Further characterization of the aggregation and fusion of chromaffin granules by synexin as a model for compound exocytosis

Isolation and Purification of Chromaffin Granules from Adrenal Glands and Cultured Neuroendocrine Cells

Chromaffin granules isolated from adrenal glands constitute a powerful experimental tool to the study of secretory vesicle components and their participation in fusion and docking processes, vesicle aggregation, and interactions with cytosolic components. Although it is possible to isolate and purify chromaffin granules from adrenal glands of different species, bovine adrenal glands are the most used tissue source due to its easy handling and the large amount of granules that can be obtained from this tissue. In this chapter, we describe an easy-to-use and short-term protocol for efficiently obtaining highly purified chromaffin granules from bovine adrenal medulla. We additionally include protocols to isolate granules from cultured bovine chromaffin cells and PC12 cells, as well as a section to obtain chromaffin granules from mouse adrenal glands.

New insights into the role of the cortical cytoskeleton in exocytosis from neuroendocrine cells

The cortical cytoskeleton is a dense network of filamentous actin (F-actin) that participates in the events associated with secretion from neuroendocrine cells. This filamentous web traps secretory vesicles, acting as a retention system that blocks the access of vesicles to secretory sites during the resting state, and it mediates their active directional transport during stimulation. The changes in the cortical cytoskeleton that drive this functional transformation have been well documented, particularly in cultured chromaffin cells. At the biochemical level, alterations in F-actin are governed by the activity of molecular motors like myosins II and V and by other calcium-dependent proteins that influence the polymerization and cross-linking of F-actin structures. In addition to modulating vesicle transport, the F-actin cortical network and its associated motor proteins also influence the late phases of the secretory process, including membrane fusion and the release of active substances through the exocytotic fusion pore. Here, we discuss the potential interactions between the F-actin cortical web and proteins such as SNAREs during secretion. We also discuss the role of the cytoskeleton in organizing the molecular elements required to sustain regulated exocytosis, forming a molecular structure that foments the efficient release of neurotransmitters and hormones.

Protein kinase C and guanosine triphosphate combine to potentiate calcium-dependent membrane fusion driven by annexin 7

Exocytotic secretion is promoted by the concerted action of calcium, guanine nucleotide, and protein kinase C. We now show that the calcium-dependent membrane fusion activity of annexin 7 in vitro is further potentiated by the combined addition of guanine nucleotide and protein kinase C. The observed increment involves the simultaneous activation of annexin 7 by these two effectors. Guanosine triphosphate (GTP) and its non-hydrolyzable analogues optimally enhance the phosphorylation of annexin 7 by protein kinase C in vitro. Reciprocally, phosphorylation by protein kinase C significantly potentiates the binding and hydrolysis of GTP by annexin 7. Only protein kinase C-dependent phosphorylation has a significant positive effect on annexin 7 GTPase, although other protein kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, and pp60(c-)(src), have been shown to label the protein with high efficiency. In vivo, the ratio of bound GDP/GTP and phosphorylation of annexin 7 change in direct proportion to the extent of catecholamine release from chromaffin cells in response to stimulation by carbachol, or to inhibition by various protein kinase C inhibitors. These results thus lead us to hypothesize that annexin 7 may serve as a common site of action for calcium, guanine nucleotide, and protein kinase C in the exocytotic membrane fusion process in chromaffin cells.

Activation of annexin 7 by protein kinase C in vitro and in vivo

Annexin 7, a Ca(2+)/GTP-activated membrane fusion protein, is preferentially phosphorylated in intact chromaffin cells, and the levels of annexin 7 phosphorylation increase quantitatively in proportion to the extent of catecholamine secretion. Consistently, various protein kinase C inhibitors proportionately reduce both secretion and phosphorylation of annexin 7 in these cells. In vitro, annexin 7 is quantitatively phosphorylated by protein kinase C to a mole ratio of 2.0, and phosphorylation is extraordinarily sensitive to variables such as pH, calcium, phospholipid, phorbol ester, and annexin 7 concentration. Phosphorylation of annexin 7 by protein kinase C significantly potentiates the ability of the protein to fuse phospholipid vesicles and lowers the half-maximal concentration of calcium needed for this fusion process. Furthermore, other protein kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, and protein-tyrosine kinase pp60(c-)(src), also label annexin 7 with high efficiency but do not have this effect on membrane fusion. In the case of pp60(c-)(src), we note that this kinase, if anything, modestly suppresses the membrane fusion activity of annexin 7. These results thus lead us to hypothesize that annexin 7 may be a positive mediator for protein kinase C action in the exocytotic membrane fusion reaction in chromaffin cells.

Biophysical and molecular properties of annexin-formed channels

The annexins are water soluble proteins possessing a hydrophilic surface, which belong to a family of proteins which (a) bind ('annex') both calcium and phospholipids, and (b) form voltage-dependent calcium channels within planar lipid bilayers. Annexins types are diverse (94 annexins in 45 species) and they belong to an enormous multigene family that ranges throughout all eukaryotic kingdoms. Although the structure of these proteins is now well known their functional and physiological roles remain largely unknown and circumstantial. Various experimental approaches provided evidence that annexins function as Ca(2+) channels that could act as regulators of membrane fusion. The identity of annexins is derived from the conserved 34 kDa C-terminal domain which comprises four repeats - except for annexin VI, with eight repeats - of a sequence of approximately seventy amino acids, which holds the area known as the 'endonexin fold', with its identifying GXGTDE. Annexins have been placed into three subgroups of (1) tetrad core and short amino terminal, (2) tetrad core and long amino terminal, and (3) octad core and short amino terminal. The repeats are highly conserved, each forming a compact alpha-helical domain comprising five alpha-helices wound in a right-handed superhelix. Four domains are formed, arranged in a nearly flat and cyclical array, with domains I and IV, and II and III respectively forming two tightly organised modules with almost twofold symmetry. A hydrophilic pore lies at the centre of the molecule, forming a prominent ion channel coated with charged and highly conserved residues. The annexin molecule is slightly curved, with both a convex and a concave face. The cation/anion permeability ratios and the selectivity sequence of the ion channels formed by several annexins confirm the selectivity of the annexins for Ca(2+) over other divalent cations, and reveals the importance of structural sites, e.g. amino acid positions 17, 78, 95 and 112 for the identification of the ion channel's position, function and regulation. Some are sensitive to low doses of the phenothiazine drugs, trifluoperazine (an anti-schizophrenia drug) and promethazine (anti nausea drug) La(3+) and Cd(2+), (blockers of voltage-gated Ca(2+) channels) nifedipine (an inhibitor of non-activating Ca(2+) channels). There are two main competing models used to explain in vitro ion channel activity of annexins: one involves changes in the conductance of ion via electrostatic disturbance of the membrane surface; the other involves a much more extensive alteration in protein structure and a correspondingly deeper penetration into the membrane.

Mouse synexin (annexin VII) polymorphisms and a phylogenetic comparison with other synexins

Two sets of cDNAs encoding mouse synexin were isolated from a liver cDNA library and sequenced. The coding regions of synexin clones show 99% identity. By contrast, the two mouse synexin cDNAs differ in a number of ways in both 5' and 3' non-coding regions. The two sets of cDNA encode a polypeptide of 463 amino acid residues which has a deduced molecular mass of 50 kDa. The amino acid sequence of mouse synexin shows a high degree of similarity to both the unique N-terminal domain and the highly conserved C-terminal domain of previously cloned human synexin. Northern-blot analysis using mouse liver polyadenylated RNA revealed two transcripts of 1.8 kb and 2.6 kb, corresponding to group I and group II respectively. Further hybridization analysis using specific sequences from each set of clones showed that the two sizes of mRNAs differ in the length of the 3' non-coding region which corresponded to the cDNAs. Both mouse liver synexin and recombinant mouse synexin expressed in Escherichia coli reacted after Western-blot analysis with a goat antibody against bovine synexin. Only in the larger group-II cDNAs do we find point mutations leading to amino acid replacements of Ser to Ala at residue 145 in the unique N-terminal domain, and of Ala to Gly at residue 304 in the transition zone between repeats II and III. We conclude from a comparison of mouse, human and Dictyostelium synexins that changes occur predominantly in the hydrophobic N-terminal domain, or, in the C-terminal region at the ends of some predicted alpha-helices, on the hydrophobic face of the amphipathic C-helices, and within a lengthy non-helical domain connecting major repeats II and III.

Pattern of repeating aromatic residues in synexin. Similarity to the cytoplasmic domain of synaptophysin

Synexin was isolated from bovine liver by high resolution cation exchange chromatography and fragmented with cyanogen bromide or trypsin. Peptides were isolated and their amino acid sequences partially determined. Twenty percent of the synexin sequence was determined in one contiguous sequence of 61 residues and a nonoverlapping sequence of 20 residues. The sequence is characterized by a hexapeptide repeat of the form YPXXXX occurring eight times in series, with phenylalanine substituting for tyrosine in two positions. The intervening amino acids (X) are predominantly proline, glycine and alanine. This pattern of periodic aromatic residues suggests the presence of a novel secondary structure and is similar to repeats present in synaptophysin, gliadin and type II keratin.

Neutrophil function disorders

The polymorphonuclear leukocyte (neutrophil) is the most important phagocytic cell that defends the host against acute bacterial infection. Disorders of neutrophil function are suggested by recurrent cutaneous, periodontal, respiratory, or soft tissue infections. Staphylococcus aureus, gram-negative bacilli, and less commonly, Candida albicans, are the causative organisms. Treatment is supportive involving surgical drainage and antibiotics. Bone marrow transplantation offers hope to some patients. The biochemical and molecular defects have been identified for some of these disorders. Identification of these defects and their physiologic consequences have improved our understanding of how the activated neutrophil is attracted and adheres to inflammatory sites, and produces toxic products that destroy bacteria. However, the activated neutrophil may also damage normal tissue and participate in diseases such as rheumatoid arthritis and the adult respiratory distress syndrome (ARDS).

Synexin-mediated fusion of bovine chromaffin granule ghosts. Effect of pH

Synexin induces chromaffin granule ghosts to fuse one to another, a process which is followed continuously and quantitatively by monitoring the mixing of the intragranular aqueous compartments. A freeze-thaw technique was used for preparing chromaffin granule ghosts loaded with a self-quenching concentration of the fluorescent, high molecular weight probe FITC-Dextran. When the loaded ghosts were mixed with empty ghosts in the presence of synexin, the two compartments fused, resulting in the dilution of the probe with the concomitant increase in fluorescence. So as to suppress possible leakage signals, anti-fluorescein antibodies which quench probe fluorescence were present in the reaction media. Synexin-mediated fusion of freeze-thaw (F/Th) ghosts and binding of 125I-synexin to these membranes were found to be dependent on Ca2+ concentration, but only in a partial manner. However, these two synexin-mediated properties were demonstrably sensitive to [H+] in the medium. A detailed pH profile of fusion revealed an apparent midpoint of activation at approx. pH 5.2, with asymptotic values at pH 4 (maximum) and pH 7.2 (minimum). In our attempt to determine whether the pH effect was on the synexin or on the membranes, we found that fusion was blocked only by treatment of the membranes with the membrane-impermeant carboxyl group modifier 1-ethyl-3-(4-azonia-4,4-dimethylpentyl)carbodiimide. These data suggest that membrane fusion evoked by synexin seems to be promoted by rendering the F/Th membranes relatively less negatively charged while the synexin becomes more positively charged. The fusion process was entirely dependent upon synexin concentration; the k1/2 under optimal conditions of pCa and pH was 85 nM. Similar to what has been previously found with intact granules, an anti-synexin polyclonal antibody partially (48%) blocked fusion, as did pretreatment of the chromaffin granules ghosts with trypsin (30%). We conclude that the coincident pCa and pH sensitivity of synexin-mediated binding to chromaffin granule membranes and their subsequent fusion might be associated with physiological changes in the concentration of both cations in the cytoplasm of secreting chromaffin cells.

Membrane capacity measurements suggest a calcium-dependent insertion of synexin into phosphatidylserine bilayers

The mechanism by which synexin mediates calcium-dependent aggregation of medullary cell chromaffin granules and fusion of granule ghosts involves specific interactions with the lipid component of the membrane. To study the details of these interactions we measured synexin-induced changes in capacitance of phosphatidylserine bilayers formed at the tip of a patch pipet using the double-dip method. Provided calcium was present in the solution filling the pipet (10-50 mM) stable phosphatidylserine bilayers were easily formed. Addition of synexin (0.1 microgram/ml) to an external medium lacking added calcium induced no measurable changes in either bilayer resistance (10-30 G omega) or displacement current across the membrane. However, addition of calcium (0.1-2.5 mM) in the presence of synexin in the external solution caused a marked increase in the size and time constant of decay of the displacement current. From the steady-state value of the current we calculated a 5-fold decrease in resistance and from the charge displaced during the voltage-clamp pulses we calculated a 10-fold increase in membrane capacitance (from 20 to 200 fF). The size of the synexin-specific charge displacement in one direction during a pulse was always equal to the charge returning to the original configuration after the pulse. The synexin-specific transfer of charge reached saturation when the pipet potential was taken to a sufficient positive or negative value. These properties of the extra charge movement support our view that in the presence of calcium the cytosolic protein synexin penetrates into the bilayer. It is possible that these properties may be related to the mechanism by which synexin promotes membrane fusion in natural membranes.

An osmotic model for the fusion of biological membranes

A molecular model for fusion-fission reactions in membranes is proposed that is based on data from studies on artificially induced cell fusion and on the behaviour of phospholipid bilayers: it is put forward as a framework for further investigations into this fundamental property of biological systems.

A fluorescence assay for monitoring and analyzing fusion biological membrane vesicles in vitro

A new technique has been developed to study fusion of biological membrane vesicles. Bovine chromaffin granule ghosts (CGG) were loaded with fluorescein isothiocyanate-dextran (FITC-dextran) at self-quenching concentrations. Loaded ghosts were then made to fuse with empty CGG. Fusion was induced by synexin, a protein previously proposed to be involved in exocytosis. The fusion process was monitored by measuring the dequenching of the fluorescence. Dequenching occurred as FITC-dextran was diluted into the increased volume due to fusion with empty ghosts. Spurious signals from leakage or breakage of vesicles were removed by including a specific anti-fluorescein antibody in the reaction medium. This new technique may prove to be of more general use for studying membrane fusion processes in other systems.

Comparison of synexin isotypes in secretory and non-secretory tissues

Three synexin isotypes were identified in bovine liver or adrenal medullary tissues by immune blotting of one- or two-dimensional SDS gels and by two-dimensional tryptic peptide mapping of gel bands or spots. These isotypes were: alpha-synexin, mass 47 kDa, pI 6.9; beta-synexin, mass 47 kDa, pI 6.5; and mu-synexin, mass 51 kDa, pI 6.1. A non-secretory tissue, bovine skeletal muscle, was found to contain only mu-synexin. The absence of alpha- and beta-synexins in a non-secretory tissue suggests these proteins may perform specific roles in the process of exocytosis.

Loss of alpha 2-macroglobulin and epidermal growth factor surface binding induced by phenothiazines and naphthalene sulfonamides

We have found that certain naphthalenesulfonamides [e.g., N-6(-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7)] and phenothiazines [e.g., trifluoperazine (TFP)] induce a loss of cell-surface receptors for alpha 2-macroglobulin, and epidermal growth factor (EGF) in fibroblasts. The loss of alpha 2-macroglobulin receptors is independent of receptor occupancy and is rapidly reversed upon removal of these agents from the culture medium. The extent of EGF receptor loss is less than for alpha 2-macroglobulin, and the EGF receptors do not reappear at the surface when W-7 is removed. Receptor loss was measured as a change in the capacity for binding iodinated ligands; no change in affinity of binding was observed. This receptor loss could reflect inactivation of receptors or internalization. W-7 did not induce a loss of cell surface beta 2-microglobulin, a membrane protein which is excluded from coated pits and which is not internalized, indicating that the effect of W-7 was specific for membrane receptors and not a result of bulk depletion of plasma membrane. The loss of alpha 2-macroglobulin and EGF receptors occurs at concentrations which do not cause an increase in the pH of endocytic vesicles or the cytoplasm, indicating that these agents act by a mechanism distinct from the effect of other weak bases. Since both TFP and W-7 are potent inhibitors of calmodulin, we investigated the possibility that inhibition of calmodulin was responsible for the loss of receptors. Three lines of evidence suggest that calmodulin inhibition is not responsible for the inhibition of binding and endocytosis: 1) Promethazine, a phenothiazine that is a poor inhibitor of calmodulin, is nearly as effective as TFP at inhibiting endocytosis; calmidazolium, a potent inhibitor of several calmodulin functions, did not cause a loss of binding; 2) the microinjection of calmodulin into cells did not reverse the effects of W-7; using pressure microinjection, we introduced up to a 100-fold excess of calmodulin over native levels into individual gerbil fibroma cells; using rhodamine-labeled alpha 2-macroglobulin, we saw that the W-7 induced inhibition of receptor-mediated endocytosis was the same in injected and uninjected cells; 3) we injected calcineurin, a calmodulin-binding protein, into cells (1-3 pg/cell) and observed no effect on the receptor-mediated endocytosis of rhodamine-labeled alpha 2-macroglobulin. These data indicated that cell surface receptor numbers can be regulated by a cellular component that is not cytoplasmic calmodulin but that shares some drug sensitivities with calmodulin.

Calcium dependence of the binding of synexin to isolated chromaffin granules

The calcium dependence of the binding of synexin to isolated chromaffin granules has been investigated. The calcium dependence was found to be pH sensitive, binding occurring at higher Ca2+ concentrations at lower values of pH. At pH 7.2 half-maximal binding occurred at 4 microM Ca2+. This is a lower Ca2+ concentration than the 200 microM that is required to give half-maximal self-association of synexin or membrane aggregation by synexin. The data therefore suggest that in the chromaffin cell stimulated to release catecholamines and proteins by exocytosis synexin first binds to membranes and then associates with itself to draw membranes together in preparation for fusion.

Differential control of insulin secretion and somatostatin-receptor recruitment in isolated pancreatic islets

Somatostatin receptors appear to be localized to secretory granules in pancreatic islet homogenates. Recruitment of these receptors to the islet-cell surfaces may mark the contact event between secretory granules and plasma membranes before release of insulin by fission. Isethionate, an impermeant anionic replacement for chloride, blocks the release step but does not affect receptor recruitment. By contrast, low concentrations of phenothiazine drugs, such as trifluoperazine and promethazine, inhibit both receptor recruitment and secretion. Scatchard analysis of phenothiazine effects on somatostatin receptors reveals that these drugs reduce the number of receptors but do not affect the affinity of the receptor for somatostatin. These data indicate that membrane contact and fission steps during exocytosis can be biochemically separated.

Role of calcium in triggering the release of transmitters at the neuromuscular junction

Calcium ions have a key role in triggering the release of packaged transmitter at the amphibian neuromuscular junction and of the chromaffin granules at the adrenal medulla. It is suggested that (i) proteins on the vesicle and plasma membranes are of particular importance in promoting membrane fusion and exocytosis (ii) they may be divalent cation-stimulated ATPases, which form the calcium-binding sites or have a specific calcium-binding protein in close molecular apposition (iii) these ATPases in synaptic vesicles and chromaffin granules also generate a protonmotive force which is associated with the uptake of transmitter (iv) the osmotic properties of the vesicle may be important during fission, but it is not suggested that chemiosmotic effects are involved in Ca2+-triggered fusion (v) the action of calcium is markedly co-operative (vi) the adrenal medullary cell and the n.m.j. may differ in the Ca2+-binding site; there is evidence for the involvement of calmodulin in granule-plasmalemma fusion in the chromaffin cells, but not at present (surprisingly) for a role of this Ca2+-binding protein at the n.m.j. (vii) exocytosis requires MgATP (viii) phosphorylation of the ATPase may well be involved; phosphorylation via cAMP does not seem to be involved in fusion in either system (ix) the ATPase may undergo configurational changes during exocytosis and is markedly sensitive to the physical state of its phospholipid environment and to the oxidation of its -SH groups.

Inhibition of synexin activity and exocytosis from chromaffin cells by phenothiazine drugs

Synexin activity was found to be inhibited by low concentrations of the phenothiazine drugs, trifluoperazine and promethazine. Both drugs were also similarly effective at blocking nicotine and veratridine-induced catecholamine secretion from cultured chromaffin cells. These data thus indicate that synexin may be a target of phenothiazine drugs when they block secretion from chromaffin cells.