Tissue distribution of SNAP-23 and its subcellular localization in 3T3-L1 cells

The SNARE hypothesis of vesicular traffic proposes that three proteins, VAMP/synaptobrevin, syntaxin, and SNAP-25, constitute a complex that docks the vesicle at the target membrane. VAMP and syntaxin isoforms have been identified outside the nervous system, and a cDNA to a SNAP-25 related protein, SNAP-23, was recently identified in human lymphocytes. Here we report the generation of isoform-specific antibodies to SNAP-23 cloned from human melanoma cells, and their use in detecting the expression and localization of the endogenous SNAP-23 protein in several tissues and cell lines. SNAP-23 was readily detected in liver, lung, kidney, and spleen, to a lesser extent in muscle and heart, and was almost undetectable in brain. The protein was also abundant in fibroblast, muscle, and fat cell lines, but relatively less enriched in neuroendocrine PC12 cells. SNAP-23 abundance did not change during differentiation of 3T3-L1 fibroblasts into adipocytes. In both, SNAP-23 was membrane-bound and below detectable levels in the cytosolic fraction. Subcellular fractionation of 3T3-L1 adipocytes revealed that the majority of the protein was associated with plasma membranes. These findings support the conclusion that a tripartite SNARE complex exists outside of the nervous system, and suggest that SNAP-23 may play a role in vesicle traffic in most cell types.

Distinct cellular locations of the syntaxin family of proteins in rat pancreatic acinar cells

Syntaxins are cytoplasmically oriented integral membrane soluble NEM-sensitive factor receptors (SNAREs; soluble NEM-sensitive factor attachment protein receptors) thought to serve as targets for the assembly of protein complexes important in regulating membrane fusion. The SNARE hypothesis predicts that the fidelity of vesicle traffic is controlled in part by the correct recognition of vesicle SNAREs with their cognate target SNARE partner. Here, we show that in the exocrine acinar cell of the pancreas, multiple syntaxin isoforms are expressed and that they appear to reside in distinct membrane compartments. Syntaxin 2 is restricted to the apical plasma membrane whereas syntaxin 4 is found most abundantly on the basolateral membranes. Surprisingly, syntaxin 3 was found to be localized to a vesicular compartment, the zymogen granule membrane. In addition, we show that these proteins are capable of specific interaction with vesicle SNARE proteins. Their nonoverlapping locations support the general principle of the SNARE hypothesis and provide new insights into the mechanisms of polarized secretion in epithelial cells.

The vesicle-associated membrane protein family of proteins in rat pancreatic and parotid acinar cells

BACKGROUND & AIMS

The vesicle-associated membrane protein (VAMP) family of proteins may play an important role in regulating enzyme secretion from pancreatic and parotid acini. The purpose of this study was to characterize the isoforms produced in pancreatic and parotid acini and determine their subcellular locations.

METHODS

Using a battery of specific antisera and recombinant tetanus toxin light chain (which cleaves VAMP-2 and cellubrevin), the presence of each VAMP molecule in the acini was determined by immunoblotting of subcellular membrane fractions; their localization was determined by confocal immunofluorescence microscopy and immunogold electron microscopy.

RESULTS

Both VAMP-2 and cellubrevin were present on both the zymogen granule membrane and plasma membrane. VAMP-1 was not present in the acinar cell but was found in the nerve endings innervating the acini. As expected, pancreatic acinar VAMP-2 and cellubrevin were sensitive to cleavage by recombinant tetanus toxin.

CONCLUSIONS

VAMP-2 and cellubrevin may play integral roles in exocytosis of the pancreatic and parotid acinar cells, whereas VAMP-1 is restricted to nerves that innervate the acini and may function to modulate exocrine activity.

Characterization of SNARE protein expression in beta cell lines and pancreatic islets

Pancreatic beta cells and cell lines were used in the present study to test the hypothesis that the molecular mechanisms controlling exocytosis from neuronal cells may be used by the beta cell to regulate insulin secretion. Using specific antisera raised against an array of synaptic proteins (SNAREs) implicated in the control of synaptic vesicle fusion and exocytosis, we have identified the expression of several SNAREs in the islet beta cell lines, beta TC6-f7 and HIT-T15, as well as in pancreatic islets. The v-SNARE vesicle-associated membrane protein (VAMP)-2 but not VAMP-1 immunoreactive proteins were detected in beta TC6-f7 and HIT-T15 cells and pancreatic islets. In these islet-derived cell lines, this 18-kDa protein comigrated with rat brain synaptic vesicle VAMP-2, which was cleaved by Tetanus toxin (TeTx). Immunofluorescence confocal microscopy and electron microscopy localized the VAMP-2 to the cytoplasmic side of insulin containing secretory granule membrane. In streptolysin O permeabilized HIT-T15 cells, TeTx inhibited Ca2+-evoked insulin release by 83 +/- 4.3%, which correlated well to the cleavage of VAMP-2. The beta cell lines were also shown to express a second vesicle (v)-SNARE, cellubrevin. The proposed neuronal target (t)-membrane SNAREs, SNAP-25, and syntaxin isoforms 1-4 were also detected by Western blotting. The beta cell 25-kDa SNAP-25 protein and syntaxin isoforms 1-3 were specifically cleaved by botulinum A and C toxins, respectively, as observed with the brain isoforms. These potential t-SNARES were localized by immunofluorescence microscopy primarily to the plasma membrane in beta cell lines as well as in islet beta cells. To determine the specific identity of the immunoreactive syntaxin-2 and -3 isoforms and to explore the possibility that these beta cells express the putative Ca2+-sensing molecule synaptotagmin III, RT-PCR was performed on the beta cell lines. These studies confirmed that betaTC6-F7 cells express syntaxin-2 isoforms, 2 and 2', but not 2'' and express syntaxin-3. They further demonstrate the expression of synaptotagmin III. DNA sequence analysis revealed that rat and mouse beta cell syntaxins 2, 2' and synaptotagmin III are highly conserved at the nucleotide and predicted amino acid levels (95-98%). The presence of VAMP-2, nSec/Munc-18, SNAP-25 and syntaxin family of proteins, along with synaptotagmin III in the islet cells and in beta cell lines provide evidence that neurons and beta cells share similar molecular mechanisms for Ca2+-regulated exocytosis. The inhibition of Ca2+-evoked insulin secretion by the proteolytic cleavage of HIT-T15 cell VAMP-2 supports the hypothesis that these proteins play an integral role in the control of insulin exocytosis.

Identification of a second homolog of N-ethylmaleimide-sensitive fusion protein that is expressed in the nervous system and secretory tissues of Drosophila

N-Ethylmaleimide-sensitive fusion protein (NSF) is an ATPase known to have an essential role in intracellular membrane transport events. Recently, cDNA clones encoding a Drosophila melanogaster homolog of this protein, named dNSF, were characterized and found to be expressed in the nervous system. We now report the identification of a second homolog of NSF, called dNSF-2 within this species and report evidence that this ubiquitous and widely utilized fusion protein belongs to a multigene family. The predicted amino acid sequence of dNSF-2 is 84.5% identical to dNSF (hereafter named dNSF-1), 59% identical to NSF from Chinese hamster, and 38.5% identical to the yeast homolog SEC18. The highest similarity was found in a region of dNSF-2 containing one of two ATP-binding sites; this region is most similar to members of a superfamily of ATPases. dNSF-2 is localized to a region between bands 87F12 and 88A3 on chromosome 3, and in situ hybridization techniques revealed expression in the nervous system during embryogenesis and in several imaginal discs and secretory structures in the larvae. Developmental modulation of dNSF-2 expression suggests that quantitative changes in the secretory apparatus are important in histogenesis.

Botulinum G neurotoxin cleaves VAMP/synaptobrevin at a single Ala-Ala peptide bond

Similarly to other serotypes, botulinum neurotoxin serotype G (BoNT/G) contains the zinc binding motif of zinc endopeptidases. Highly purified preparations of BoNT/G show a zinc-dependent protease activity specific for VAMP/synaptobrevin, a membrane protein of synaptic vesicles. The two neuronal VAMP isoforms are cleaved with similar rates at one Ala-Ala peptide bond present in the same region, out of the several such peptide bonds present in their sequences. This site of cleavage is unique among the eight clostridial neurotoxins. VAMP proteolysis is displayed only after reduction of the single interchain disulfide bond present in the toxin, and it is inhibited by EDTA, o-phenanthroline and captopril.

Tetanus toxin light chain cleaves a vesicle-associated membrane protein (VAMP) isoform 2 in rat pancreatic zymogen granules and inhibits enzyme secretion

Pancreatic acinar cells, a model cell type for the study of exocytosis in non-excitable cells, were used here to test the hypothesis that molecular mechanisms regulating exocytosis from neuronal and neuroendocrine cells may also be utilized in exocrine cells. Using specific antisera raised against vesicle-associated membrane protein (VAMP) isoforms 1 and 2, we have identified VAMP-2, but not VAMP-1, immunoreactive proteins on rat pancreatic acinar cell secretory (zymogen) granules. This 18-kDa protein co-migrated with rat brain synaptic vesicle VAMP-2. Tetanus toxin (TeTx) light chain caused complete cleavage of this protein, which was prevented by the addition of EDTA. This toxin also inhibited in a dose-dependent manner Ca(2+)-stimulated enzyme secretion by up to approximately 30% in streptolysin O-permeabilized acini. This effect of TeTx on secretion was prevented by the zinc endopeptidase inhibitor captopril or by boiling the toxin. Incomplete inhibition of secretion by TeTx may suggest that TeTx-insensitive or VAMP-2-independent mechanisms also regulate exocytosis. In support, TeTx light chain was without effect on Rab3AL (effector domain peptide of Rab3)-mediated potentiation of Ca(2+)-stimulated secretion. These results indicate that a TeTx-sensitive VAMP-2-like protein on zymogen granules participates in Ca(2+)-regulated pancreatic enzyme secretion and that undefined Rab3AL-activated mechanisms may act independently to regulate exocytosis.

Synaptic regulation of glial protein expression in vivo

We investigated signaling between individual nerve terminals and perisynaptic Schwann cells, the teloglial cells that cover neuromuscular junctions. When deprived of neuronal activity in vivo, either by motor nerve transection or tetrodotoxin injection, perisynaptic Schwann cells rapidly up-regulated glial fibrillary acidic protein. Addition of transcription or translation inhibitors to excised muscles prevented this increase. Stimulation of cut nerves prevented glial fibrillary acidic protein increases even when postsynaptic nicotinic receptors were blocked, but not when neurotransmitter release was blocked with omega-conotoxin GVIA. We conclude that there is a nerve terminal to glial signal, requiring presynaptic neurotransmitter release, which regulates perisynaptic Schwann cell genes. This may be a general principle since many types of glial are sensitive to transmitters applied in vitro or released in situ.

Analysis of the structure and expression of the VAMP family of synaptic vesicle proteins

VAMP/synaptobrevin proteins were first discovered as small integral membrane proteins in synaptic vesicles of vertebrates and invertebrates. At least two isoforms are expressed in the central nervous system of mammals in non-overlapping patterns. Biochemical studies have revealed that the VAMP synaptic vesicle proteins are the specific target in the presynaptic nerve terminal of botulinum B neurotoxin and tetanus toxin metalloendoprotease activities. The fact that these toxins rapidly and completely abrogate neurotransmission suggests that VAMP proteins play an essential role in this process. More recently, immunologically related proteins have been identified in non-neuronal cells such as adipocytes. In addition, molecular genetic studies of yeast secretion have identified VAMP-related proteins as playing important roles in vesicular transport between the endoreticulum and Golgi. Taken together, these results suggest that the VAMP proteins found on synaptic vesicles might represent specialized forms of proteins which participate in general aspects of cell membrane trafficking.

Members of the VAMP family of synaptic vesicle proteins are components of glucose transporter-containing vesicles from rat adipocytes

Existing data support the hypothesis that insulin triggers the exocytosis of small vesicles containing the GluT4 isoform of the glucose transporter. The data also suggest that these vesicles reform through endocytosis of GluT4. These processes resemble those described for synaptic vesicles after depolarization of nerve cells. To determine whether GluT4 vesicles are related to synaptic vesicles, rat adipocyte low density microsomes (LDM), which are rich in GluT4 vesicles, were screened for the synaptic vesicle proteins synaptotagmin, synaptophysin, SV2, p29, rab3, and VAMP (synaptobrevin) by immunoblotting. Two polypeptides that reacted with antibodies against the VAMPs were identified, one with the same apparent size as the two isoforms of VAMP in the brain (18 kDa) and one that was slightly smaller (17 kDa). These members of the VAMP family were highly enriched in GluT4 vesicles isolated by immunoadsorption and translocated from the LDM to the plasma membrane in response to insulin. With the exception of rab3, which was observed in the LDM but was not localized in the GluT4 vesicles, the other synaptic vesicle proteins were not detected. The presence of the VAMPs in both GluT4 and synaptic vesicles suggests that the genesis and/or exocytosis of these two types of vesicles involve shared processes.

Distinct patterns of expression of two VAMP genes within the rat brain

VAMPs are synaptic vesicle-specific proteins composed of a carboxy-terminal hydrophobic membrane anchor and an approximately 100 amino acid domain oriented towards the cytoplasm. In rat, two 77% homologous VAMP genes are expressed in the CNS. To precisely localize the neurons expressing these 2 forms of VAMP, we have used RNA blotting and in situ hybridization histochemistry with RNA probes specific for the 3' untranslated regions of the transcripts. These experiments revealed that the 2 genes are expressed in distict, but slightly overlapping, patterns in the rat brain. VAMP-1 expression is localized to a limited number of nuclei, particularly those involved in modulating somatomotor functions, while VAMP-2 expression is more ubiquitous, being found in nuclei associated with autonomic, sensory, and integrative roles. These data suggest that the specific structural features of individual VAMPs may play an important role in synaptic vesicle metabolism.

Two vesicle-associated membrane protein genes are differentially expressed in the rat central nervous system

Vesicle-associated membrane protein (VAMP) 1 is a 120-amino acid protein which co-purifies with cholinergic synaptic vesicles from the marine ray Torpedo californica. We used the Torpedo gene to isolate two independent classes of VAMP cDNA clones from rat brain. Nucleotide sequence analysis of the cDNAs predicts proteins which are 84 and 75% homologous to Torpedo VAMP-1. The amino-terminal 24-28 amino acid residues which comprise the proline-rich head are only about 50% homologous between the different VAMPs, yet the proline-rich character is maintained. The 69 amino acids which comprise the hydrophilic core are highly homologous to Torpedo VAMP-1, with only 2 amino acid substitutions in rat VAMP-1 and 6 in rat VAMP-2. The carboxyl-terminal 23 amino acids of all of the VAMP proteins maintain the hydrophobic character necessary to serve as a membrane anchor. Both VAMP transcripts are expressed differentially in the rat central nervous system. Whereas VAMP-2 is more highly expressed in the whole brain, VAMP-1 is expressed at a higher level in the spinal cord.

VAMP-1: a synaptic vesicle-associated integral membrane protein

Several proteins are associated with, or are integral components of, the lipid bilayer that forms the delineating membrane of neuronal synaptic vesicles. To characterize these molecules, we used a polyclonal antiserum raised against purified cholinergic synaptic vesicles from Torpedo to screen a cDNA expression library constructed from mRNA of the electromotor nucleus. One clone encodes VAMP-1 (vesicle-associated membrane protein 1), a nervous-system-specific protein of 120 amino acids whose primary sequence can be divided into three domains: a proline-rich amino terminus, a highly charged internal region, and a hydrophobic carboxyl-terminal domain that is predicted to comprise a membrane anchor. Tryptic digestion of intact and lysed vesicles suggests that the protein faces the cytoplasm, where it may play a role in packaging, transport, or release of neurotransmitters.

Deletion analysis of the c-Ha-ras oncogene promoter

Using transient assays of chloramphenicol acetyltransferase expression in CV-1 cells, we have localized the human c-Ha-ras oncogene promoter to a 550 bp fragment located about 1 kb upstream from the ras coding sequence. Deletion analysis has revealed that a 100 nucleotide region 200 base pairs upstream of the putative initiation sites for transcription is essential for high levels of expression. Within this sequence lie two Spl binding sites and a consensus CCAAT box.

Morphological transformation and tumorigenicity in C3H/10T1/2 cells transformed with an inducible c-Ha-ras oncogene

A C3H/10T1/2 cell line containing an inducible metallothionein-ras hybrid oncogene was conditionally and reversibly transformed upon exposure to zinc ions. Interestingly, although the cell line was fully malignant when expressing only low levels of ras, complete morphological transformation required much higher levels.

Enhanced lytic susceptibility of Ha-ras transformants after oncogene induction is specific to activated NK cells

C3H 10T1/2 mouse fibroblasts were transfected with a plasmid vector composed of EJ, the mutated c-Ha-ras, and a metallothionein promotor that induced amplified ras expression when activated by culture in the presence of zinc. Experiments were conducted to compare the effect of induction on killing by activated natural killer (NK) cells, cytotoxic T lymphocytes, activated macrophages, and antibody plus complement. The only effector that recognized increased ras expression and exhibited high-inducible cytolysis was an activated NK cell. The effectors from spleen were poly I.C. boostable, Lyt-1.1 negative, NK 1.2 positive, and asialo GM1 positive. Spleen cells from T cell-deficient nude mice, but not NK-deficient beige mice, exhibited high levels of killing activity, and experiments with NK cell clones demonstrated that these lines were also highly cytolytic and killed Ha-ras transfectants in parallel to YAC. Transfection of the same fibroblast line with c-myc did not alter the level of activated NK sensitivity. Cold target competition experiments revealed that Ha-ras-transfected and non-transfected 10T1/2 fibroblasts competed equally for lysis of either YAC or Ha-ras transfectants. Rat-1 fibroblasts did not compete, but gained this capacity when transformed with the v-Ki-ras oncogene but not v-fps. These data suggest that Ha-ras acts in target cells at a post-binding step, whereas Ki-ras may affect expression of target-effector binding structures. The findings that activated NK cell lysis may be specifically influenced by ras expression support a role for NK cells in host surveillance against early neoplastic changes.

Enhanced lytic susceptibility of Ha-ras transformants after oncogene induction is specific to activated NK cells

C3H 10T1/2 mouse fibroblasts were transfected with a plasmid vector composed of EJ, the mutated c-Ha-ras, and a metallothionein promotor that induced amplified ras expression when activated by culture in the presence of zinc. Experiments were conducted to compare the effect of induction on killing by activated natural killer (NK) cells, cytotoxic T lymphocytes, activated macrophages, and antibody plus complement. The only effector that recognized increased ras expression and exhibited high-inducible cytolysis was an activated NK cell. The effectors from spleen were poly I.C. boostable, Lyt-1.1 negative, NK 1.2 positive, and asialo GM1 positive. Spleen cells from T cell-deficient nude mice, but not NK-deficient beige mice, exhibited high levels of killing activity, and experiments with NK cell clones demonstrated that these lines were also highly cytolytic and killed Ha-ras transfectants in parallel to YAC. Transfection of the same fibroblast line with c-myc did not alter the level of activated NK sensitivity. Cold target competition experiments revealed that Ha-ras-transfected and non-transfected 10T1/2 fibroblasts competed equally for lysis of either YAC or Ha-ras transfectants. Rat-1 fibroblasts did not compete, but gained this capacity when transformed with the v-Ki-ras oncogene but not v-fps. These data suggest that Ha-ras acts in target cells at a post-binding step, whereas Ki-ras may affect expression of target-effector binding structures. The findings that activated NK cell lysis may be specifically influenced by ras expression support a role for NK cells in host surveillance against early neoplastic changes.

Analysis of hybridoma mutants defective in synthesis of immunoglobulin M

Hybridoma mutants defective in the expression of IgM have been analyzed by molecular and somatic cell hybridization techniques. The frequency of kappa light-chain mutants in the hybridoma PC7 was much higher than for other cell lines. In contrast to the mutations which we observed previously, the kappa mutants examined here resulted from complete or partial deletion of the kappa gene. Mutants defective in mu chain synthesis were of more diverse types including deletions, gross rearrangements, and more subtle changes. One mutant containing a cis-acting mutation resulting in reduced expression of the mu chain had an associated partial duplication of the mu gene, while others making low or undetectable levels of mu had no gross alterations in genetic structure. The usefulness of this approach to the study of gene structure and expression is discussed.

Inducible cellular transformation by a metallothionein-ras hybrid oncogene leads to natural killer cell susceptibility

Natural killer (NK) cells are lymphoid effector cells possessing spontaneous cytolytic activity against a variety of tumour targets. The fact that NK cells pre-exist at high frequency and require no lengthy activation, and the observation that differentiated and metastatic tumour cells often have a decreased sensitivity to NK cytolysis have led to the hypothesis that these cells may be involved in the earliest stages of antitumour surveillance. Central to this model is the prediction that NK sensitivity must arise during cellular transformation. To test this prediction directly, we have constructed a vector containing the transforming gene from the EJ bladder carcinoma cell line under the transcriptional control of the mouse metallothionein-I promoter. When induced with heavy metal ions, mouse fibroblast lines containing this vector become dramatically sensitive to NK-mediated cytolysis concomitant with the expression of the cellular Harvey ras (c-Ha-ras) p21 protein and with cellular transformation.

Transposition of intracisternal A-particle genes in mouse hybridomas

A nonfunctional immunoglobulin kappa-chain gene previously shown to contain intracisternal A-particle (IAP) sequences within one of its introns was further characterized by DNA and RNA blot analysis. The results of these experiments indicate that the joining of the IAP sequences to the kappa-chain gene was a consequence of an insertion event which presumably involved a complete IAP gene. It is postulated that the insertion of the IAP gene has altered the secondary or tertiary structure of the kappa-chain pre-mRNA, resulting in the utilization of a "cryptic" 5' splice site. The detection of numerous IAPs in isogenous wild-type hybridoma cells by electron microscopy suggests that transposition of IAP genes might proceed via the reverse transcription of IAP-associated RNA. These findings raise the possibility that transposition of IAP genes might also occur in other IAP-positive mouse cells. In particular, insertional mutagenesis by IAP genes in IAP-producing tumor cells could play a role in tumor cell heterogeneity or tumor progression. We have also investigated the possibility that IAP-related sequences might be present in the human genome. Using nonstringent hybridization conditions, multiple discrete restriction fragments could be detected in human DNA with several mouse IAP-specific probes.