DNA driven self-assembly of micron-sized rods using DNA-grafted bacteriophage fd virions

We have functionalized the sides of fd bacteriophage virions with oligonucleotides to induce DNA hybridization driven self-assembly of high aspect ratio filamentous particles. Potential impacts of this new structure range from an entirely new building block in DNA origami structures, inclusion of virions in DNA nanostructures and nanomachines, to a new means of adding thermotropic control to lyotropic liquid crystal systems. A protocol for producing the virions in bulk is reviewed. Thiolated oligonucleotides are attached to the viral capsid using a heterobifunctional chemical linker. A commonly used system is utilized, where a sticky, single-stranded DNA strand is connected to an inert double-stranded spacer to increase inter-particle connectivity. Solutions of fd virions carrying complementary strands are mixed, annealed, and their aggregation is studied using dynamic light scattering (DLS), fluorescence microscopy, and atomic force microscopy (AFM). Aggregation is clearly observed on cooling, with some degree of local order, and is reversible when temperature is cycled through the DNA hybridization transition.

P2X4 receptors interact with both P2X2 and P2X7 receptors in the form of homotrimers

BACKGROUND AND PURPOSE

The P2X receptor family consists of seven subunit types - P2X1-P2X7. All but P2X6 are able to assemble as homotrimers. In addition, various subunit permutations have been reported to form heterotrimers. Evidence for heterotrimer formation includes co-localization, co-immunoprecipitation and the generation of receptors with novel functional properties; however, direct structural evidence for heteromer formation, such as chemical cross-linking and single-molecule imaging, is available in only a few cases. Here we examined the nature of the interaction between two pairs of subunits - P2X2 and P2X4, and P2X4 and P2X7.

EXPERIMENTAL APPROACH

We used several experimental approaches, including in situ proximity ligation, co-immunoprecipitation, co-isolation on affinity beads, chemical cross-linking and atomic force microscopy (AFM) imaging.

KEY RESULTS

Both pairs of subunits co-localize upon co-transfection, interact intimately within cells, and can be co-immunoprecipitated and co-isolated from cell extracts. Despite this, chemical cross-linking failed to show evidence for heteromer formation. AFM imaging of isolated receptors showed that all three subunits had the propensity to form receptor dimers. This self-association is likely to account for the observed close interaction between the subunit pairs, in the absence of true heteromer formation.

CONCLUSIONS AND IMPLICATIONS

We conclude that both pairs of receptors interact in the form of distinct homomers. We urge caution in the interpretation of biochemical evidence indicating heteromer formation in other cases.

DNA translocation by type III restriction enzymes: a comparison of current models of their operation derived from ensemble and single-molecule measurements

Much insight into the interactions of DNA and enzymes has been obtained using a number of single-molecule techniques. However, recent results generated using two of these techniques-atomic force microscopy (AFM) and magnetic tweezers (MT)-have produced apparently contradictory results when applied to the action of the ATP-dependent type III restriction endonucleases on DNA. The AFM images show extensive looping of the DNA brought about by the existence of multiple DNA binding sites on each enzyme and enzyme dimerisation. The MT experiments show no evidence for looping being a requirement for DNA cleavage, but instead support a diffusive sliding of the enzyme on the DNA until an enzyme-enzyme collision occurs, leading to cleavage. Not only do these two methods appear to disagree, but also the models derived from them have difficulty explaining some ensemble biochemical results on DNA cleavage. In this 'Survey and Summary', we describe several different models put forward for the action of type III restriction enzymes and their inadequacies. We also attempt to reconcile the different models and indicate areas for further experimentation to elucidate the mechanism of these enzymes.

Intragranular targeting of syncollin, but not a syncollinGFP chimera, inhibits regulated insulin exocytosis in pancreatic beta-cells

Several proteins play a role in the mechanism of insulin exocytosis. However, these 'exocytotic proteins' have yet to account for the regulated aspect of insulin exocytosis, and other factors are involved. In pancreatic exocrine cells, the intralumenal zymogen granule protein, syncollin, is required for efficient regulated exocytosis, but it is not known whether intragranular peptides similarly influence regulated insulin exocytosis. Here, this issue has been addressed using expression of syncollin and a syncollin-green fluorescent protein (syncollinGFP) chimera in rat islet beta-cells as experimental tools. Syncollin is not normally expressed in beta-cells but adenoviral-mediated expression of both syncollin and syncollinGFP indicated that these were specifically targeted to the lumen of beta-granules. Syncollin expression in isolated rat islets had no effect on basal insulin secretion but significantly inhibited regulated insulin secretion stimulated by glucose (16.7 mM), glucagon-like peptide-1 (GLP-1) (10 nM) and glyburide (5 microM). Consistent with specific localization of syncollin to beta-granules, constitutive secretion was unchanged by syncollin expression in rat islets. Syncollin-mediated inhibition of insulin secretion was not due to inadequate insulin production. Moreover, secretagogue-induced increases in cytosolic intracellular Ca2+, which is a prerequisite for triggering insulin exocytosis, were unaffected in syncollin-expressing islets. Therefore, syncollin was most likely acting downstream of secondary signals at the level of insulin exocytosis. Thus, syncollin expression in beta-cells has highlighted the importance of intralumenal beta-granule peptide factors playing a role in the control of insulin exocytosis. In contrast to syncollin, syncollinGFP had no effect on insulin secretion, underlining its usefulness as a 'fluorescent tag' to track beta-granule transport and exocytosis in real time.

Syntaxin is efficiently excluded from sphingomyelin-enriched domains in supported lipid bilayers containing cholesterol

Formation of a trans-complex between the three SNARE proteins syntaxin, synaptobrevin and SNAP-25 drives membrane fusion. The structure of the core SNARE complex has been studied extensively. Here we have used atomic force microscopy to study the behavior of recombinant syntaxin 1A both in detergent extracts and in a lipid environment. Full-length syntaxin in detergent extracts had a marked tendency to aggregate, which was countered by addition of munc-18. In contrast, syntaxin lacking its transmembrane region was predominantly monomeric. Syntaxin could be integrated into liposomes, which formed lipid bilayers when deposited on a mica support. Supported bilayers were decorated with lipid vesicles in the presence, but not the absence, of full-length syntaxin, indicating that formation of syntaxin complexes in trans could mediate vesicle docking. Syntaxin complexes remained at the sites of docking following detergent solubilization of the lipids. Raised lipid domains could be seen in bilayers containing sphingomyelin, and these domains were devoid of syntaxin and docked vesicles in the presence, but not the absence, of cholesterol. Our results demonstrate that syntaxin is excluded from sphingomyelin-enriched domains in a cholesterol-dependent manner.

Syncollin homo-oligomers associate with lipid bilayers in the form of doughnut-shaped structures

Syncollin is a 16-kDa protein that is associated with the luminal surface of the zymogen granule membrane in the pancreatic acinar cell. Detergent-solubilized, purified syncollin migrates on sucrose density gradients as a large (120-kDa) protein, suggesting that it exists naturally as a homo-oligomer. In this study, we investigated the structure of the syncollin oligomer. Chemical cross-linking of syncollin produced a ladder of bands, the sizes of which are consistent with discrete species from monomers up to hexamers. Electron microscopy of negatively stained syncollin revealed doughnut-shaped structures of outer diameter 10 nm and inner diameter 3 nm. Atomic force microscopy (AFM) of syncollin on mica supports at pH 7.6 showed particles of molecular volume 155 nm(3). Smaller particles were observed either at alkaline pH (11.0), or in the presence of a reducing agent (dithiothreitol), conditions that cause dissociation of the oligomer. AFM imaging of syncollin attached to supported lipid bilayers again revealed doughnut-shaped structures (outer diameter 31 nm, inner diameter 6 nm) protruding 1 nm from the bilayer. Finally, addition of syncollin to liposomes rendered them permeable to the water-soluble fluorescent probe 5(6)-carboxyfluorescein. These results are discussed in relation to the possible physiological role of syncollin.

Structure of Ocr from bacteriophage T7, a protein that mimics B-form DNA

We have solved, by X-ray crystallography to a resolution of 1.8 A, the structure of a protein capable of mimicking approximately 20 base pairs of B-form DNA. This ocr protein, encoded by gene 0.3 of bacteriophage T7, mimics the size and shape of a bent DNA molecule and the arrangement of negative charges along the phosphate backbone of B-form DNA. We also demonstrate that ocr is an efficient inhibitor in vivo of all known families of the complex type I DNA restriction enzymes. Using atomic force microscopy, we have also observed that type I enzymes induce a bend in DNA of similar magnitude to the bend in the ocr molecule. This first structure of an antirestriction protein demonstrates the construction of structural mimetics of long segments of B-form DNA.

Trafficking of green fluorescent protein-tagged muscarinic M4 receptors in NG108-15 cells

The muscarinic M4 receptor for acetylcholine was tagged at its C terminus with green fluorescent protein (GFP) and expressed in NG108-15 cells, which normally express this receptor subtype. The binding affinity of the antagonist N-methylscopolamine was not significantly affected by the presence of the GFP tag, whereas the affinity of the receptor for the agonist carbachol was reduced by four-fold. Stimulation of the tagged receptor resulted in inhibition of adenylyl cyclase. Following agonist stimulation, the tagged receptor was slowly internalized, and became partially co-localized with the endosomal marker Texas Red-transferrin after 30 min. There was little co-localization with the lysosomal marker 1gp120 even after 60 min of internalization. Finally, the tagged receptor, unlike the endogenous receptor, failed to recycle to the plasma membrane on removal of the agonist. We conclude that the GFP-tagged muscarinic M4 receptor does not traffic normally in NG108-15 cells, most likely because of its gross overexpression.

Abnormalities in the synaptic vesicle fusion machinery in Huntington's disease

We have recently described the progressive and selective loss of the presynaptic protein complexin II in brains of mice (R6/2) transgenic for the Huntington's disease (HD) mutation. Here we have determined the expression of components of the synaptic vesicle fusion machinery in the striatum and hippocampus from post-mortem brains of HD cases and neurologically normal controls. As in the brains of R6/2 mice, complexin II was markedly depleted in the HD striatum; the depletion was compartmentally organized, with complexin II-poor regions corresponding with areas of low immunoreactivity toward the matrix marker calbindin D(28K). Decreases in the levels of the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) protein synaptobrevin 2 and of rab3A were also seen, but none of the other proteins tested was significantly affected. In the hippocampus, levels of complexin II, synaptobrevin 2, rab3A, and also of alpha-SNAP, were markedly elevated in HD brains. We suggest that the observed abnormalities in the expression of proteins known to be involved in the control of neurotransmitter release, including both modulators and core components of the vesicle fusion machinery, might account for at least some of the functional abnormalities seen in HD.

Cholesterol-dependent interaction of syncollin with the membrane of the pancreatic zymogen granule

Syncollin is a protein of the pancreatic zymogen granule that was isolated through its ability to bind to syntaxin. Despite this in vitro interaction, it is now clear that syncollin is present on the luminal side of the zymogen granule membrane. Here we show that there are two pools of syncollin within the zymogen granule: one free in the lumen and the other tightly associated with the granule membrane. When unheated or cross-linked samples of membrane-derived syncollin are analysed by SDS/PAGE, higher-order forms are seen in addition to the monomer, which has an apparent molecular mass of 16 kDa. Extraction of cholesterol from the granule membrane by treatment with methyl-beta-cyclodextrin causes the detachment of syncollin, and this effect is enhanced at a high salt concentration. Purified syncollin is able to bind to brain liposomes at pH 5.0, but not at pH 11.0, a condition that also causes its extraction from granule membranes. Syncollin binds only poorly to dioleoyl phosphatidylcholine liposomes, but binding is dramatically enhanced by the inclusion of cholesterol. Finally, cholesterol can be co-immunoprecipitated with syncollin. We conclude that syncollin is able to interact directly with membrane lipids, and to insert into the granule membrane in a cholesterol-dependent manner. Membrane-associated syncollin apparently exists as a homo-oligomer, possibly consisting of six subunits, and its association with the membrane may be stabilized by electrostatic interactions with either other proteins or phospholipids.

Progressive depletion of complexin II in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor, emotional and cognitive dysfunction. There is no treatment or cure for this disease, and after the onset of symptoms, usually in the fourth decade of life, there is an inexorable decline to death. In many patients there is a complex deterioration of function before the onset of neuronal loss and, at least in mouse models, abnormalities in neurotransmission represent early events in the development of the disease. Here we describe the specific and progressive loss of complexin II from the brains of mice carrying the HD mutation (R6/2 line), and the later appearance of this protein in a subpopulation of neuronal intranuclear inclusions. Although the precise role of complexin II is still unclear, it is known to bind to the SNARE complex, and is therefore likely to be involved in the control of exocytosis. Our results suggest that changes in neurotransmitter release might contribute to the neuronal dysfunction seen in these mice.

Real-time studies of zymogen granule exocytosis in intact rat pancreatic acinar cells

An adequate understanding of secretion requires the measurement of exocytosis on the same time scale as that used for second messenger dynamics. To investigate the kinetics of ACh-evoked secretion in pancreatic acinar cells, exocytosis of zymogen granules was quantified by continuous, time-differential analysis of digital images. The validity of this method was confirmed by simultaneous fluorescence imaging of quinacrine-loaded zymogen granules. Basal rates of exocytosis were low (0.2 events min(-1)). ACh stimulated a biphasic increase in secretory activity, maximal rates exceeding 20 events min(-1) after 10 s of ACh application (10 microM). Over the next 15 s the rate of exocytosis fell to less than 4 events min(-1); then began a second phase of secretion that peaked 15 s later at approximately 11 events min(-1), but subsequently declined in the continued presence of agonist. Measurements of fura-2 fluorescence demonstrated a biphasic increase in intracellular [Ca2+] ([Ca2+]i). Comparison of the [Ca2+]i records and time-differential analysis revealed that the fall in exocytotic rate following the initial burst occurred despite the fact that [Ca2+]i remained high. The second phase of secretion depended on both [Ca2+]i and [ACh]. At 10 microM ACh there was a decrease in the steepness of the relationship between [Ca2+]i and exocytosis that led to an enhancement of the slow secretory phase. We propose that acinar cells contain two pools of secretory vesicles: a small pool of granules that is exocytosed rapidly, but is quickly depleted; and a reserve pool of granules that can be recruited by ACh in a process that is modulated by second messengers other than calcium.

Targeting of the zymogen-granule protein syncollin in AR42J and AtT-20 cells

Syncollin is a 13-kDa protein associated with the membranes of pancreatic zymogen granules. Here we determine the in situ localization of syncollin in pancreatic acinar cells from adult and neonatal rats, and study the targeting of green fluorescent protein-(GFP-) and His(6)-tagged syncollin chimaeras in model exocrine and endocrine secretory cells. Immunocytochemical analysis of the distribution of syncollin in fully differentiated and neonatal acinar cells revealed a granular pattern that corresponded with that of the zymogen-granule markers synaptobrevin 2 and amylase. In fully differentiated acinar cells syncollin-positive vesicles were detected in the apical region of the cells, whereas in neonatal acinar cells they were found clustered near the cell nucleus. Both GFP- and His(6)-tagged syncollin entered the secretory pathway when transiently expressed in AR42J or AtT-20 cells. Syncollin-GFP was found predominantly in amylase-positive granules in AR42J cells and in adrenocorticotrophic hormone- (ACTH-) positive granules in AtT-20 cells. Syncollin-GFP was also present in the Golgi complex in AR42J cells. Syncollin-His(6) became localized in ACTH-containing granules in the neuritic processes of AtT-20 cells. In AR42J cells syncollin-His(6) did not co-localize with amylase, but was detected in acidic vesicles. These results show that the exocrine protein syncollin contains intrinsic cell-type-independent targeting information that is retained in both exocrine and endocrine cells after fusion to the GFP tag. In contrast, His(6)-tagged syncollin is efficiently targeted to secretory granules only in AtT-20 cells and not in AR42J cells.

Translocation-independent dimerization of the EcoKI endonuclease visualized by atomic force microscopy

Bacterial type I restriction/modification systems are capable of performing multiple actions in response to the methylation pattern on their DNA recognition sequences. The enzymes making up these systems serve to protect the bacterial cells against viral infection by binding to their recognition sequences on the invading DNA and degrading it after extensive ATP-driven translocation. DNA cleavage has been thought to occur as the result of a collision between two translocating enzyme complexes. Using atomic force microscopy (AFM), we show here that EcoKI dimerizes rapidly when bound to a plasmid containing two recognition sites for the enzyme. Dimerization proceeds in the absence of ATP and is also seen with an EcoKI mutant (K477R) that is unable to translocate DNA. Only monomers are seen when the enzyme complex binds to a plasmid containing a single recognition site. Based on our results, we propose that the binding of EcoKI to specific DNA target sequences is accompanied by a conformational change that leads rapidly to dimerization. This event is followed by ATP-dependent translocation and cleavage of the DNA.

Analysis of the association of syncollin with the membrane of the pancreatic zymogen granule

Syncollin is a pancreatic zymogen granule protein that was isolated through its ability to bind to syntaxin. Here we show that syncollin has a cleavable signal sequence and can be removed from granule membranes by washing with sodium carbonate. When membranes were subjected to Triton X-114 partitioning, syncollin was found predominantly in the aqueous phase, indicating that it is not sufficiently hydrophobic to be embedded in the membrane. Syncollin has intramolecular disulfide bonds and was accessible to water-soluble cross-linking and biotinylating reagents only when granules were lysed by sonication. These results indicate that syncollin is tightly bound to the luminal surface of the granule membrane. In situ, syncollin was resistant to proteases such as trypsin. When granule membranes were solubilized in ionic detergents such as deoxycholate, this trypsin resistance was maintained, and syncollin migrated on sucrose density gradients as a large (150 kDa) protein. In contrast, in non-ionic detergents such as Triton X-100, syncollin became partially sensitive to trypsin and behaved as a monomer. Syncollin in alkaline extracts of granule membranes was also monomeric. However, reduction of the pH regenerated the oligomeric form, which was insoluble. We conclude that syncollin exists as a homo-oligomer and that its ability to self-associate can be reversibly modulated via changes in pH. In light of our findings, we reassess the likely role of syncollin in the pancreatic acinar cell.

Effect of G protein-coupled receptor kinase 2 on the sensitivity of M4 muscarinic acetylcholine receptors to agonist-induced internalization and desensitization in NG108-15 cells

NG108-15 cells express predominantly the M4 subtype of the muscarinic receptor for acetylcholine. Stimulation of these receptors by the agonist carbachol causes an inhibition of cellular adenylyl cyclase and a consequent fall in the intracellular cyclic AMP concentration. Pretreatment of the cells with carbachol caused both internalization and desensitization of the M4 receptor. Overexpression of G protein-coupled receptor kinase (GRK) 2 caused an increase in the rate constant for receptor endocytosis (from 0.06 to 0.18 min(-1)) and a decrease in the EC50 for carbachol stimulation of internalization (from 15 to 3 microM). Overexpression of a dominant negative form of GRK2 had more modest effects, reducing the rate constant for endocytosis (from 0.11 to 0.07 min(-1)) and increasing the EC50 for carbachol stimulation of internalization (from 8 to 17 microM). Neither GRK2 nor dominant negative GRK2 overexpression had any effect on the rate constant for receptor recycling following agonist removal. The time course and extent of receptor desensitization in control cells were identical to the corresponding values for receptor internalization, and the rate and extent of desensitization were again increased by GRK2 overexpression. Exposure of the cells to hyperosmolar sucrose (0.6 M) almost completely blocked agonist-induced receptor internalization in both control and GRK2-overexpressing cells. Sucrose treatment also blocked agonist-induced desensitization. We conclude that the internalization and desensitization of the M4 muscarinic receptor in NG108-15 cells can be modulated in response to changes in GRK2 activity and also that internalization plays a key role in desensitization.

Identification of SNAREs involved in regulated exocytosis in the pancreatic acinar cell

The molecular basis of exocytotic membrane fusion in the pancreatic acinar cell was investigated using an in vitro assay that measures both zymogen granule-plasma membrane fusion and granule-granule fusion. These two fusion events were differentially sensitive to Ca(2+), suggesting that they are controlled by different Ca(2+)-sensing mechanisms. Botulinum neurotoxin C (BoNT/C) treatment of the plasma membranes caused cleavage of syntaxin 2, the apical isoform of this Q-SNARE, but did not affect syntaxin 4, the basolateral isoform. BoNT/C also cleaved syntaxin 3, the zymogen granule isoform. BoNT/C treatment of plasma membranes abolished granule-plasma membrane fusion, whereas toxin treatment of the granules reduced granule-plasma membrane fusion and abolished granule-granule fusion. Tetanus toxin cleaved granule-associated synaptobrevin 2 but caused only a small reduction in both granule-plasma membrane fusion and granule-granule fusion. Our results indicate that syntaxin 2 is the isoform that mediates fusion between zymogen granules and the apical plasma membrane of the acinar cell. Syntaxin 3 mediates granule-granule fusion, which might be involved in compound exocytosis. In contrast, the major R-SNARE on the zymogen granule remains to be identified.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Endocytosis and recycling of muscarinic receptors

Agonist stimulation causes the endocytosis of many G protein-coupled receptors, including muscarinic acetylcholine receptors. In this study we have investigated the agonist-triggered trafficking of the M3 muscarinic receptor expressed in SH-SY5Y human neuroblastoma cells. We have compared the ability of a series of agonists to generate the second messenger Ins(1,4,5)P3 with their ability to stimulate receptor endocytosis. We show that there is a good correlation between the intrinsic activity of the agonists and their ability to increase the rate constant for receptor endocytosis. Furthermore, on the basis of our results, we predict that even very weak partial agonists should under some circumstances be able to cause substantial receptor internalization. Receptor endocytosis occurs too slowly to account for the rapid desensitization of the Ca2+ response to carbachol. Instead, receptor endocytosis and recycling appear to play an important role in resensitization. After an initial agonist challenge, the response to carbachol is fully recovered when only about half of the receptors have been recycled to the cell surface, suggesting that there is a receptor reserve of about 50%. Removal of this reserve by receptor alkylation significantly reduces the extent of resensitization. Resensitization is also reduced by inhibitors of either endocytosis alone (concanavalin A) or of endocytosis and recycling (nigericin). Finally, the protein phosphatase inhibitor calyculin A also reduces resensitization, possibly by blocking the dephosphorylation of the receptors in an endosomal compartment.

Investigation of protein partnerships using atomic force microscopy

The origin of contrast in atomic force microscopy (AFM) lies in the probe's response to forces between itself and the sample. These forces most commonly result from changes in height as the tip is scanned over the surface, but can also originate in properties inherent in the sample. These have been exploited as further means of contrast and have spawned an array of similar imaging techniques, such as chemical force microscopy, magnetic force microscopy, and frictional force microscopy. All of these techniques use AFM as an extremely sensitive gauge to map forces at discrete sites on the surface. A natural extension of this approach is to map forces in an array, in order to create a force map. AFM can be used in aqueous or fluid environments, thus allowing the exploration of forces in biological systems under physiologically relevant conditions. By immobilizing one half of an interacting pair of proteins onto the tip and the other half onto the substrate, it is possible to investigate the electrostatic and hydrophobic interactions between them. We employed these techniques to examine the interaction between a pair of proteins of known affinity that are involved in exocytosis (NSF and alpha-SNAP) and separately to demonstrate how two-dimensional force mapping can be applied to the nuclear envelope to identify nuclear pore complexes.

Fates of endocytosed somatostatin sst2 receptors and associated agonists

Somatostatin agonists are rapidly and efficiently internalized with the somatostatin sst2 receptor. The fate of internalized agonists and receptors is of critical importance because the rate of ligand recycling back to the cell surface can limit the amount of radioligand accumulated inside the cells, whereas receptor recycling might be of vital importance in providing the cell surface with dephosphorylated, resensitized receptors. Furthermore the accumulation of radioisotope-conjugated somatostatin agonists inside cancer cells resulting from receptor-mediated internalization has been used as a treatment for cancers that overexpress somatostatin receptors. In the present study, radio-iodinated agonists at the sst2 somatostatin receptor were employed to allow quantitative analysis of the fate of endocytosed agonist. After endocytosis, recycling back to the cell surface was the main pathway for both 125I-labelled somatostatin-14 (SRIF-14) and the more stable agonist 125I-labelled cyclo(N-Me-Ala-Tyr-d-Trp-Lys-Abu-Phe) (BIM-23027; Abu stands for aminobutyric acid), accounting for 75-85% of internalized ligand when re-endocytosis of radioligand was prevented. We have shown that there is a dynamic cycling of both somatostatin agonist ligands and receptors between the cell surface and internal compartments both during agonist treatment and after surface-bound agonist has been removed, unless steps are taken to prevent the re-activation of receptors by recycled agonist. Internalization leads to increased degradation of 125I-labelled SRIF-14 but not 125I-labelled BIM-23027. The concentration of recycled agonist accumulating in the extracellular medium was sufficient to re-activate the receptor, as measured both by the inhibition of forskolin-stimulated adenylate cyclase and the recovery of surface receptor number after internalization.

Diagnosis of mitochondrial disease: assessment of mitochondrial DNA heteroplasmy in blood

Mitochondrial DNA (mtDNA) mutations are an important cause of neurological disease. The identification of causative mtDNA mutations may be particularly troublesome in blood where there are often low levels of mutant mtDNA. This is evident from a recent study in which heteroplasmic mtDNA mutations in cytochrome c oxidase genes were incorrectly thought to be linked to Alzheimer's disease. We wished to explore whether analysis of blood mtDNA, prepared by a number of DNA extraction procedures, influenced the diagnosis of mtDNA disease. DNA was extracted by different procedures from 4 patients with heteroplasmic mtDNA mutations, and the level of heteroplasmy investigated by radioactive PCR-RFLP analysis. Whilst there was no consistent decrease in the level of mtDNA heteroplasmy, we observed the coamplification of a novel mtDNA pseudogene from DNA samples extracted by a simple 'boiling' procedure using primers designed to screen for the tRNALeu(UUR) A3243G mutation. This pseudogene was readily amplified from DNA extracted from rho degrees (mtDNA-less) cells, confirming its nuclear location. We believe that mtDNA pseudogenes may therefore present significant difficulties in the accurate identification of pathogenic heteroplasmic mtDNA mutations in blood.

A cell-free system for Ca2+-regulated exocytosis

The reconstitution of a membrane fusion event in a cell-free system makes possible a biochemical investigation of the molecular mechanisms underlying it. We have developed an in vitro assay for the fusion of pancreatic zymogen granules with the plasma membrane. The lipid-soluble fluorescent probe octadecylrhodamine is loaded into the granule membrane, and the granules are then incubated with unlabeled plasma membranes. Membrane fusion results in a dilution of the probe, which is detected through the dequenching of its fluorescence. The properties of the in vitro fusion event are impressively similar to those of exocytosis from permeabilized pancreatic acini, indicating that dequenching is detecting a physiologically relevant process. In particular, exocytotic membrane fusion both in vitro and in permeabilized acini is stimulated by Ca2+ with an EC50 of 1 microM, and enhanced by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) with an EC50 of 10-20 microM. Another parallel between the two systems is the incomplete inhibition of fusion/exocytosis by tetanus toxin, despite complete cleavage of synaptobrevin 2 on the zymogen granule membrane. Recently, the in vitro assay for membrane fusion has been used to indicate a role in the control of exocytosis for syncollin, a granule membrane protein that binds to syntaxin in a Ca2+-sensitive manner. The assay should continue to provide information about this exocytotic membrane fusion event in the future.

Membrane fusion: all done with SNAREpins?

SNARE proteins are sufficient to fuse artificial membranes together. In the cell, vesicle transport may rely on fusion mediated by interaction between vesicle (v) and target (t) SNAREs, whereas the homotypic fusion of organelle biogenesis may be mediated by t-SNARE-t-SNARE interaction.

The relationship between agonist intrinsic activity and the rate of endocytosis of muscarinic receptors in a human neuroblastoma cell line

The molecular mechanisms underlying the internalization of G protein-coupled receptors are still poorly understood. Normally agonists but not antagonists cause internalization (defined here as a reduction in the number of receptors at the cell surface), suggesting a functional relationship between agonist activity and internalization. In this study we investigated the effects of eight muscarinic ligands on the rate constants for endocytosis and recycling of m3 muscarinic acetylcholine receptors in human SH-SY5Y neuroblastoma cells. We found that there was a linear correlation between the intrinsic activity of the ligand and its ability to increase the rate constant for endocytosis, suggesting that the same active conformation of the receptor is responsible for stimulating both second messenger generation and receptor endocytosis. In contrast, the rate constant for recycling did not depend on which agonist had triggered receptor endocytosis, suggesting that recycling is a purely constitutive process. Because receptor internalization depends on the rate constants for both endocytosis and recycling, the relationship between internalization and intrinsic activity is nonlinear. In particular, mathematical modeling of receptor trafficking revealed that under certain conditions very small (3% or less) increases in the rate constant for endocytosis are sufficient to cause substantial receptor internalization. An important implication of this analysis is that extremely weak partial agonists (which may in practice be indistinguishable from antagonists) may produce significant receptor internalization.

Involvement of receptor cycling and receptor reserve in resensitization of muscarinic responses in SH-SY5Y human neuroblastoma cells

Preexposure of SH-SY5Y cells to the muscarinic agonist carbachol caused a rapid desensitization of subsequent carbachol-stimulated intracellular Ca2+ responses and a slower decrease in the number of receptors at the plasma membrane. Desensitization (to 30% of the control response) was maximal after 1 min of exposure to agonist, whereas the number of cell surface receptors reached a minimum (33% of control) only after 5 min. Following agonist washout, the recovery of response was complete within 12 min, whereas the recovery of surface receptor number reached a plateau at 65% of control after 30 min. Treatment with inhibitors of endocytosis (concanavalin A) or recycling (nigericin) did not affect rapid desensitization but did decrease resensitization, suggesting that receptor cycling is involved in resensitization. Experiments with the irreversible antagonist propylbenzilylcholine mustard demonstrated that the receptor reserve for the Ca2+ response to 1 mM carbachol is approximately 50%. Removal of this receptor reserve led to a decrease in the rate of resensitization. We propose that the existence of a receptor reserve might explain the poor correlation between functional response and surface receptor number, and that one of its roles might be to permit rapid resensitization after a significant agonist-induced decrease in surface receptor number. The purpose of receptor cycling might be to allow dephosphorylation (and reactivation) of receptors that have become phosphorylated (and inactivated) in response to agonist stimulation, because the protein phosphatase inhibitor calyculin A significantly reduced resensitization.

Endocytosis and recycling of G protein-coupled receptors

Agonist stimulation of G protein-coupled receptors causes a dramatic reorganization of their intracellular distribution. Activation of receptors triggers receptor endocytosis and, since receptors recycle back to the surface continuously, a new steady state is reached where a significant proportion of receptors is located internally. Although this movement of receptors is remarkable, its role has been enigmatic. Recent developments have provided insight into the compartments through which the receptors move, the nature of the signals that trigger receptor translocation, and the significance of receptor cycling for cell function. In this article, Jennifer Koenig and Michael Edwardson review recent progress in this field and place receptor cycling into a mathematical framework that reveals the extent and rate of intracellular receptor movement.

The secretory granule protein syncollin binds to syntaxin in a Ca2(+)-sensitive manner

The membrane proteins synaptobrevin, syntaxin, and SNAP-25 form the core of a ubiquitous fusion machine that interacts with the soluble proteins NSF and alpha-SNAP. During regulated exocytosis, membrane fusion is usually strictly controlled by Ca2+ ions. However, the mechanism by which Ca2+ regulates exocytosis is still unclear. Here we show that the membranes of exocrine secretory granules contain an 18-kDa protein, syncollin, that binds to syntaxin at low Ca2+ concentrations and dissociates at concentrations known to stimulate exocytosis. Syncollin has a single hydrophobic domain at its N-terminus and shows no significant homology with any known protein. Recombinant syncollin inhibits fusion in vitro between zymogen granules and pancreatic plasma membranes, and its potency falls as Ca2+ concentration rises. We suggest that syncollin acts as a Ca2(+)-sensitive regulator of exocytosis in exocrine tissues.

Evidence for interaction of the fusion protein alpha-SNAP with membrane lipid

alpha-SNAP [soluble N-ethylmaleimide-sensitive fusion protein (NSF)-attachment protein] is required for fusion of transport vesicles with their target membrane. In this study, we have examined the membrane-binding properties of alpha-SNAP. We have found that in several tissues a much larger amount of alpha-SNAP per unit weight of protein is bound to membranes than is free in the cytosol. Biochemical analysis shows that a fraction of alpha-SNAP behaves in ways characteristic of hydrophobic, lipid-associated proteins. These findings suggest that membrane binding may be accounted for, at least in part, by interaction with membrane lipid. Consistent with this idea, binding of newly synthesized alpha-SNAP to brain membranes was found to be independent of functional SNAP receptors and could be accounted for by direct binding of alpha-SNAP to membrane lipid. Furthermore, membrane lipid enhanced the ability of alpha-SNAP to stimulate NSF-dependent ATPase activity.

Somatostatin receptors in Neuro2A neuroblastoma cells: ligand internalization

1. Receptor-dependent internalization of somatostatin (SRIF) agonists has been a matter of controversy probably because [125I]Tyr11-SRIF-14 is rapidly degraded. We have studied the internalization of a stable somatostatin analogue, [125I]-BIM-23027, in a neuronal cell line, Neuro2A, which natively expresses somatostatin sst2 receptors. 2. Incubation of Neuro2A cells with [125I]-BIM-23027 at 37 degrees C resulted in a time-dependent internalization of the ligand, which reached a maximum at 30 min. Acid-washing showed that cell-surface binding of the ligand accounted for only 34% of total binding at this time. Internalization was dramatically reduced at 15 degrees C. 3. Internalization of [125I]-BIM-23027 was prevented by inclusion of unlabelled somatostatin receptor agonists in a concentration-dependent manner. The IC50 values for inhibition of [125I]-BIM-23027 internalization were approximately 100 fold lower than for inhibition of [125I]-BIM-23027 binding to membrane homogenates but followed the same rank order of potencies. 4. Disruption of G-protein coupling by treatment with pertussis toxin caused a 60% reduction in internalization of ligand. A combination of antimycin (50 nM) and deoxyglucose (50 mM) pretreatment, which leads to a depletion of cellular ATP, decreased internalization of [125I]-BIM-23027 by 66% of control and increased the proportion of surface-bound ligand. Hypertonic sucrose, which prevents clathrin-mediated endocytosis, reversibly abolished the internalization of ligand without increasing the proportion bound at the cell surface. 5. After internalization of [125I]-BIM-23027, approximately half of the ligand was recycled back to the extracellular medium within 20 min at 37 degrees C. This finding suggests that the intracellular content of [125I]-BIM-23027 reaches a steady state which is determined by the rates of both internalization and recycling of the ligand. In contrast to studies in which the internalization of [125I]-Tyr11-SRIF-14 was examined, neither internalized nor recycled [125I]-BIM-23027 was degraded to its component amino acids. 6. These findings indicate that the somatostatin agonist, [125I]-BIM-23027, is internalized in a receptor-dependent manner which involves clathrin-coated pits in Neuro2A cells. Furthermore, much of the internalized ligand is rapidly recycled back to the extracellular medium without undergoing significant degradation.

Somatostatin receptors in Neuro2A neuroblastoma cells: operational characteristics

1. We have used somatostatin (SRIF) receptor subtype-selective ligands to determine some of the operational characteristics of somatostatin receptors in Neuro2A mouse neuroblastoma cells. The potent SRIF1-receptor selective ligand, BIM-23027, was able to displace completely the specific binding of radioiodinated somatostatin, [125I]-Tyr11-SRIF-14, with a pIC50 of 10.3, suggesting that Neuro2A cells contain predominantly receptors of the SRIF1 receptor group. The rank order of affinities for several somatostatin analogues tested in competition studies, together with the high affinity of BIM-23027, indicate that the majority of receptors in Neuro2A cells are of the sst2 subtype. 2. The stable radioligand, [125I]-BIM-23027, bound with high affinity (Kd = 13 pM, Bmax = 0.2 pmol mg-1 protein) to Neuro2A cell membranes, but its binding was only partially reversible at room temperature and below. Thus at 4 degrees C, only 36% of the bound ligand dissociated within 2 h. In contrast, 60% of the ligand dissociated at 15 degrees C and 89% of the ligand dissociated at 37 degrees C. 3. Equilibrium binding of [125I]-BIM-23027 was partially (25%) inhibited by 10 microM GTP, and by 120 mM NaCl (42% inhibition) but this inhibition was increased to 75% when sodium chloride and GTP were added together. This effect of GTP and sodium chloride was also seen in dissociation experiments. After incubation to equilibrium with [125I]-BIM-23027, dissociation was initiated with excess unlabelled ligand in the presence of GTP (10 microM) and sodium chloride (120 mM). Under these conditions 67% of the ligand dissociated at 4 degrees C, 81% at 15 degrees C and 93% at 37 degrees C. Binding was totally inhibited by pretreatment of cells with pertussis toxin. 4. Functionally, BIM-23027 inhibited forskolin-stimulated cyclic AMP accumulation in a concentration-dependent manner with an IC50 of 1.0 nM and a maximal inhibition of 37%. This effect was abolished by pretreatment of the cells with pertussis toxin. However, unlike in studies reported with the recombinant sst2 receptor, no rise in intracellular calcium concentration was observed with SRIF-14. 5. We conclude that Neuro2A cells provide a stable neuronal cell line for the study of functionally coupled endogenous somatostatin receptors of the sst2 type. In addition, we have found that activation of the receptor is associated with ligand-receptor internalisation.

Association of nucleoside diphosphate kinase with pancreatic zymogen granules: effects of local GTP generation on granule membrane characteristics

It is well established that both GTP-binding proteins and phosphoproteins are involved in the control of exocytosis in the exocrine pancreas. Exocytotic membrane fusion is stimulated by guanosine 5'-[gamma-thio]triphosphate, and the phosphorylation states of several proteins, including at least one on the zymogen granule membrane, are known to change during exocytosis. We show here that a nucleoside diphosphate kinase is associated with the cytoplasmic face of pancreatic zymogen granules. This enzyme behaves as a phosphoprotein of apparent molecular mass 21 kDa on SDS/polyacrylamide gels, and is able to produce GTP by using ATP to phosphorylate endogenous GDP. GTP production by nucleoside diphosphate kinase is stimulated by the wasp venom peptide mastoparan, both through a direct action on the enzyme and through its ability to increase the availability of endogenous GDP. Two effects of the GTP produced by nucleoside diphosphate kinase are demonstrated: phosphorylation of a 37 kDa zymogen granule protein on histidine residues, and stimulation of the fusion of zymogen granules with pancreatic plasma membranes in vitro. These results suggest that granule-associated nucleoside diphosphate kinase is able to maintain local GTP concentrations, and raise the possibility that it might be involved in the control of exocytosis in the pancreatic acinar cell.

A novel function for the second C2 domain of synaptotagmin. Ca2+-triggered dimerization

Synaptotagmin serves as the major Ca2+ sensor for regulated exocytosis from neurons. While the mechanism by which synaptotagmin regulates membrane fusion remains unknown, studies using Drosophila indicate that the molecule functions as a multimeric complex and that its second C2 domain is essential for efficient excitation-secretion coupling. Here we describe biochemical data that may account for these phenomena. We report that Ca2+ causes synaptotagmin to oligomerize, primarily forming dimers, via its second C2 domain. This effect is specific for divalent cations that can stimulate exocytosis of synaptic vesicles (Ca2+ >> Ba2+, Sr2+ >> Mg2+) and occurs with an EC50 value of 3-10 microM Ca2+. In contrast, a separate Ca2+-dependent interaction between synaptotagmin and syntaxin, a component of the fusion apparatus, occurs with an EC50 value of approximately 100 microM Ca2+ and involves the synergistic action of both C2 domains of synaptotagmin. We propose that Ca2+ triggers two consecutive protein-protein interactions: the formation of synaptotagmin dimers at low Ca2+ concentrations followed by the association of synaptotagmin dimers with syntaxin at higher Ca2+-concentrations. Our findings, in conjunction with physiological studies, indicate that the Ca2+-induced dimerization of synaptotagmin is important for the efficient regulation of exocytosis by Ca2+.

Intracellular trafficking of the muscarinic acetylcholine receptor: importance of subtype and cell type

Agonist-induced decrease of surface muscarinic receptor number occurs in a number of cell lines. Recent work has suggested that some muscarinic receptor subtypes undergo internalization, whereas others do not. We investigated the agonist-induced trafficking of various muscarinic receptor subtypes transfected into CHO cells and compared it with the trafficking of receptors expressed natively in neuronal cells, fibroblasts, or epithelial cells. SH-SY5Y neuroblastoma cells, which express predominantly the m3 receptor subtype, show qualitatively similar changes in surface receptor number in response to agonist stimulation to those occurring in NG108-15 cells, which express predominantly the m4 subtype. The rate constants for internalization, however, were considerably different, indicating that receptors in SH-SY5Y cells show a much faster turnover than those in NG108-15 cells. In the transfected cells, the muscarinic receptor subtypes m1 and m3, which are coupled to second messenger systems via Gq/11, showed little agonist-induced loss of surface receptors. In contrast, the muscarinic receptor subtypes m2 and m4, which are coupled via Gi or G(o), showed a substantial loss of surface receptors after treatment with agonist. An interesting implication of this result is that agonist-induced receptor trafficking can still occur efficiently, even at very high receptor densities. Significant agonist-induced internalization also occurs in a fibroblast line (HeLa) and an epithelial cell line (HT29), both of which express predominantly m3 receptors. Our results suggest that the extent and rate of the loss of receptors from the cell surface in response to agonist stimulation are governed by both the receptor subtype and the cell type in which it is expressed.

Kinetic analysis of the trafficking of muscarinic acetylcholine receptors between the plasma membrane and intracellular compartments

We have studied the kinetics of muscarinic receptor trafficking between the plasma membrane and intracellular compartments in NG108-15 cells. A model of trafficking is proposed that includes four transport steps, delivery to the plasma membrane from the Golgi complex (linear) and endosomes (exponential) internalization from the plasma membrane into endosomes (exponential), and a degradative route from endosomes into lysosomes (linear). Based upon this model, a general equation has been derived that describes changes in receptor number at the plasma membrane and in endosomes in response to agonist stimulation. By following the movement of receptors into and out of the plasma membrane under various experimental conditions, it has been possible to determine the values of the four rate constants in the general equation and also the size of the endosomal receptor pool created by agonist stimulation. The consequences of changes in these constants for receptor trafficking are demonstrated. The model accounts for the effect of varying the duration of agonist stimulation on the size of the endosomal receptor pool and also permits an estimation of the receptor trafficking that underlies the well-established phenomenon of agonist-induced receptor internalization.

Routes of delivery of muscarinic acetylcholine receptors to the plasma membrane in NG108-15 cells

1. We have examined the routes of delivery of muscarinic acetylcholine receptors to the plasma membrane in unstimulated and agonist-stimulated NG108-15 cells. Delivery of receptors to the plasma membrane was measured by irreversible alkylating receptors already at the cell surface with propylbenzilylcholine mustard (PrBCM) and then following the recovery of binding of the polar radioligand [3H]-N-methylscopolamine ([3H]-NMS) in intact cells. 2. In unstimulated cells, recovery of [3H]-NMS binding after 2 h of incubation at 37 degrees C was 30% of binding in control cells. Binding affinity of [3H]-NMS was unchanged. In cells that had been pre-exposed to carbachol (0.5 mM) for 30 min, initial [3H]-NMS binding was reduced by 38% but recovery of binding was increased from 30% to 43% of control binding. 3. When the cells were pre-incubated for 1 h with the protein synthesis inhibitor cycloheximide (20 micrograms ml-1), recovery of [3H]-NMS binding was reduced by similar extents in unstimulated (30% to 9%) and carbachol-stimulated (43% to 19%) cells. Incubation of the cells at 20 degrees C instead of 37 degrees C reduced recovery of [3H]-NMS binding from 30% to 9% in unstimulated cells and from 43% to 23% in carbachol-stimulated cells. 4. Depletion of cellular ATP by addition of antimycin (50 nM) and deoxyglucose (50 mM), reduced recovery of binding to 12% in unstimulated cells and to 6% in carbachol-stimulated cells. 5. These results indicate that in unstimulated NG108-15 cells, delivery of muscarinic receptors to the plasma membrane is almost exclusively through the synthetic pathway. In agonist-stimulated cells,receptor sequestration into an intracellular compartment (probably endosomes) occurs. Under these circumstances, delivery of receptors to the plasma membrane along the synthetic route is unaffected but an additional route of delivery (recycling) now operates.

Pancreatic plasma membranes: promiscuous partners in membrane fusion

We have developed a system in which the fusion of pancreatic plasma membranes with zymogen granules can be studied in vitro. We show here that pancreatic plasma membranes fuse not only with pancreatic zymogen granules but also with parotid secretory granules. In contrast, parotid membranes fuse only with parotid granules and not with pancreatic granules. The extent of fusion is insensitive to Ca2+ for all combinations of plasma membranes and granules. Guanosine 5'-[gamma-thio]triphosphate (GTP[S]), on the other hand, stimulates fusion of pancreatic membranes with both pancreatic granules and parotid granules, but inhibits fusion between parotid membranes and parotid granules.

Involvement of a phosphoprotein on the zymogen granule membrane in the control of regulated exocytosis in the exocrine pancreas

The pancreatic acinar cell is one of a number of cell types in which phosphoproteins are believed to be involved in the control of regulated exocytosis. We have examined the effects of three agents that affect secretion in the acinar cell on the phosphorylation states of proteins on the zymogen granule membrane. We show that Ca2+ and GTP gamma S, which stimulate secretion, also stimulate the phosphorylation of a protein of M(r) 45,000 (p45) on isolated zymogen granules. On the other hand, the protein kinase inhibitor genistein inhibits both secretion and phosphorylation of p45. For all three agents, p45 phosphorylation is affected over concentration ranges identical to those that affect secretion. The stimulatory effect of GTP gamma S and the inhibitory effect of genistein are also seen when the phosphorylation state of p45 on granules within permeabilized cells is examined. Ca2+, however, over the same concentration range, now causes dephosphorylation of p45. Furthermore, the time-course of this effect is similar to that of Ca(2+)-triggered secretion. Phosphorylation of p45 is exclusively on serine, with no detectable phosphorylation on either threonine or tyrosine. We propose that exocytosis in pancreatic acini is controlled at least in part through the phosphorylation/dephosphorylation of p45, with dephosphorylation acting as a trigger for exocytosis.

Stimulation of exocytotic membrane fusion by modified peptides of the rab3 effector domain: re-evaluation of the role of rab3 in regulated exocytosis

We have shown previously that fusion between pancreatic zymogen granules and plasma membranes is stimulated by a peptide corresponding to the putative effector domain of rab3. Here we show that this stimulatory effect persists when the amino acid sequence of the peptide is substantially modified. We also show that an antibody raised against rab3a recognizes a protein of appropriate size on the zymogen-granule membrane, but has no effect on membrane fusion. We suggest that rab3 is not directly involved in the control of this membrane fusion event, and that the peptides are stimulating fusion by a mechanism unrelated to rab3.

Effects of the immunosuppressants cyclosporin A and FK 506 on exocytosis in the rat exocrine pancreas in vitro

1. We have examined the effects of the immunosuppressive drugs cyclosporin A (CsA) and FK 506 on exocytosis in two in vitro preparations of the exocrine pancreas-lobules and dispersed acini. 2. In lobules taken from starved rats and stimulated with the secretagogue caerulein, both CsA and FK 506, given shortly before stimulation, caused a dose-dependent inhibition of amylase secretion. In lobules from rats that had been pretreated in vivo with the protease inhibitor FOY-305 to stimulate secretion maximally, both CsA and FK 506 inhibited secretion of newly synthesized proteins, whereas only FK 506 inhibited caerulein-stimulated amylase release. 3. These different effects of the immunosuppressants on amylase release were reflected in their effects on degranulation, as revealed by electron microscopy. Control acinar cells in lobules from FOY-305-treated rats were almost completely degranulated, whereas treatment with FK 506, but not CsA, caused the accumulation of zymogen granules close to the apical plasma membrane. 4. In dispersed acini, stimulated with the cholinomimetic secretagogue bethanechol, both CsA and FK 506 reduced the secretory response, to about 45% of control; IC50 values were 50 nM and 3 nM, respectively. A similar partial inhibition of exocytosis was seen in acini permeabilized with the bacterial toxin streptolysin O and stimulated with 10 microM Ca2+. 5. These results demonstrate that the immunosuppressants cause an inhibition of exocytosis in the exocrine pancreas that is both rapid in onset and potent. The loss of the inhibitory effect of CsA on amylase release in lobules taken from FOY-305-treated rats may reveal a change in the characteristics of exocytosis as a consequence of the high level of stimulation, and also indicates that CsA and FK 506 have subtly different effects on secretion. We suggest that these drugs might be useful tools in the dissection of the molecular mechanisms of exocytosis.

Synthetic peptides of the rab3 effector domain stimulate a membrane fusion event involved in regulated exocytosis

We have developed a system in which the fusion of pancreatic zymogen granules with plasma membranes can be studied in vitro. Here we show that this membrane fusion event is stimulated specifically by peptides of the effector domain of rab3, a small, monomeric GTP-binding protein. In addition, we demonstrate that the stimulatory effect of the peptides involves their binding to a target on the plasma membrane, and is both qualitatively and quantitatively different from the effect of GTP gamma S, which also enhances membrane fusion. We suggest that regulated exocytosis in the pancreatic acinar cell may be under the control of more than one type of GTP-binding protein.

Inhibition of alloxan-induced hyperglycaemia by compounds of similar molecular structure

In this study we have shown that a range of compounds that are structurally similar to alloxan are able to protect mice against the diabetogenic effect of alloxan. The compounds include a group of five barbiturates, a group of five hydantoins, the methylxanthines caffeine and theophylline, the related compound uric acid, and ethosuximide. They were injected intraperitoneally prior to intravenous injection of alloxan, and blood glucose concentration was used as an index of alloxan toxicity. The salient structural feature possessed by all of these protective compounds is a pair of carbonyl oxygen atoms separated by a distance of 4.5 A and projecting from an approximately planar heterocyclic five- or six-membered ring; in all cases the carbonyl groups are separated by a ring nitrogen. We suggest that this feature is required for the protective effect of these compounds. In order to test further the requirement for two ring carbonyl groups, we also examined the effects of two compounds containing hydroxyl groups projecting from a six-membered ring, inositol and glucuronic acid. In agreement with previous studies on hexoses, we found that the effects of compounds such as these are unpredictable, with inositol protecting against alloxan toxicity but glucuronic acid not. We are unable to identify the critical difference in structure between these two compounds.

Fusion between rat pancreatic zymogen granules and plasma membranes. Modulation by a GTP-binding protein

At the moment, little is known about the molecular characteristics of the final step in the process of regulated exocytosis, i.e. the fusion of the membrane of a secretory vesicle with the plasma membrane. We have reconstituted this fusion event in vitro, using zymogen granules and plasma membranes from the exocrine pancreas of the rat. The membranes of zymogen granules were loaded with the lipid-soluble fluorescent probe octadecylrhodamine B, at a concentration that resulted in self-quenching of its fluorescence. The granules were then incubated with pancreatic plasma membranes at 37 degrees C, and fusion was measured through the dilution-dependent de-quenching of the fluorescence of the probe. Zymogen granules fused with pancreatic plasma membranes, but not with plasma membranes from liver or chromaffin cells; granules also fused with unlabelled granule membranes. The fusion of granules with plasma membranes was unaffected by variation of the Ca2+ concentration over a wide range, but fusion of granules with both plasma membranes and zymogen granule membranes was stimulated by GTP and, more potently, by guanosine 5'-[gamma-thio]triphosphate (GTP[S]). The effect of GTP[S] was to increase the extent of fusion occurring at low concentrations of plasma membranes, without affecting the maximum signal obtained at high membrane concentrations. Pre-incubation of the plasma membranes with GTP[S] also enhanced their ability to fuse with zymogen granules. Our results indicate that membrane fusion during exocytosis may be under the direct control of a GTP-binding protein.

Reconstitution in vitro of a membrane-fusion event involved in constitutive exocytosis. A role for cytosolic proteins and a GTP-binding protein, but not for Ca2+

The fusion of post-Golgi transport vesicles with the plasma membrane is perhaps the least well understood step in the network of intracellular membrane traffic. We have used an 'in vitro' system to study this membrane-fusion event. We show here that fusion requires the presence of cytosolic proteins, but not Ca2+, and is inhibited by the non-hydrolysable GTP analogue guanosine 5'-[gamma-thio]triphosphate, which indicates the involvement of a GTP-binding protein.

Differential recognition of secretory vesicles by annexins. European Molecular Biology Organization Course "Advanced Techniques for Studying Secretion"

The interactions of synexin, calpactin, and p68 (annexins VII, II, and VI) with zymogen granules and adrenal medullary chromaffin granules were compared. Synexin promoted the aggregation and fusion of both types of organelles. Calpactin inhibited the action of synexin on zymogen granules but would aggregate only chromaffin granules. p68 inhibited aggregation of chromaffin granules by calpactin and fusion of zymogen granules by synexin. The results demonstrate that these annexins, all of which bind acidic phospholipids, interact differently with different biological membranes and therefore may play specific and non-overlapping roles in cells where they are co-expressed.

Rat pancreatic acini permeabilised with streptolysin O secrete amylase at Ca2+ concentrations in the micromolar range, when provided with ATP and GTP gamma S

The aim of this study was to define the conditions required for exocytosis in pancreatic acini permeabilised with the bacterial toxin streptolysin O. Treatment of a suspension of acini with streptolysin O caused the release of both the cytoplasmic enzyme lactate dehydrogenase and the zymogen granule enzyme amylase. The release of amylase occurred more quickly than that of lactate dehydrogenase and was smaller in magnitude. In addition, a component of amylase release occurred only in the presence of Ca2+ (at concentrations in the micromolar range), ATP and GTP gamma S. We conclude that this component represents an exocytotic event, but that the release of lactate dehydrogenase occurs through toxin-generated lesions. The concentrations of Ca2+, ATP and GTP gamma S causing half-maximal exocytosis were 0.7 microM, 0.2 mM and 10 microM, respectively. This system should permit a study of the mechanisms underlying regulated exocytosis in this cell type.

Characteristics of a cell-free assay for the delivery of proteins to the plasma membrane

We have previously described the reconstitution, in a cell-free system, of the constitutive delivery of a newly synthesized protein, influenza neuraminidase, to the plasma membrane in BHK cells. Here we report some of the characteristics of this in vitro membrane fusion event. We show that fusion requires ATP hydrolysis, and exploit this requirement to distinguish the time-course of fusion from that of neuraminidase action. In addition, we present evidence for the occurrence of multiple fusions between hybrid membrane vesicles.