin 1A (Syn-1A) is a plasma membrane protein important for membrane fusion during exocytosis of insulin granules. Here, we show that Syn-1A and K(ATP) channels endogenously expressed in the insulin-secreting cell INS-1 interact. Upregulation of Syn-1A by overexpression in INS-1 leads to a decrease, whereas downregulation of Syn-1A by small interfering RNA (siRNA) leads to an increase, in surface expression of K(ATP) channels. Using COSm6 cells as a heterologous expression system for mechanistic investigation, we found that Syn-1A interacts with SUR1 but not Kir6.2. Furthermore, Syn-1A decreases surface expression of K(ATP) channels via two mechanisms. One mechanism involves accelerated endocytosis of surface channels. The other involves decreased biogenesis and processing of channels in the early secretory pathway. This regulation is K(ATP) channel specific as Syn-1A has no effect on another inward rectifier potassium channel Kir3.1/3.4. Our results demonstrate that in addition to a previously documented role in modulating K(ATP) channel gating, Syn-1A also regulates K(ATP) channel expression in β-cells. We propose that physiological or pathological changes in Syn-1A expression may modulate insulin secretion by altering glucose-secretion coupling via changes in K(ATP) channel expression.

Syntaxin 5 is required for copper homeostasis in Drosophila and mammals

Copper is essential for aerobic life, but many aspects of its cellular uptake and distribution remain to be fully elucidated. A genome-wide screen for copper homeostasis genes in Drosophila melanogaster identified the SNARE gene Syntaxin 5 (Syx5) as playing an important role in copper regulation; flies heterozygous for a null mutation in Syx5 display increased tolerance to high dietary copper. The phenotype is shown here to be due to a decrease in copper accumulation, a mechanism also observed in both Drosophila and human cell lines. Studies in adult Drosophila tissue suggest that very low levels of Syx5 result in neuronal defects and lethality, and increased levels also generate neuronal defects. In contrast, mild suppression generates a phenotype typical of copper-deficiency in viable, fertile flies and is exacerbated by co-suppression of the copper uptake gene Ctr1A. Reduced copper uptake appears to be due to reduced levels at the plasma membrane of the copper uptake transporter, Ctr1. Thus Syx5 plays an essential role in copper homeostasis and is a candidate gene for copper-related disease in humans.

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.

Integrity of the early secretory pathway promotes, but is not required for, severe acute respiratory syndrome coronavirus RNA synthesis and virus-induced remodeling of endoplasmic reticulum membranes

To accommodate its RNA synthesis in the infected cell, severe acute respiratory syndrome coronavirus (SARS-CoV) induces a cytoplasmic reticulovesicular network (RVN) that is derived from endoplasmic reticulum (ER) membranes. We set out to investigate how the early secretory pathway interacts with the RVN and the viral replication/transcription complex (RTC) that is anchored to it. When the secretory pathway was disrupted by brefeldin A (BFA) treatment at the start of infection, RVN formation and viral RTC activity were not blocked and continued up to 11 h postinfection, although RNA synthesis was reduced by ca. 80%. In vitro RTC assays, using membrane fractions from infected cells, demonstrated that BFA does not directly interfere with the activity of the viral RNA-synthesizing enzymes. Confocal microscopy studies showed that early secretory pathway components are not associated with SARS-CoV-induced replication sites, although our studies revealed that infection induces a remarkable redistribution of the translocon subunit Sec61alpha. Ultrastructural studies, including electron tomography, revealed that the formation of the RVN and all its previously documented features can occur in the presence of BFA, despite differences in the volume and morphology of the network. We therefore conclude that early secretory pathway proteins do not play a direct role in RVN morphogenesis or the functionality of the SARS-CoV RTC. The BFA-induced disruption of ER integrity and functionality probably affects the overall quality of the membrane scaffold that is needed to support the viral RTC and/or the availability of specific host factors, which in turn compromises viral RNA synthesis.

Role of syntaxin 18 in the organization of endoplasmic reticulum subdomains

The presence of subdomains in the endoplasmic reticulum (ER) enables this organelle to perform a variety of functions, yet the mechanisms underlying their organization are poorly understood. In the present study, we show that syntaxin 18, a SNAP (soluble NSF attachment protein) receptor localized in the ER, is important for the organization of two ER subdomains, smooth/rough ER membranes and ER exit sites. Knockdown of syntaxin 18 caused a global change in ER membrane architecture, leading to the segregation of the smooth and rough ER. Furthermore, the organization of ER exit sites was markedly changed concomitantly with dispersion of the ER-Golgi intermediate compartment and the Golgi complex. These morphological changes in the ER were substantially recovered by treatment of syntaxin-18-depleted cells with brefeldin A, a reagent that stimulates retrograde membrane flow to the ER. These results suggest that syntaxin 18 has an important role in ER subdomain organization by mediating the fusion of retrograde membrane carriers with the ER membrane.

A VAMP7/Vti1a SNARE complex distinguishes a non-conventional traffic route to the cell surface used by KChIP1 and Kv4 potassium channels

The KChIPs (K(+) channel-interacting proteins) are EF hand-containing proteins required for the traffic of channel-forming Kv4 K(+) subunits to the plasma membrane. KChIP1 is targeted, through N-terminal myristoylation, to intracellular vesicles that appear to be trafficking intermediates from the ER (endoplasmic reticulum) to the Golgi but differ from those underlying conventional ER-Golgi traffic. To define KChIP1 vesicles and the traffic pathway followed by Kv4/KChIP1 traffic, we examined their relationship to potential SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins mediating the trafficking step. To distinguish Kv4/KChIP1 from conventional constitutive traffic, we compared it to the traffic of the VSVG (vesicular-stomatitis virus G-protein). Expression of KChIP with single or triple EF hand mutations quantitatively inhibited Kv4/KChIP1 traffic to the cell surface but had no effect on VSVG traffic. KChIP1-expressing vesicles co-localized with the SNARE proteins Vti1a and VAMP7 (vesicle-associated membrane protein 7), but not with the components of two other ER-Golgi SNARE complexes. siRNA (small interfering RNA)-mediated knockdown of Vti1a or VAMP7 inhibited Kv4/KChIP1traffic to the plasma membrane in HeLa and Neuro2A cells. Vti1a and VAMP7 siRNA had no effect on VSVG traffic or that of Kv4.2 when stimulated by KChIP2, a KChIP with different intrinsic membrane targeting compared with KChIP1. The present results suggest that a SNARE complex containing VAMP7 and Vti1a defines a novel traffic pathway to the cell surface in both neuronal and non-neuronal cells.

Poliovirus infection blocks ERGIC-to-Golgi trafficking and induces microtubule-dependent disruption of the Golgi complex

Cells infected with poliovirus exhibit a rapid inhibition of protein secretion and disruption of the Golgi complex. Neither the precise step at which the virus inhibits protein secretion nor the fate of the Golgi complex during infection has been determined. We find that transport-vesicle exit from the endoplasmic reticulum (ER) and trafficking to the ER-Golgi intermediate compartment (ERGIC) are unaffected in the poliovirus-infected cell. By contrast, poliovirus infection blocks transport from the ERGIC to the Golgi complex. Poliovirus infection also induces fragmentation of the Golgi complex resulting in diffuse distribution of both large and small vesicles throughout the cell. Pre-treatment with nocodazole prevents complete fragmentation, indicating that microtubules are required for poliovirus-induced Golgi dispersion. However, virally induced inhibition of the secretory pathway is not affected by nocodazole, and Golgi dispersion was found to occur during infection with mutant viruses with reduce ability to inhibit protein secretion. We conclude that the dispersion of the Golgi complex is not in itself the cause of inhibition of traffic between the ERGIC and the Golgi. Instead, these phenomena are independent effects of poliovirus infection on the host secretory complex.

Syntaxin 16 and syntaxin 5 are required for efficient retrograde transport of several exogenous and endogenous cargo proteins

Retrograde transport allows proteins and lipids to leave the endocytic pathway to reach other intracellular compartments, such as trans-Golgi network (TGN)/Golgi membranes, the endoplasmic reticulum and, in some instances, the cytosol. Here, we have used RNA interference against the SNARE proteins syntaxin 5 and syntaxin 16, combined with recently developed quantitative trafficking assays, morphological approaches and cell intoxication analysis to show that these SNARE proteins are not only required for efficient retrograde transport of Shiga toxin, but also for that of an endogenous cargo protein - the mannose 6-phosphate receptor - and for the productive trafficking into cells of cholera toxin and ricin. We have found that the function of syntaxin 16 was specifically required for, and restricted to, the retrograde pathway. Strikingly, syntaxin 5 RNA interference protected cells particularly strongly against Shiga toxin. Since our trafficking analysis showed that apart from inhibiting retrograde endosome-to-TGN transport, the silencing of syntaxin 5 had no additional effect on Shiga toxin endocytosis or trafficking from TGN/Golgi membranes to the endoplasmic reticulum, we hypothesize that syntaxin 5 also has trafficking-independent functions. In summary, our data demonstrate that several cellular and exogenous cargo proteins use elements of the same SNARE machinery for efficient retrograde transport between early/recycling endosomes and TGN/Golgi membranes.

Copper homeostasis gene discovery in Drosophila melanogaster

Recent studies have shown a high level of conservation between Drosophila melanogaster and mammalian copper homeostasis mechanisms. These studies have also demonstrated the efficiency with which this species can be used to characterize novel genes, at both the cellular and whole organism level. As a versatile and inexpensive model organism, Drosophila is also particularly useful for gene discovery applications and thus has the potential to be extremely useful in identifying novel copper homeostasis genes and putative disease genes. In order to assess the suitability of Drosophila for this purpose, three screening approaches have been investigated. These include an analysis of the global transcriptional response to copper in both adult flies and an embryonic cell line using DNA microarray analysis. Two mutagenesis-based screens were also utilized. Several candidate copper homeostasis genes have been identified through this work. In addition, the results of each screen were carefully analyzed to identify any factors influencing efficiency and sensitivity. These are discussed here with the aim of maximizing the efficiency of future screens and the most suitable approaches are outlined. Building on this information, there is great potential for the further use of Drosophila for copper homeostasis gene discovery.

Toxicity mediated by soluble oligomers of beta-amyloid(1-42) on cholinergic SN56.B5.G4 cells

Alzheimer's disease (AD) is characterized by cholinergic dysfunction and progressive basal forebrain cell loss which has been assumed to be as a result of the extensive accumulation of beta-amyloid (Abeta). In addition to Abeta fibrillar assemblies, there are pre-fibrillar forms that have been shown to be neurotoxic, although their role in cholinergic degeneration is still not known. Using the cholinergic cell line SN56.B5.G4, we investigated the effect of different Abeta(1-42) aggregates on cell viability. In our model, only soluble oligomeric but not fibrillar Abeta(1-42) forms induced toxicity in cholinergic cells. To determine whether the neurotoxicity of oligomeric Abeta(1-42) was caused by its oxidative potential, we performed microarray analysis of SN56.B5.G4 cells treated either with oligomeric Abeta(1-42) or H(2)O(2). We showed that genes affected by Abeta(1-42) differed from those affected by non-specific oxidative stress. Many of the genes affected by Abeta(1-42) were present in the endoplasmic reticulum (ER), Golgi apparatus and/or otherwise involved in protein modification and degradation (chaperones, ATF6), indicating a possible role for ER-mediated stress in Abeta-mediated toxicity. Moreover, a number of genes, which are known to be involved in AD (clusterin, Slc18a3), were identified. This study provides important leads for the understanding of oligomeric Abeta(1-42) toxicity in cholinergic cells, which may account in part for cholinergic degeneration in AD.

Vesicle-associated membrane protein 7 is expressed in intestinal ER

Intestinal dietary triacylglycerol absorption is a multi-step process. Triacylglycerol exit from the endoplasmic reticulum (ER) is the rate-limiting step in the progress of the lipid from its apical absorption to its basolateral membrane export. Triacylglycerol is transported from the ER to the cis Golgi in a specialized vesicle, the pre-chylomicron transport vesicle (PCTV). The vesicle-associated membrane protein 7 (VAMP7) was found to be more concentrated on PCTVs compared with ER membranes. VAMP7 has been previously identified associated with post-Golgi sites in eukaryotes. To examine the potential role of VAMP7 in PCTV trafficking, antibodies were generated that identified a 25 kDa band consistent with VAMP7 but did not crossreact with VAMP1,2. VAMP7 was concentrated on intestinal ER by immunofluorescence microscopy. Immunoelectron microscopy showed that the ER proteins Sar1 and rBet1 were present on PCTVs and colocalized with VAMP7. Iodixanol gradient centrifugation showed VAMP7 to be isodense with ER and endosomes. Although VAMP7 localized to intestinal ER, it was not present in the ER of liver and kidney. Anti-VAMP7 antibodies reduced the transfer of triacylglycerol, but not newly synthesized proteins, from the ER to the Golgi by 85%. We conclude that VAMP7 is enriched in intestinal ER and that it plays a functional role in the delivery of triacylglycerol from the ER to the Golgi.

Identification of a Sed5 -like SNARE Gene LjSYP32-1 that Contributes to Nodule Tissue Formation of Lotus japonicus

We identified a Sed5-like clone LjSYP32-1 which contributes to nodule tissue formation and plant growth in Lotus japonicus. In the L. japonicus expressed sequence tag (EST) clone databases of Kazusa DNA Research Institute, another syntaxin-related clone (LjSYP32-2) was also detected, and the nucleotide and amino acid sequences of these two clone are very similar to each other. Real-time PCR and promoter analysis indicated that expression of LjSYP32-1 was dominant compared with LjSYP32-2 in the various plant organs. Promoter analysis and in situ hybridization revealed that LjSYP32-1 was expressed significantly in the inner cortex cell layer surrounding the infected zone of young nodules and in the meristem area of developing lateral root. To explore the function and physiological role of LjSYP32-1 in nodules and other plant organs, stable transformation lines of L. japonicus expressing either sense or antisense LjSYP32-1 were prepared. The antisense plants showed a significantly retarded plant growth phenotype, suggesting a role for LjSYP32-1 in supporting plant growth. In the same transgenic lines, the plants were capable of forming nodules, but the acetylene reduction activity was reduced by around 50% per plant. The nodules were much smaller and some nodules were fused to each other by sharing the inner cortex. The rate of occurrence of such irregular nodules was twice that observed in wild-type plants. The data suggest that LjSYP32-1 contributes to the support of plant growth and normal nodule tissue differentiation.

Syntaxin 5 interacts specifically with presenilin holoproteins and affects processing of betaAPP in neuronal cells

The specific roles of syntaxin 5 (Syx 5) in the interaction with presenilin (PS) and the accumulation of beta-amyloid precursor protein (betaAPP), as well as the secretion of beta-amyloid peptide (Abeta peptide) were examined in NG108-15 cells. Syx 5, which localizes from the endoplasmic reticulum (ER) to the Golgi, bound to PS holoproteins, while the other Syxs studied did not. Among familial Alzheimer's disease (FAD)-linked PS mutants, PS1deltaE9, which lacks the endoproteolytic cleavage site, showed markedly decreased binding to Syx 5. The interaction domains in Syx 5 were mapped to the transmembrane region and to the cytoplasmic region containing the alpha-helical domains, which are distinct from the H3 (SNARE motif). Among all of the Syxs examined, only overexpression of Syx 5 resulted in the accumulation of betaAPP in the ER to cis-Golgi compartment, an attenuation of the amount of the C-terminal fragment (APP-CTF) of betaAPP, and a reduction in the secretion of Abeta peptides. Furthermore, co-expression of Syx 5 with C99 resulted in an increase in APP-CTF and suppressed Abeta secretion. Taken together, these results indicate that Syx 5 may play a specific role in the modulation of processing and/or trafficking of FAD-related proteins in neuronal cells by interaction with PS holoproteins in the early secretory compartment of neuronal cells.

RNA interference-mediated silencing of the syntaxin 5 gene induces Golgi fragmentation but capable of transporting vesicles

It has been suggested that syntaxin 5 (Syx5) participates in vesicular transport. We examined the effects of Syx5 down-regulation on the morphology of the Golgi apparatus and the transport of vesicles in mammalian cells. Knockdown of the Syx5 gene resulted in Golgi fragmentation without changing the level of endoplasmic reticulum (ER)-resident proteins, other Golgi-SNAREs (soluble N-ethylmaleimide-sensitive factor-attachment protein receptors), and coatmer proteins. Strikingly, a major decrease in Syx5 expression barely affected the anterograde transport of vesicular stomatitis virus G (VSVG) protein to the plasma membrane. These results suggest that Syx5 is required for the maintenance of the Golgi structures but may not play a major role in the transport of vesicles carrying VSVG between the ER and the Golgi compartment.

dGRASP localization and function in the early exocytic pathway in Drosophila S2 cells

The de novo model for Golgi stack biogenesis predicts that membrane exiting the ER at transitional ER (tER) sites contains and recruits all the necessary molecules to form a Golgi stack, including the Golgi matrix proteins, p115, GM130, and GRASP65/55. These proteins leave the tER sites faster than Golgi transmembrane resident enzymes, suggesting that they act as a template nucleating the formation of the Golgi apparatus. However, the localization of the Golgi matrix proteins at tER sites is only shown under conditions where exit from the ER is blocked. Here, we show in Drosophila S2 cells, that dGRASP, the single Drosophila homologue of GRASP65/55, localizes both to the Golgi membranes and the tER sites at steady state and that the myristoylation of glycine 2 is essential for the localization to both compartments. Its depletion for 96 h by RNAi gave an effect on the architecture of the Golgi stacks in 30% of the cells, but a double depletion of dGRASP and dGM130 led to the quantitative conversion of Golgi stacks into clusters of vesicles and tubules, often featuring single cisternae. This disruption of Golgi architecture was not accompanied by the disorganization of tER sites or the inhibition of anterograde transport. This shows that, at least in Drosophila, the structural integrity of the Golgi stacks is not required for efficient transport. Overall, dGRASP exhibits a dynamic association to the membrane of the early exocytic pathway and is involved in Golgi stack architecture.

Inhibition of ROMK potassium channel by syntaxin 1A

ROMK potassium channels are present in the cortical collecting duct (CCD) of the kidney and serve as apical exit pathways for K+secretion in this nephron segment. K+secretion in the CCD is regulated by multiple factors. In this study, we show that syntaxin 1A, but not syntaxin 3 or 4, inhibited whole cell ROMK currents in Xenopus laevis oocytes. Syntaxin 1A, but not syntaxin 3 or 4, interacted with the COOH-terminal cytoplasmic domain of ROMK in intro. Coexpression with synaptobrevin 2 reversed inhibition of whole cell ROMK currents by syntaxin 1A. In excised inside-out membranes of oocytes, application of fusion proteins containing the cytoplasmic region of syntaxin 1A to the cytoplasmic face caused a dose-dependent inhibition of ROMK. We further examined regulation of the K+channels in the CCD by syntaxin 1A. Application of botulinum toxin C1 to the excised inside-out membranes of the CCD caused an increase in the activity of K+channels. In contrast, application of toxin B had no effects. These results suggest that syntaxin 1A causes a tonic inhibition of the K+channels in the apical membrane of the CCD. Binding of synaptobrevin 2 to syntaxin 1A during docking and fusion of transport vesicles to the plasma membranes of CCD may lead to activation of these channels.

Coronin proteins as multifunctional regulators of the cytoskeleton and membrane trafficking

Coronins constitute an evolutionarily conserved family of WD-repeat actin-binding proteins, which can be clearly classified into two distinct groups based on their structural features. All coronins possess a conserved basic N-terminal motif and three to ten WD repeats clustered in one or two core domains. Dictyostelium and mammalian coronins are important regulators of the actin cytoskeleton, while the fly Dpod1 and the yeast coronin proteins crosslink both actin and microtubules. Apart from that, several coronins have been shown to be involved in vesicular transport. C. elegans POD-1 and Drosophila coro regulate the actin cytoskeleton, but also govern vesicular trafficking as indicated by mutant phenotypes. In both organisms, defects in cytoskeleton and trafficking lead to severe developmental defects ranging from abnormal cell division to aberrant formation of morphogen gradients. Finally, mammalian coronin 7 appears not to execute any cytoskeleton-related functions, but rather participates in regulating Golgi trafficking. Here, we review recent data providing more insight into molecular mechanisms underlying the regulation of F-actin structures, cytoskeletal rearrangements and intracellular membrane transport by coronin proteins and the way that they might link cytoskeleton with trafficking in development and disease.

Syntaxin 5 interacts with presenilin holoproteins, but not with their N- or C-terminal fragments, and affects beta-amyloid peptide production

Mutations in presenilins 1 and 2 (PS1 and PS2) account for the majority of cases of early-onset familial Alzheimer's disease. However, the trafficking and interaction of PSs with other proteins in the early secretory pathways are poorly understood. Using co-immunoprecipitation, we found that PS bound to Syx5 (syntaxin 5), which is a target-soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor involved in endoplasmic reticulum (ER)-Golgi vesicular transport in vivo. Syx5 interacted only with the full-length PS holoproteins and not with the naturally occurring N- or C-terminal fragments. The PS holoproteins co-immunoprecipitated with the mutant Syx5, which localized to the ER and Golgi compartments, despite the substitution of the transmembrane region with that of syntaxin 1A. In contrast, the transmembrane deletion mutant that localized to the cytosol, but not to the ER or Golgi compartments, did not co-immunoprecipitate the PS holoproteins. The PS1 variant linked to familial Alzheimer's disease (PS1DeltaE9), lacking the region that contains the endoproteolytic cleavage site in the cytoplasmic loop, showed markedly decreased binding to Syx5. Immunofluorescence and sucrose-density-gradient fractionation analyses showed that the full-length PS holoproteins co-localized with Syx5 to the ER and cis-Golgi compartments. Furthermore, Syx5 overexpression resulted in the accumulation of PS holoproteins and the beta-amyloid precursor protein, and reduced the secretion of the Abeta (amyloid beta) peptide in COS-7 cells. In summary, these results indicate that Syx5 binds to full-length PSs and affects the processing and trafficking of beta-amyloid precursor protein in the early secretory compartments.

Morphine-induced alterations in gene expression of calbindin immunopositive neurons in nucleus accumbens shell and core

Chronic opiate administration induces a number of biochemical alterations within the mesolimbic dopamine system that may mediate various aspects of the addictive process. In the present study, rats were administered morphine (1.0 mg/infusion) for 20 days (17.6+/-3.0 infusions/day) based on infusion histories of self-administering rats. Calbindin-D28K immunoreactive neurons were microdissected from the nucleus accumbens (NAc) shell and core subregions and gene expression was assessed using cDNA macroarrays. Comparison of gene expression between the shell and core subregions of vehicle-treated rats revealed significantly higher relative abundance of GABA-A alpha1, Galphai2 and post-synaptic density protein 95 transcript (PSD-95) mRNA levels in the shell, whereas Ggamma2 and synuclein 1 were more abundant in the core of the NAc. In the NAc shell, morphine administration resulted in upregulation of caspace 9, NF-kappaB, NF-H, tau, GABA-A delta subunit, FGFR1, Ggamma2, synuclein 1, syntaxin 5 and 13, GRK5, and c-fos mRNAs. Caspace 1, D2 dopamine receptor, GABA-A alpha1 subunit, GRIA 1/3/4, Galphai2, PSD-95 and CREB were down-regulated in the NAc shell with morphine administration. In the core, neuronal apoptotic inhibitory protein (NAIP), GABA-A alpha1 subunit, GRIN2C, GRIA1, mGluR1, D4 dopamine receptor and PSD-95 were upregulated by morphine administration whereas bax, bcl-x, cox-1 and MAP2 were decreased. These data demonstrate that morphine administration alters gene expression differentially in NAc subregions. Specifically, GABA-A alpha1 subunit, GRIA1 subunit and PSD-95 mRNAs were decreased in the shell but increased in the core following morphine administration. In addition, these results provide potential targets for further evaluation in models of morphine reinforcement as well as novel mechanisms of action in morphine-induced pathophysiology.

Participation of the syntaxin 5/Ykt6/GS28/GS15 SNARE complex in transport from the early/recycling endosome to the trans-Golgi network

An in vitro transport assay, established with a modified Shiga toxin B subunit (STxB) as a marker, has proved to be useful for the study of transport from the early/recycling endosome (EE/RE) to the trans-Golgi network (TGN). Here, we modified this assay to test antibodies to all known soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) that have been shown to localize in the Golgi and found that syntaxin 5, GS28, Ykt6, and GS15 antibodies specifically inhibited STxB transport. Because syntaxin 5, GS28, Ykt6, and GS15 exist as a unique SNARE complex, our observation indicates that these four SNAREs function as a complex in EE/RE-TGN transport. The importance of GS15 in EE/RE-TGN transport was further demonstrated by a block in recombinant STxB transport in HeLa cells when GS15 expression was knocked down by its small interfering iRNA. Morphological analyses showed that some GS15 and Ykt6 were redistributed from the Golgi to the endosomes when the recycling endosome was perturbed by SNX3-overexpression, suggesting that GS15 and Ykt6 might cycle between the endosomes and the Golgi apparatus. Further studies indicated that syntaxin 5 and syntaxin 16 exerted their role in EE/RE-TGN transport in an additive manner. The kinetics of inhibition exhibited by syntaxin 16 and syntaxin 5 antibodies is similar.

Caspase-mediated cleavage of syntaxin 5 and giantin accompanies inhibition of secretory traffic during apoptosis

We report the caspase-dependent cleavage of two Golgi-associated transport factors during apoptosis. The tethering factor giantin is rapidly cleaved both in vitro and in vivo at a conserved site, to generate a stable membrane-anchored domain and a soluble domain that is subject to further caspase-dependent cleavage. The t-SNARE syntaxin 5 is also cleaved rapidly, resulting in the separation of the catalytic membrane-proximal domain from an N-terminal regulatory domain. Cleavage of giantin and syntaxin 5 is accompanied by a cessation of vesicular transport between the ER and the Golgi complex, which first manifests itself as a block in ER exit. The contribution that such an inhibition of trafficking may make towards the generation of an apoptotic phenotype is discussed.

Identification of mouse Vps16 and biochemical characterization of mammalian class C Vps complex

Many multiprotein complexes mediate the fusion of the intracellular membranes. The question how the specificity of the membrane fusion is controlled has not been fully elucidated. Here we report the identification of a mouse homologue Vps16p (mVps16), which exhibits a high homology to the yeast Vps16p, a component of Class C vacuolar protein sorting (Vps) complex implicated in the yeast vacuole membrane fusion. Northern and Western blot analyses reveal that mVps16 is ubiquitously expressed in the mouse peripheral tissues. Biochemical analyses show that mammalian Class C Vps proteins interact with multiple syntaxins and Vps45p, which localizes in the endosomal compartments. The internalization of transferrin (Tf) is not affected by the overexpression of mammalian class C Vps proteins, but the recycling was inhibited. Taken together, this study provides biochemical characteristics of mVps16p in mammalian cells and the potential roles of mammalian Class C Vps proteins in membrane trafficking.

Deregulation of gene expression in fetal oocytes exposed to doxorubicin

Doxorubicin is an effective anticancer drug but its use is limited due to its adverse side effects such as infertility and cardiomyopathy. Some possible mechanisms of the action of doxorubicin have been postulated, but the initial gene deregulation response has not been investigated. Fetal life stages are critical periods in mammalian oogenesis. This study analyzes gene expression alterations in mouse fetal oocytes exposed in vitro to this anticancer agent. cDNA libraries were generated from isolated fetal oocytes and differential screenings performed with cDNAs from in vitro doxorubicin-treated and -untreated oocytes. Differentially expressed genes were assessed by real-time RT-PCR to quantify the extent of their transcriptional control in doxorubicin-exposed oocytes. The results show that doxorubicin alters the expression of genes involved in the mitochondrial respiratory chain, intracellular transport and cell differentiation. Finally, the up-regulation of a differentially expressed gene (metaxin) mediated by its promoter was evaluated in a functional assay. When treated with doxorubicin, somatic cells transfected with a genetic construct including the promoter of metaxin and a reporter gene showed increases in expression similar to those observed in fetal oocytes. This demonstrates the direct effect of agent on the regulation of a specific gene.

Syntaxin 1A regulates ENaC via domain-specific interactions

The epithelial sodium channel (ENaC) is a heterotrimeric protein responsible for Na(+) absorption across the apical membranes of several absorptive epithelia. The rate of Na(+) absorption is governed in part by regulated membrane trafficking mechanisms that control the apical membrane ENaC density. Previous reports have implicated a role for the t-SNARE protein, syntaxin 1A (S1A), in the regulation of ENaC current (I(Na)). In the present study, we examine the structure-function relations influencing S1A-ENaC interactions. In vitro pull-down assays demonstrated that S1A directly interacts with the C termini of the alpha-, beta-, and gamma-ENaC subunits but not with the N terminus of any ENaC subunit. The H3 domain of S1A is the critical motif mediating S1A-ENaC binding. Functional studies in ENaC expressing Xenopus oocytes revealed that deletion of the H3 domain of co-expressed S1A eliminated its inhibition of I(Na), and acute injection of a GST-H3 fusion protein into ENaC expressing oocytes inhibited I(Na) to the same extent as S1A co-expression. In cell surface ENaC labeling experiments, reductions in plasma membrane ENaC accounted for the H3 domain inhibition of I(Na). Individually substituting C terminus-truncated alpha-, beta-, or gamma-ENaC subunits for their wild-type counterparts reversed the S1A-induced inhibition of I(Na), and oocytes expressing ENaC comprised of three C terminus-truncated subunits showed no S1A inhibition of I(Na). C terminus truncation or disruption of the C terminus beta-subunit PY motif increases I(Na) by interfering with ENaC endocytosis. In contrast to subunit truncation, a beta-ENaC PY mutation did not relieve S1A inhibition of I(Na), suggesting that S1A does not perturb Nedd4 interactions that lead to ENaC endocytosis/degradation. This study provides support for the concept that S1A inhibits ENaC-mediated Na(+) transport by decreasing cell surface channel number via direct protein-protein interactions at the ENaC C termini.

COPII proteins are required for Golgi fusion but not for endoplasmic reticulum budding of the pre-chylomicron transport vesicle

The budding of vesicles from endoplasmic reticulum (ER) that contains nascent proteins is regulated by COPII proteins. The mechanisms that regulate lipid-carrying pre-chylomicron transport vesicles (PCTVs) budding from the ER are unknown. To study the dependence of PCTV-ER budding on COPII proteins we examined protein and PCTV budding by using ER prepared from rat small intestinal mucosal cells prelabeled with (3)H-oleate or (14)C-oleate and (3)H-leucine. Budded (3)H-oleate-containing PCTVs were separated by sucrose density centrifugation and were revealed by electron microscopy as 142-500 nm vesicles. Our results showed the following: (1) Proteinase K treatment did not degrade the PCTV cargo protein, apolipoprotein B-48, unless Triton X-100 was added. (2) PCTV budding was dependent on cytosol and ATP. (3) The COPII proteins Sar1, Sec24 and Sec13/31 and the membrane proteins syntaxin 5 and rBet1 were associated with PCTVs. (4) Isolated PCTVs were able to fuse with intestinal Golgi. (5) Antibodies to Sar1 completely inhibited protein vesicle budding but increased the generation of PCTV; these changes were reversed by the addition of recombinant Sar1. (6) PCTVs formed in the absence of Sar1 did not contain the COPII proteins Sar1, Sec24 or Sec31 and did not fuse with the Golgi complex. Together, these findings suggest that COPII proteins may not be required for the exit of membrane-bound chylomicrons from the ER but that they or other proteins may be necessary for PCTV fusion with the Golgi.