Calmodulin and protein kinase C increase Ca(2+)-stimulated secretion by modulating membrane-attached exocytic machinery

Identification of protein kinase Calpha as an essential, but not sufficient, cytosolic factor for Ca2+-induced alpha- and dense-core granule secretion in platelets

Upon activation, platelets release many active substances. Here, we have analyzed the mechanism governing Ca(2+)-induced secretion of von Willebrand factor stored in alpha-granules and 5-hydroxytryptamine in dense-core granules in permeabilized human platelets. Both secretions were dependent on ATP and cytosol. An essential factor for both granule secretions was purified from rat brain cytosol and identified to be protein kinase Calpha (PKCalpha) by partial amino acid sequencing. Purified PKCalpha efficiently stimulated both secretions in the presence of cytosol, whereas PKCalpha alone did not support the secretion of either type of granules, suggesting that PKCalpha is not a sufficient factor. Finally, in human platelet cytosol fractionated by a gel filtration column, the stimulatory activity for dense-core granule secretion paralleled with the concentration of PKC, suggesting that PKC could also be such a stimulatory factor in platelet cytosol. Thus, we identified PKCalpha as an essential, but not sufficient, cytosolic factor for the Ca(2+)-induced secretions of both alpha- and dense-core granules in platelets.

Fusion of docked membranes requires the armadillo repeat protein Vac8p

The discovery of molecules required for membrane fusion has revealed a remarkably conserved mechanism that centers upon the formation of a complex of SNARE proteins. However, whether the SNARE proteins or other components catalyze the final steps of membrane fusion in vivo remains unclear. Understanding this last step depends on the identification of molecules that act late in the fusion process. Here we demonstrate that in Saccharomyces cerevisiae, Vac8p, a myristoylated and palmitoylated armadillo repeat protein, is required for homotypic vacuole fusion. Vac8p is palmitoylated during the fusion reaction, and the ability of Vac8p to be palmitoylated appears to be necessary for its function in fusion. Both in vivo and in vitro analyses show that Vac8p functions after both Rab-dependent vacuole docking and the formation of trans-SNARE pairs. We propose that Vac8p may bind the fusion machinery through its armadillo repeats and that palmitoylation brings this machinery to a specialized lipid domain that facilitates bilayer mixing.

Secretion in alveolar type II cells at the interface of constitutive and regulated exocytosis

Long-term, simultaneous, measurements of cytoplasmic free Ca(2+) concentrations and single exocytotic fusion events in surfactant-secreting type II cells were performed. All fusion (constitutive, phorbol ester-induced, and agonist-induced) was Ca(2+)-dependent. Kinetic analysis revealed that agonist (adenosine triphosphate [ATP])-induced fusion exhibited a kinetic pattern that correlated well with the Ca(2+) signal. The effects of Ca(2+) release from intracellular stores (early) and Ca(2+) entry (late) could be demonstrated for the first time by dissecting the slow (10-to-15-min) fusion response to ATP into these two components. Bath Ba(2+) or Sr(2+) could replace Ca(2+) to elicit a fusion response in thapsigargin-pretreated cells lacking ATP-induced Ca(2+) release from stores. Although the late response was partially inhibited by interrupting the phospholipase D-protein kinase C axis, a high Ca(2+) dependence of the entire secretory course was demonstrated by a significant correlation between the integrated Ca(2+) signal and the fusion response. There was also a highly significant correlation between constitutive and ATP-stimulated fusion activity in individual cells. We propose a common mechanistic model for all types of fusion in this slow secretory cell, in which constitutive and regulated forms of exocytosis are subject to the same principles of regulation.

Phospholipase D1: a key factor for the exocytotic machinery in neuroendocrine cells

Phospholipase D (PLD) has been proposed to mediate cytoskeletal remodeling and vesicular trafficking along the secretory pathway. We recently described the activation of an ADP ribosylation factor-regulated PLD at the plasma membrane of chromaffin cells undergoing secretagogue-stimulated exocytosis. We show here that the isoform involved is PLD1b, and, using a real-time assay for individual cells, that PLD activation and exocytosis are closely correlated. Moreover, overexpressed PLD1, but not PLD2, increases stimulated exocytosis in a phosphatidylinositol 4,5-bisphosphate-dependent manner, whereas catalytically inactive PLD1 inhibits it. These results provide the first direct evidence that PLD1 is an important component of the exocytotic machinery in neuroendocrine cells.

Calcium Regulation of Exocytosis in PC12 Cells

The calcium (Ca(2+)) regulation of neurotransmitter release is poorly understood. Here we investigated several aspects of this process in PC12 cells. We first showed that osmotic shock by 1 m sucrose stimulated rapid release of neurotransmitters from intact PC12 cells, indicating that most of the vesicles were docked at the plasma membrane. Second, we further investigated the mechanism of rescue of botulinum neurotoxin E inhibition of release by recombinant SNAP-25 COOH-terminal coil, which is known to be required in the triggering stage. We confirmed here that Ca(2+) was required simultaneously with the SNAP-25 peptide, with no significant increase in release if either the peptide or Ca(2+) was present during the priming stage as well as the triggering, suggesting that SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) complex assembly was involved in the final Ca(2+)-triggered event. Using this rescue system, we also identified a series of acidic surface SNAP-25 residues that rescued better than wild-type when mutated, due to broadened Ca(2+) sensitivity, suggesting that this charged patch may interact electrostatically with a negative regulator of membrane fusion. Finally, we showed that the previously demonstrated stimulation of exocytosis in this system by calmodulin required calcium binding, since calmodulin mutants defective in Ca(2+)-binding were not able to enhance release.

A Discontinuous SNAP-25 C-terminal Coil Supports Exocytosis

Membrane fusion requires the formation of four-helical bundles comprised of the SNARE proteins syntaxin, vesicle-associated membrane protein (VAMP), and the synaptosomal-associated protein of 25 kDa (SNAP-25). Botulinum neurotoxin E cleaves the C-terminal coil of SNAP-25, inhibiting exocytosis of norepinephrine from permeabilized PC12 cells. Addition of a 26-mer peptide comprising the C terminus of SNAP-25 that is cleaved by the toxin restores exocytosis, demonstrating that continuity of the SNAP-25 C-terminal helix is not critical for its function. By contrast, vesicle-associated membrane protein peptides could not rescue botulinum neurotoxin D-treated cells, suggesting that helix continuity is critical for VAMP function. Much higher concentrations of the SNAP-25 C-terminal peptide are required for rescuing exocytosis (K(assembly) = approximately 460 microm) than for binding to other SNAREs in vitro (Kd < 5 microm). Each residue of the peptide was mutated to alanine to assess its functional importance. Whereas most mutants rescue exocytosis with lower efficiency than the wild type peptide, D186A rescues with higher efficiency, and kinetic analysis suggests this is because of higher affinity for the cellular binding site. This is consistent with Asp-186 contributing to negative regulation of the fusion process.

Ca2+-independent protein kinase C Apl II mediates the serotonin-induced facilitation at depressed aplysia sensorimotor synapses

At nondepressed Aplysia sensory to motor synapses, serotonin (5-HT) facilitates transmitter release primarily through a protein kinase A pathway. In contrast, at depressed Aplysia sensory to motor synapses, 5-HT facilitates transmitter release primarily through a protein kinase C (PKC)-dependent pathway. It is known that only two phorbol ester-activated PKC isoforms, the Ca(2+)-dependent PKC Apl I and the Ca(2+)-independent PKC Apl II, exist in the Aplysia nervous system. For the first time, we have now been able to functionally determine which isoform of PKC is involved in a particular form of plasticity. We microinjected cultured sensorimotor pairs of neurons with various PKC constructs tagged with the enhanced green fluorescent protein as a reporter for successful plasmid expression. Our results demonstrate that short-term facilitation of depressed synapses is mediated by PKC Apl II. Dominant-negative PKC Apl II, but not dominant-negative PKC Apl I, disrupted the normal kinetics of 5-HT-induced facilitation by completely blocking its rapid onset. This effect was specific to depressed synapses, because dominant-negative PKC Apl II did not inhibit 5-HT-mediated facilitation of nondepressed synapses. Our results suggest that not only different signal transduction pathways but also different isoforms of a specific cascade may mediate physiological responses according to the state of a synapse.

Synaptotagmin VII as a plasma membrane Ca(2+) sensor in exocytosis

Synaptotagmins I and II are Ca(2+) binding proteins of synaptic vesicles essential for fast Ca(2+)-triggered neurotransmitter release. However, central synapses and neuroendocrine cells lacking these synaptotagmins still exhibit Ca(2+)-evoked exocytosis. We now propose that synaptotagmin VII functions as a plasma membrane Ca(2+) sensor in synaptic exocytosis complementary to vesicular synaptotagmins. We show that alternatively spliced forms of synaptotagmin VII are expressed in a developmentally regulated pattern in brain and are concentrated in presynaptic active zones of central synapses. In neuroendocrine PC12 cells, the C(2)A and C(2)B domains of synaptotagmin VII are potent inhibitors of Ca(2+)-dependent exocytosis, but only when they bind Ca(2+). Our data suggest that in synaptic vesicle exocytosis, distinct synaptotagmins function as independent Ca(2+) sensors on the two fusion partners, the plasma membrane (synaptotagmin VII) versus synaptic vesicles (synaptotagmins I and II).

Mycobacterium tuberculosis phagosomes exhibit altered calmodulin-dependent signal transduction: contribution to inhibition of phagosome-lysosome fusion and intracellular survival in human macrophages

Mycobacterium tuberculosis successfully parasitizes macrophages by disrupting the maturation of its phagosome, creating an intracellular compartment with endosomal rather than lysosomal characteristics. We have recently demonstrated that live M. tuberculosis infect human macrophages in the absence of an increase in cytosolic Ca(2+) ([Ca(2+)](c)), which correlates with inhibition of phagosome-lysosome fusion and intracellular viability. In contrast, killed M. tuberculosis induces an elevation in [Ca(2+)](c) that is coupled to phagosome-lysosome fusion. We tested the hypothesis that defective activation of the Ca(2+)-dependent effector proteins calmodulin (CaM) and CaM-dependent protein kinase II (CaMKII) contributes to the intracellular pathogenesis of tuberculosis. Phagosomes containing live M. tuberculosis exhibited decreased levels of CaM and the activated form of CaMKII compared with phagosomes encompassing killed tubercle bacilli. Furthermore, ionophore-induced elevations in [Ca(2+)](c) resulted in recruitment of CaM and activation of CaMKII on phagosomes containing live M. tuberculosis. Specific inhibitors of CaM or CaMKII blocked Ca(2+) ionophore-induced phagosomal maturation and enhanced the bacilli's intracellular viability. These results demonstrate a novel role for CaM and CaMKII in the regulation of phagosome-lysosome fusion and suggest that defective activation of these Ca(2+)-activated signaling components contributes to the successful parasitism of human macrophages by M. tuberculosis.

SNARE-mediated membrane fusion

SNARE proteins have been proposed to mediate all intracellular membrane fusion events. There are over 30 SNARE family members in mammalian cells and each is found in a distinct subcellular compartment. It is likely that SNAREs encode aspects of membrane transport specificity but the mechanism by which this specificity is achieved remains controversial. Functional studies have provided exciting insights into how SNARE proteins interact with each other to generate the driving force needed to fuse lipid bilayers.

Trans-complex formation by proteolipid channels in the terminal phase of membrane fusion

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) and Rab-GTPases, together with their cofactors, mediate the attachment step in the membrane fusion of vesicles. But how bilayer mixing--the subsequent core process of fusion--is catalysed remains unclear. Ca2+/calmodulin controls this terminal process in many intracellular fusion events. Here we identify V0, the membrane-integral sector of the vacuolar H+-ATPase, as a target of calmodulin on yeast vacuoles. Between docking and bilayer fusion, V0 sectors from opposing membranes form complexes. V0 trans-complex formation occurs downstream from trans-SNARE pairing, and depends on both the Rab-GTPase Ypt7 and calmodulin. The maintenance of existing complexes and completion of fusion are independent of trans-SNARE pairs. Reconstituted proteolipids form sealed channels, which can expand to form aqueous pores in a Ca2+/calmodulin-dependent fashion. V0 trans-complexes may therefore form a continuous, proteolipid-lined channel at the fusion site. We propose that radial expansion of such a protein pore may be a mechanism for intracellular membrane fusion.

Stable SNARE complex prior to evoked synaptic vesicle fusion revealed by fluorescence resonance energy transfer

Although it is clear that soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) complex plays an essential role in synaptic vesicle fusion, the dynamics of SNARE assembly during vesicle fusion remain to be determined. In this report, we employ fluorescence resonance energy transfer technique to study the formation of SNARE complexes. Donor/acceptor pair variants of green fluorescent protein (GFP), cyan fluorescent protein (CFP), and yellow fluorescent protein (YFP) are fused with the N termini of SNAP-25 and synaptobrevin, respectively. In vitro assembly of SNARE core complex in the presence of syntaxin shows strong fluorescence resonance energy transfer (FRET) between the CFP-SNAP-25 and YFP-synaptobrevin. Under the same conditions, CFP fused to the C terminus of SNAP-25, and YFP- synaptobrevin have no FRET. Adenovirus-mediated gene transfer is used to express the fusion proteins in PC12 cells and cultured rat cerebellar granule cells. Strong FRET is associated with neurite membranes and vesicular structures in PC12 cells co-expressing CFP-SNAP-25 and YFP-synaptobrevin. In cultured rat cerebellar granule cells, FRET between CFP-SNAP-25 and YFP-synaptobrevin is mostly associated with sites presumed to be synaptic junctions. Neurosecretion in PC12 cells initiated by KCl depolarization leads to an increase in the extent of FRET. These results demonstrate that significant amounts of stable SNARE complex exist prior to evoked synaptic vesicle fusion and that the assembly of SNARE complex occurs during vesicle docking/priming stage. Moreover, it demonstrates that FRET can be used as an effective tool for investigating dynamic SNARE interactions during synaptic vesicle fusion.

Two mechanisms of synaptic vesicle recycling in rat brain nerve terminals

KCl and 4-aminopyridine (4-AP) evoke glutamate release from rat brain cortical nerve terminals by voltage clamping or by Na(+) channel-generated repetitive action potentials, respectively. Stimulation by 4-AP but not KCl is largely mediated by protein kinase C (PKC). To determine whether KCl and 4-AP utilise the same mechanism to release glutamate, we correlated glutamate release with release of the hydrophobic synaptic vesicle (SV) marker FM2-10. A strong correlation was observed for increasing concentrations of KCl and after application of phorbol 12-myristate 13-acetate (PMA) or staurosporine. The parallel increase in exocytosis measured by two approaches suggested it occurred by a PKC-independent mechanism involving complete fusion of SVs with the plasma membrane. At low concentrations of 4-AP, alone or with staurosporine, glutamate and FM2-10 release also correlated. However, higher concentrations of 4-AP or of 4-AP plus PMA greatly increased glutamate release but did not further increase FM2-10 release. This divergence suggests that 4-AP recruits an additional mechanism of release during strong stimulation that is PKC dependent and is superimposed upon the first mechanism. This second mechanism is characteristic of kiss-and-run, which is not detectable by styryl dyes. Our data suggest that glutamate release in nerve terminals occurs via two mechanisms: (1) complete SV fusion, which is PKC independent; and (2) a kiss-and-run-like mechanism, which is PKC dependent. Recruitment of a second release mechanism may be a widespread means to facilitate neurotransmitter release in central neurons.

Protein kinase C phosphorylation of syntaxin 4 in thrombin-activated human platelets

We postulated that the syntaxins, because of their key role in SNARE complex formation and exocytosis, could be important targets for signaling by intracellular kinases involved in secretion. We found that syntaxin 4 was phosphorylated in human platelets treated with a physiologic agent that induces secretion (thrombin) but not when they were treated with an agent that prevents secretion (prostacyclin). Syntaxin 4 phosphorylation was blocked by inhibitors of activated protein kinase C (PKC), and, in parallel assays, PKC inhibitors also blocked secretion from thrombin-activated platelets. In platelets, cellular activation by thrombin or phorbol 12-myristate 13-acetate decreased the binding of syntaxin 4 with SNAP-23, another platelet t-SNARE. Phosphatase inhibitors increased syntaxin 4 phosphorylation and further decreased syntaxin 4-SNAP-23 binding induced by cell activation. Conversely, a PKC inhibitor blocked syntaxin 4 phosphorylation and returned binding of syntaxin 4-SNAP-23 to that seen in nonstimulated platelets. In vitro, PKC directly phosphorylated platelet syntaxin 4 and recombinant syntaxin 4. PKC phosphorylation in vitro inhibited (71 +/- 8%) the binding of syntaxin 4 to SNAP-23. These results provide evidence that extracellular activation can be coupled through intracellular PKC signaling so as to modulate SNARE protein interactions involved in platelet exocytosis.

Ca2+-dependent regulation of synaptic SNARE complex assembly via a calmodulin- and phospholipid-binding domain of synaptobrevin

Synaptic core complex formation is an essential step in exocytosis, and assembly into a superhelical structure may drive synaptic vesicle fusion. To ascertain how Ca(2+) could regulate this process, we examined calmodulin binding to recombinant core complex components. Surface plasmon resonance and pull-down assays revealed Ca(2+)-dependent calmodulin binding (K(d) = 500 nM) to glutathione S-transferase fusion proteins containing synaptobrevin (VAMP 2) domains but not to syntaxin 1 or synaptosomal-associated protein of 25 kDa (SNAP-25). Deletion mutations, tetanus toxin cleavage, and peptide synthesis localized the calmodulin-binding domain to VAMP(77-94), immediately C-terminal to the tetanus toxin cleavage site (Q(76)-F(77)). In isolated synaptic vesicles, Ca(2+)/calmodulin protected native membrane-inserted VAMP from proteolysis by tetanus toxin. Assembly of a (35)S-SNAP-25, syntaxin 1 GST-VAMP(1-96) complex was inhibited by Ca(2+)/calmodulin, but assembly did not mask subsequent accessibility of the calmodulin-binding domain. The same domain contains a predicted phospholipid interaction site. SPR revealed calcium-independent interactions between VAMP(77-94) and liposomes containing phosphatidylserine, which blocked calmodulin binding. Circular dichroism spectroscopy demonstrated that the calmodulin/phospholipid-binding peptide displayed a significant increase in alphahelical content in a hydrophobic environment. These data provide insight into the mechanisms by which Ca(2+) may regulate synaptic core complex assembly and protein interactions with membrane bilayers during exocytosis.

Dynamic, Rho1p-dependent localization of Pkc1p to sites of polarized growth

In eukaryotes, the Rho GTPases and their effectors are key regulators of the actin cytoskeleton, membrane trafficking and secretion, cell growth, cell cycle progression and cytokinesis. Budding yeast Pkc1p, a protein kinase C-like enzyme involved in cell wall biosynthesis and cytoskeletal polarity, is structurally and functionally related to the Rho-associated kinases (PRK/ROCK) of mammalian cells. In this study, localization of Pkc1p was monitored in live cells using a GFP fusion (Pkc1p-GFP). Pkc1p-GFP showed dynamic spatial and temporal localization at sites of polarized growth. Early in the cell cycle, Pkc1p-GFP was found at the pre-bud site and bud tips, becoming delocalized as the cell progressed further and finally relocalizing around the mother-daughter bud neck in an incomplete ring, which persisted until cell separation. Bud localization was actin-dependent but stability of Pkc1p-GFP at the neck was actin-independent, although localization at both sites required functional Rho1p. In addition, Pkc1p-GFP showed rapid relocalization after cell wall damage. These results suggest that the roles of Pkc1p in both polarized growth and the response to cell wall stress are mediated by dynamic changes in its localization, and suggest an additional potential role in cytokinesis.

Cellular mechanism of sodium oleate-stimulated secretion of cholecystokinin and secretin

Long-chain fatty acids are potent stimulants of secretin and CCK release. The cellular mechanisms of fatty acid-stimulated secretion of these two hormones are not clear. We studied the stimulatory effect and mechanism of sodium oleate (SO) on secretin- and CCK-producing cells. SO stimulated the release of secretin or CCK from isolated rat mucosal cell preparations enriched in either secretin- or CCK-producing cells, respectively. SO also time- and dose-dependently stimulated secretin and CCK release from STC-1 cells. In STC-1 cells, SO-stimulated secretin and CCK release was potentiated by IBMX and inhibited by a protein kinase A-selective inhibitor and a cAMP-specific antagonist. SO-stimulated releases of the two hormones were also inhibited by downregulation or inhibitors of protein kinase C, a calmodulin antagonist and an inhibitor of calmodulin-dependent protein kinase II. Chelating of extracellular Ca(2+) or addition of an L-type calcium channel blocker diminished SO-stimulated hormone releases. SO caused an increase in intracellular Ca(2+) concentration that was partially reversed by diltiazem but had no effect on production of cAMP, cGMP, or inositol-1,4,5-triphosphate. These results indicate that SO acts on secretin- and CCK-producing cells. Its stimulatory effect is potentiated by endogenous protein kinase A and mediated by activation of Ca(2+) influx through the L-type channels and of protein kinase C and Ca(2+)/calmodulin-dependent protein kinase II.

Beta-granule transport and exocytosis

Regulated beta -granule exocytosis is critical for the ability of the beta -cell to finely control body glucose homeostasis. This is now understood to be a multistage process whereby beta -granules are transported from biosynthetic/storage sites in the cell cytoplasm and targeted to specific regions of the plasma membrane. Exocytosis is achieved when these granules are triggered to fuse with the membrane by an elevated cytosolic Ca(2+). Dramatic advances have been made recently in our understanding of the protein-protein interactions and regulatory signals that govern intracellular transport and fusion. Although best understood for exocytosis from neurons and neuroendocrine cells, similar processes are thought to be conserved in the beta -cell.

Priming in exocytosis: attaining fusion-competence after vesicle docking

Membrane contact established by tethering or docking mechanisms is not a sufficient condition for membrane fusion. In neural and neuroendocrine cells, only a small fraction of secretory vesicles docked at the plasma membrane are fusion-competent and undergo rapid ATP-independent fusion in response to Ca(2+) elevations. Additional biochemical events termed 'priming' are essential to render vesicles competent for Ca(2+)-triggered fusion. The priming of vesicles is ATP-dependent and a number of ATP-dependent priming reactions have been characterized in permeable neuroendocrine cells. These involve NSF-mediated priming of SNARE protein complexes, the ATP-dependent synthesis of phosphoinositides, and protein kinase-mediated protein phosphorylation. In addition, munc13 is an important protein involved in priming synaptic vesicles. An emphasis in this review is on recent work indicating that priming events identified in the pathways of regulated exocytosis share many features with pre-fusion processes characterized in constitutive fusion pathways.

Molecular mechanisms of platelet exocytosis: requirements for alpha-granule release

Platelets function by secreting components necessary for primary clot formation. This report describes an in vitro assay that measures alpha-granule secretion. Using permeabilized platelets, it is possible to recreate Ca(2+)-stimulated release of platelet factor 4 (PF4) that is ATP- and temperature-dependent. Though other divalent cations can replace Ca(2+) (i.e., Sr(2+), Mn(2+), Zn(2+)), there is no effect of Ba(2+). Analysis by electron microscopy indicates that the in vitro assay also mimics the cytoskeletal rearrangements and granule centralization that occurs upon platelet activation in vivo. Antibody inhibition studies show that PF4 release requires the general membrane fusion protein N-ethylmaleimide-sensitive factor (NSF) and well as the target membrane SNAP receptors (t-SNAREs), syntaxin 2, 4, and SNAP-23. As shown by electron microscopy, the anti-t-SNARE antibodies block granule to target membrane fusion. This finding is unique in that it is the first report of a role for two syntaxins in the same exocytosis event.