Immunofluorescent staining of septins in primary cilia.

Primary cilia are cellular antennae that receive and transduce extracellular cues. These microtubule-rich structures are comprised of at least three distinct ciliary compartments: basal bodies, transition zone, and axoneme. Septins have been implicated in cilia function at the transition zone, but accumulating evidence suggests that they localize predominantly within the axoneme. Here, we describe three fixation conditions that preserve the substructure of primary cilia and demonstrate known ciliary proteins that localize to these distinct ciliary substructures. Finally, we show immunostaining and live microscopy methods to detect septins within the axoneme.

Expression of two membrane fusion proteins, synaptosome-associated protein of 25 kDa and vesicle-associated membrane protein, in choroid plexus epithelium.

In addition to being the major site of cerebrospinal fluid formation, the choroid plexus epithelium emerges as an important source of polypeptides in the brain. Physiologically regulated release of some polypeptides synthesized by the choroid plexus has been shown. The molecular mechanisms underlying this polypeptide secretion have not been characterized, however. In the present study, synaptosome-associated protein of 25 kDa and vesicle-associated membrane protein, two membrane fusion proteins playing a critical role in exocytosis in neurons and endocrine cells, were found to be expressed in the choroid plexus epithelium. It was also shown that in choroidal epithelium, synaptosome-associated protein of 25 kDa and vesicle-associated membrane protein stably interact. Two members of the vesicle-associated membrane protein family, vesicle-associated membrane protein-1 and vesicle-associated membrane protein-2, were expressed in the rat choroid plexus at the messenger RNA and protein level. However, their newly discovered isoforms, vesicle-associated membrane protein-1b and vesicle-associated membrane protein-2b, produced by alternative RNA splicing, were not detected in choroidal tissue. Immunohistochemistry demonstrated that vesicle-associated membrane protein is confined to the cytoplasm of choroidal epithelium, whereas synaptosome-associated protein of 25 kDa is associated with plasma membranes, albeit with a varied cellular distribution among species studied. Specifically, in the rat choroid plexus, synaptosome-associated protein of 25 kDa was localized to the basolateral membrane domain of choroidal epithelium and was expressed in small groups of cells. In comparison, in ovine and human choroidal tissues, apical staining for synaptosome-associated protein of 25 kDa was found in the majority of epithelial cells. These species-related differences in cellular synaptosome-associated protein of 25 kDa distribution suggested that the synaptosome-associated protein of 25 kDa homologue, synaptosome-associated protein of 23 kDa, is also expressed in the rat choroid plexus, which was confirmed by reverse-transcriptase polymerase chain reaction. Our findings suggest that synaptosome-associated protein of 25 kDa and vesicle-associated membrane protein are involved in secretion of polypeptides from the choroid plexus epithelium. The presence of synaptosome-associated protein of 25 kDa and its homologue as well as multiple isoforms of vesicle-associated membrane protein in choroidal epithelium may play a role in the apical versus basolateral targeting of secretory vesicles.

Supramaximal cholecystokinin displaces Munc18c from the pancreatic acinar basal surface, redirecting apical exocytosis to the basal membrane.

Exocytosis at the apical surface of pancreatic acinar cells occurs in the presence of physiological concentrations of cholecystokinin (CCK) but is inhibited at high concentrations. Here we show that Munc18c is localized predominantly to the basal membranes of acinar cells. Supramaximal but not submaximal CCK stimulation caused Munc18c to dissociate from the plasma membrane, and this displacement was blocked by protein kinase C (PKC) inhibitors. Conversely, whereas the CCK analog CCK-OPE alone failed to displace Munc18c from the membrane, this agent caused Munc18c displacement following minimal PKC activation. To determine the physiological significance of this displacement, we used the fluorescent dye FM1-43 to visualize individual exocytosis events in real-time from rat acinar cells in culture. We showed that supramaximal CCK inhibition of secretion resulted from impaired apical secretion and a redirection of exocytic events to restricted basal membrane sites. In contrast, CCK-OPE evoked apical exocytosis and could only induce basolateral exocytosis following activation of PKC. Infusion of supraphysiological concentrations of CCK in rats, a treatment that induced tissue changes reminiscent of mild acute pancreatitis, likewise resulted in rapid displacement of Munc18c from the basal membrane in vivo.

Membrane dynamics in phagocytosis.

Engulfment of particles by phagocytes involves remodeling of the plasma membrane. We review recent work that suggests that focal exocytosis of endomembranes plays an important role in pseudopod extension during phagocytosis.

Ca(2+) influx and cAMP elevation overcame botulinum toxin A but not tetanus toxin inhibition of insulin exocytosis.

Previous reports showed that cleavage of vesicle-associated membrane protein-2 (VAMP-2) and synaptosomal-associated protein of 25 kDa (SNAP-25) by clostridial neurotoxins in permeabilized insulin-secreting beta-cells inhibited Ca(2+)-evoked insulin secretion. In these reports, the soluble N-ethylmaleimide-sensitive factor attachment protein target receptor proteins might have formed complexes, which preclude full accessibility of the putative sites for neurotoxin cleavage. In this work, VAMP-2 and SNAP-25 were effectively cleaved before they formed toxin-insensitive complexes by transient transfection of insulinoma HIT or INS-1 cells with tetanus toxin (TeTx) or botulinum neurotoxin A (BoNT/A), as shown by immunoblotting and immunofluorescence microscopy. This resulted in an inhibition of Ca(2+) (glucose or KCl)-evoked insulin release proportionate to the transfection efficiency (40-50%) and an accumulation of insulin granules. With the use of patch-clamp capacitance measurements, Ca(2+)-evoked exocytosis by membrane depolarization to -10 mV was abolished by TeTx (6% of control) but only moderately inhibited by BoNT/A (30% of control). Depolarization to 0 mV to maximize Ca(2+) influx partially overcame BoNT/A (50% of control) but not TeTx inhibition. Of note, cAMP activation potentiated Ca(2+)-evoked secretion by 129% in control cells but only 55% in BoNT/A-transfected cells and had negligible effects in TeTx-transfected cells. These results indicate that, whereas VAMP-2 is absolutely necessary for insulin exocytosis, the effects of SNAP-25 depletion on exocytosis, perhaps on insulin granule pool priming or mobilization steps, could be partially reversed by higher levels of Ca(2+) or cAMP potentiation.

Cholecystokinin-regulated exocytosis in rat pancreatic acinar cells is inhibited by a C-terminus truncated mutant of SNAP-23.

INTRODUCTION: Exocytosis is thought to result from the fusion of vesicle and plasma membranes caused by the formation of a trans-complex between proteins of the vesicle-associated membrane protein (VAMP) family on the vesicle with members of the syntaxin and synaptosomal-associated protein of 25 kd (SNAP-25) families on the plasma membrane. In the pancreatic acinar cell, synaptosomal-associated protein of 23 kd (SNAP-23) is the major SNAP-25 isoform expressed in pancreatic acinar cells, but its role in acinar cell exocytosis has not been determined. AIMS: To examine the role of SNAP-23 in regulated exocytosis in acinar cells, we subcloned into adenoviral vectors SNAP-23, SNAP-25, and dominant negative mutants in which the C-terminal domains corresponding to the botulinum neurotoxin A cleavage sites are deleted. METHODOLOGY AND RESULTS: High-efficiency infection of rat pancreatic acini in culture with these adenoviruses by subcellular fractionation showed that the overexpressed SNAP-23, SNAP-25, and their truncated mutant proteins were uniformly targeted to the zymogen granules and plasma membrane. To maximally stimulate apical exocytosis from these infected acini, we used the cholecystokinin-phenylethyl ester analog (CCK-OPE), which does not show inhibition of secretion from maximal levels at high doses. CCK-OPE-stimulated amylase release from adenovirus-cytomegalovirus (AdCMV)-SNAP-23 or AdCMV-SNAP-25-infected acini to the same extent as from acini infected with the empty vector. In contrast, CCK-OPE-evoked enzyme secretion from AdCMV-SNAP-23deltaC8- and AdCMV-SNAP-25(1-197)-infected acini were inhibited by 60% and 40%, respectively. The identical targeting of the mutant SNAP-23 and SNAP-25 proteins to the same membrane compartments as SNAP-23 suggests that the inhibition of secretion was a result of their competition against endogenous SNAP-23. This is supported by the fact that this inhibition by the mutant proteins was partially reversed or rescued when the AdCMV-SNAP-25AC8- or AdCMV-SNAP-25(1-197)-infected acini were co-infected with wild-type SNAP-23 or SNAP-25. CONCLUSION: From these results, we conclude that SNAP-23 plays a role in CCK-evoked regulated exocytosis in the acinar cells.

VAP-A binds promiscuously to both v- and tSNAREs.

Proteins that bind to SNAREs may regulate their function. One such protein, VAP-33, was first discovered in Aplysia californica and has two mammalian homologues, VAP-A and VAP-B. VAP-A has been implicated in vesicle targeting to the plasma membrane based on its location in polarized cells and its ability to bind VAMP in vitro. Here, we demonstrate that VAP-A is a widely expressed resident of the ER/Golgi intermediate compartment in COS-7 cells. Moreover, we demonstrate that VAMP-binding and VAP-dimerization require both the N- and C-terminal domains of VAP-A and also that VAP-A binds to a wide range of SNAREs and fusion-related proteins including syntaxin 1A, rbet1, rsec22, alphaSNAP, and NSF. Together, these results suggest that VAP-A is not a regulator of a specific VAMP, but rather may play a more general role in SNARE-mediated vesicle traffic between the ER and Golgi in nonpolarized cells.

Analysis of the mutant Drosophila N-ethylmaleimide sensitive fusion-1 protein in comatose reveals molecular correlates of the behavioural paralysis.

NEM-sensitive fusion protein (NSF) is an ATPase required for many intracellular membrane trafficking steps. Recent studies have suggested that NSF alters the conformation of the SNAP receptors (SNAREs) to permit their interaction, or to uncouple them after they interact. Most organisms have a single NSF gene product but Drosophila express two highly related isoforms, dNSF-1 and dNSF-2. dNSF-1 is encoded by the gene comatose (comt), first identified as the locus of a temperature-sensitive paralytic mutation. Here we show that dNSF-1 is most abundant in the nervous system and can be detected in larval and adult CNS. Subcellular fractionation revealed that dNSF-1 was enriched in a vesicle fraction along with the synaptic vesicle protein synaptotagmin. comt flies maintained at the non-permissive temperature rapidly accumulate sodium dodecyl sulfate (SDS)-resistant SNARE complexes at the restrictive temperature, with concomitant translocation of dNSF-1 from cytosol and membrane fractions into a Triton X-100 insoluble fraction. The long recovery of comt flies after heat shock induced paralysis correlated with the irreversibility of this translocation. Interestingly, while dNSF-1 also translocates in comt(TP7) larvae, there is no associated neurophysiological phenotype at the neuromuscular junction (nmj) or accumulation of SDS-resistant complexes in the CNS. Together, these results suggest that dNSF-1 is required for adult neuronal function, but that in the larval nmj function may be maintained by other isoforms.

SNARE-dependent signaling at the Drosophila wing margin.

The wing of Drosophila melanogaster has long been used as a model system to characterize intermolecular interactions important in development. Implicit in our understanding of developmental processes is the proper trafficking and sorting of signaling molecules, although the precise mechanisms that regulate membrane trafficking in a developmental context are not well studied. We have therefore chosen the Drosophila wing to assess the importance of SNARE-dependent membrane trafficking during development. N-Ethylmaleimide-sensitive fusion protein (NSF) is a key component of the membrane-trafficking machinery and we constructed a mutant form of NSF whose expression we directed to the developing wing margin. This resulted in a notched-wing phenotype, the severity of which was enhanced when combined with mutants of VAMP/Synaptobrevin or Syntaxin, indicating that it results from impaired membrane trafficking. Importantly, we find that the phenotype is also enhanced by mutations in genes for wingless and components of the Notch signaling pathway, suggesting that these signaling pathways were disrupted. Finally, we used this phenotype to conduct a screen for interacting genes, uncovering two Notch pathway components that had not previously been linked to wing development. We conclude that SNARE-mediated membrane trafficking is an important component of wing margin development and that dosage-sensitive developmental pathways will act as a sensitive reporter of partial membrane-trafficking disruption.

Activation of synaptic NMDA receptors induces membrane insertion of new AMPA receptors and LTP in cultured hippocampal neurons.

Long-term potentiation (LTP) of excitatory transmission in the hippocampus likely contributes to learning and memory. The mechanisms underlying LTP at these synapses are not well understood, although phosphorylation and redistribution of AMPA receptors may be responsible for this form of synaptic plasticity. We show here that miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons reliably demonstrate LTP when postsynaptic NMDA receptors are briefly stimulated with glycine. LTP of these synapses is accompanied by a rapid insertion of native AMPA receptors and by increased clustering of AMPA receptors at the surface of dendritic membranes. Both LTP and glycine-facilitated AMPA receptor insertion are blocked by intracellular tetanus toxin (TeTx), providing evidence that AMPA receptors are inserted into excitatory synapses via a SNARE-dependent exocytosis during LTP.

Requirement for N-ethylmaleimide-sensitive factor activity at different stages of bacterial invasion and phagocytosis.

Bacterial invasion, like the process of phagocytosis, involves extensive and localized protrusion of the host cell plasma membrane. To examine the molecular mechanisms of the membrane remodeling that accompanies bacterial invasion, soluble NSF attachment protein receptor (SNARE)-mediated membrane traffic was studied in cultured cells during infection by Salmonella typhimurium. A green fluorescent protein-tagged chimera of VAMP3, a SNARE characteristic of recycling endosomes, was found to accumulate at sites of Salmonella invasion. To analyze the possible role of SNARE-mediated membrane traffic in bacterial infection, invasion was measured in cells expressing a dominant-negative form of N-ethylmaleimide-sensitive factor (NSF), an essential regulator of membrane fusion. Inhibition of NSF activity did not affect cellular invasion by S. typhimurium nor the associated membrane remodeling. By contrast, Fcgamma receptor-mediated phagocytosis was greatly reduced in the presence of the mutant NSF. Most important, dominant-negative NSF significantly impaired the fusion of Salmonella-containing vacuoles with endomembranes. These observations indicate that the membrane protrusions elicited by Salmonella invasion, unlike those involved in phagocytosis, occur via an NSF-independent mechanism, whereas maturation of Salmonella-containing vacuoles is NSF-dependent.

SNARE proteins contribute to calcium cooperativity of synaptic transmission.

A hallmark of calcium-triggered synaptic transmission is the cooperative relationship between calcium and the amount of transmitter released. This relationship is thought to be important for improving the efficiency of synaptic vesicle exocytosis. Although it is generally held that cooperativity arises from the interaction of multiple calcium ions with a single calcium-sensing molecule, the precise molecular basis of this phenomenon is not known. The SNARE proteins are known to be critical for synaptic vesicle exocytosis. We therefore tested for a contribution of SNARE proteins to cooperativity by genetically reducing the levels of syntaxin IA and neuronal-synaptobrevin in Drosophila. Surprisingly, we found that reducing these SNARE proteins also reduced Ca(2+) cooperativity. Thus, SNARE proteins are important for determining the cooperative relationship between calcium and synaptic transmission.

VAMP2, but not VAMP3/cellubrevin, mediates insulin-dependent incorporation of GLUT4 into the plasma membrane of L6 myoblasts.

Like neuronal synaptic vesicles, intracellular GLUT4-containing vesicles must dock and fuse with the plasma membrane, thereby facilitating insulin-regulated glucose uptake into muscle and fat cells. GLUT4 colocalizes in part with the vesicle SNAREs VAMP2 and VAMP3. In this study, we used a single-cell fluorescence-based assay to compare the functional involvement of VAMP2 and VAMP3 in GLUT4 translocation. Transient transfection of proteolytically active tetanus toxin light chain cleaved both VAMP2 and VAMP3 proteins in L6 myoblasts stably expressing exofacially myc-tagged GLUT4 protein and inhibited insulin-stimulated GLUT4 translocation. Tetanus toxin also caused accumulation of the remaining C-terminal VAMP2 and VAMP3 portions in Golgi elements. This behavior was exclusive to these proteins, because the localization of intracellular myc-tagged GLUT4 protein was not affected by the toxin. Upon cotransfection of tetanus toxin with individual vesicle SNARE constructs, only toxin-resistant VAMP2 rescued the inhibition of insulin-dependent GLUT4 translocation by tetanus toxin. Moreover, insulin caused a cortical actin filament reorganization in which GLUT4 and VAMP2, but not VAMP3, were clustered. We propose that VAMP2 is a resident protein of the insulin-sensitive GLUT4 compartment and that the integrity of this protein is required for GLUT4 vesicle incorporation into the cell surface in response to insulin.

Focal exocytosis of VAMP3-containing vesicles at sites of phagosome formation.

Phagocytosis involves the receptor-mediated extension of plasmalemmal protrusions, called pseudopods, which fuse at their tip to engulf a particle. Actin polymerizes under the nascent phagosome and may propel the protrusion of pseudopods. Alternatively, membrane extension could result from the localized insertion of intracellular membranes into the plasmalemma next to the particle. Here we show focal accumulation of VAMP3-containing vesicles, likely derived from recycling endosomes, in the vicinity of the nascent phagosome. Using green fluorescent protein (GFP) as both a fluorescent indicator and an exofacial epitope tag, we show that polarized fusion of VAMP3 vesicles precedes phagosome sealing. It is therefore likely that targeted delivery of endomembranes contributes to the elongation of pseudopods. In addition to mediating pseudopod formation, receptor-triggered focal secretion of endosomes may contribute to polarized membrane extension in processes such as lamellipodial elongation or chemotaxis.

A functional role for VAP-33 in insulin-stimulated GLUT4 traffic.

Soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) are critical proteins in membrane fusion, in both regulated and constitutive vesicular traffic. In addition, proteins that interact with the SNAREs are thought to regulate fusion. Vesicle-associated membrane protein-2 (VAMP-2) is a SNARE protein involved in insulin-dependent glucose transporter 4 (GLUT4) traffic. VAMP-2 is required for productive GLUT4 incorporation into the plasma membrane. VAMP-associated protein of 33 kDa (VAP-33) is an integral membrane protein that binds VAMPs in vitro, and is hypothesized to be a regulator of VAMPs. In L6 skeletal myoblasts, which display insulin-dependent traffic of GLUT4, we show that VAP-33 colocalized significantly with VAMP-2 using indirect confocal immunofluorescence and biochemical cosegregation. Overexpression of wild-type VAP-33 in L6 myoblasts attenuated the insulin-dependent incorporation of myc-tagged GLUT4 into the plasma membrane, and this response was restored by co-overexpression of VAMP-2 linked to green fluorescent protein. Antibodies to VAP-33 microinjected into 3T3-L1 adipocytes abrogated the insulin-stimulated translocation of GLUT4 to the plasma membrane, as measured in adhered plasma membrane lawns. Immunopurified VAMP-2-containing compartments from L6 myotubes and 3T3-L1 adipocytes showed significant levels of VAP-33. We propose that VAP-33 may be a regulator of VAMP-2 availability for GLUT4 traffic and other vesicle fusion events.

Phosphatidylinositol polyphosphate binding to the mammalian septin H5 is modulated by GTP.

BACKGROUND: Septins are members of a conserved family of GTPases found in organisms as diverse as budding yeast and mammals. In budding yeast, septins form hetero-oligomeric filaments that lie adjacent to the membrane at the mother-bud neck, whereas in mammals, they concentrate at the cleavage furrow of mitotic cells; in both cases, septins provide a required function for cytokinesis. What directs the location and determines the stability of septin filaments, however, remains unknown. RESULTS: Here we show that the mammalian septin H5 is associated with the plasma membrane and specifically binds the phospholipids phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P(2)) and phosphatidylinositol 3,4, 5-trisphosphate (PtdIns(3,4,5)P(3)). Deletion analysis revealed that this binding occurs at a site rich in basic residues that is conserved in most septins and is located adjacent to the GTP-binding motif. Phosphoinositide binding was inhibited by mutations within this motif and was also blocked by agents known to associate with PtdInsP(2) or by a peptide corresponding to the predicted PtdInsP(2)-binding sequence of H5. GTP binding and hydrolysis by H5 significantly reduced its PtdInsP(2)-binding capability. Treatment of cells with agents that occluded, dephosphorylated or degraded PtdInsP(2) altered the appearance and localization of H5. CONCLUSIONS: These results indicate that the interaction of septins with PtdInsP(2) might be an important cellular mechanism for the spatial and temporal control of septin accumulation.

Identification of the endoplasmic reticulum targeting signal in vesicle-associated membrane proteins.

The vesicle-associated membrane proteins (Vamp(s)) function as soluble N-ethylmaleimide-sensitive factor attachment receptor proteins in the intracellular trafficking of vesicles. The membrane attachment of Vamps requires a carboxyl-terminal hydrophobic sequence termed an insertion sequence. Unlike other insertion sequence-containing proteins, targeting of the highly homologous Vamp1 and Vamp2 to the endoplasmic reticulum requires ATP and a membrane-bound receptor. To determine if this mechanism of targeting to the endoplasmic reticulum extends to other Vamps, we compared the membrane binding of Vamp1 and Vamp2 with the distantly related Vamp8. Similar to the other Vamps, Vamp8 requires both ATP and a membrane component to target to the endoplasmic reticulum. Furthermore, binding curves for the three Vamps overlap, suggesting a common receptor-mediated process. We identified a minimal endoplasmic reticulum targeting domain that is both necessary and sufficient to confer receptor-mediated, ATP-dependent, binding of a heterologous protein to microsomes. Surprisingly, this conserved sequence includes four positively charged amino acids spaced along an amphipathic sequence, which unlike the carboxyl-terminal targeting sequence in mitochondrial Vamp isoforms, is amino-terminal to the insertion sequence. Because Vamps do not bind to phospholipid vesicles, it is likely that these residues mediate an interaction with a protein, rather than bind to acidic phospholipids. Therefore, we suggest that a bipartite motif is required for the specific targeting and integration of Vamps into the endoplasmic reticulum with receptor-mediated recognition of specifically configured positive residues leading to the insertion of the hydrophobic tail into the membrane.

Snare protein expression and adenoviral transfection of amphicrine AR42J.

The amphicrine AR42J acinar cell line is an excellent model to study both exocrine and neuroendocrine exocytotic mechanisms. As a first step toward this goal, we determined the specific isoforms of the v- and t-SNARE and Munc18 families expressed in these cells. In addition, we show that dexamethasone-induced differentiation toward the exocrine phenotype causes an upregulation of several of these proteins. AR42J is notoriously difficult to transfect, limiting its usefulness as a model. However, we have now overcome this obstacle by acheiving high efficiency expression of a beta-galactosidase reporter gene and truncated SNAP-25 gene using adenoviral infection techniques. The AR42J cells can now be used to pursue and elucidate the distinct functions of individual SNARE isoforms used in endocrine and exocrine secretion within a single cell line.

The septin CDCrel-1 binds syntaxin and inhibits exocytosis.

Septins are GTPases required for the completion of cytokinesis in diverse organisms, yet their roles in cytokinesis or other cellular processes remain unknown. Here we describe studies of a newly identified septin, CDCrel-1, which is predominantly expressed in the nervous system. This protein was associated with membrane fractions, and a significant fraction of the protein copurified and coprecipitated with synaptic vesicles. In detergent extracts, CDCrel-1 and another septin, Nedd5, immunoprecipitated with the SNARE protein syntaxin by directly binding to syntaxin via the SNARE interaction domain. Transfection of HIT-T15 cells with wild-type CDCrel-1 inhibited secretion, whereas GTPase dominant-negative mutants enhanced secretion. These data suggest that septins may regulate vesicle dynamics through interactions with syntaxin.