Mutations that disrupt PHOXB interaction with the neuronal calcium sensor HPCAL1 impede cellular differentiation in neuroblastoma

Heterozygous germline mutations in PHOX2B, a transcriptional regulator of sympathetic neuronal differentiation, predispose to diseases of the sympathetic nervous system, including neuroblastoma and congenital central hypoventilation syndrome (CCHS). Although the PHOX2B variants in CCHS largely involve expansions of the second polyalanine repeat within the C-terminus of the protein, those associated with neuroblastic tumors are nearly always frameshift and truncation mutations. To test the hypothesis that the neuroblastoma-associated variants exert their effects through loss or gain of protein-protein interactions, we performed a large-scale yeast two-hybrid screen using both wild-type (WT) and six different mutant PHOX2B proteins against over 10 000 human genes. The neuronal calcium sensor protein HPCAL1 (VILIP-3) exhibited strong binding to WT PHOX2B and a CCHS-associated polyalanine expansion mutant but only weakly or not at all to neuroblastoma-associated frameshift and truncation variants. We demonstrate that both WT PHOX2B and the neuroblastoma-associated R100L missense and the CCHS-associated alanine expansion variants induce nuclear translocation of HPCAL1 in a Ca(2+)-independent manner, while the neuroblastoma-associated 676delG frameshift and K155X truncation mutants impair subcellular localization of HPCAL1, causing it to remain in the cytoplasm. HPCAL1 did not appreciably influence the ability of WT PHOX2B to transactivate the DBH promoter, nor did it alter the decreased transactivation potential of PHOX2B variants in 293T cells. Abrogation of the PHOX2B-HPCAL1 interaction by shRNA knockdown of HPCAL1 in neuroblastoma cells expressing PHOX2B led to impaired neurite outgrowth with transcriptional profiles indicative of inhibited sympathetic neuronal differentiation. Our results suggest that certain PHOX2B variants associated with neuroblastoma pathogenesis, because of their inability to bind to key interacting proteins such as HPCAL1, may predispose to this malignancy by impeding the differentiation of immature sympathetic neurons.

Decoding glutamate receptor activation by the Ca2+ sensor protein hippocalcin in rat hippocampal neurons

Hippocalcin is a Ca²⁺-binding protein that belongs to a family of neuronal Ca²⁺sensors and is a key mediator of many cellular functions including synaptic plasticity and learning. However, the molecular mechanisms involved in hippocalcin signalling remain illusive. Here we studied whether glutamate receptor activation induced by locally applied or synaptically released glutamate can be decoded by hippocalcin translocation. Local AMPA receptor activation resulted in fast hippocalcin-YFP translocation to specific sites within a dendritic tree mainly due to AMPA receptor-dependent depolarization and following Ca²⁺influx via voltage-operated calcium channels. Short local NMDA receptor activation induced fast hippocalcin-YFP translocation in a dendritic shaft at the application site due to direct Ca²⁺influx via NMDA receptor channels. Intrinsic network bursting produced hippocalcin-YFP translocation to a set of dendritic spines when they were subjected to several successive synaptic vesicle releases during a given burst whereas no translocation to spines was observed in response to a single synaptic vesicle release and to back-propagating action potentials. The translocation to spines required Ca²⁺influx via synaptic NMDA receptors in which Mg²⁺ block is relieved by postsynaptic depolarization. This synaptic translocation was restricted to spine heads and even closely (within 1–2 μm) located spines on the same dendritic branch signalled independently. Thus, we conclude that hippocalcin may differentially decode various spatiotemporal patterns of glutamate receptor activation into site- and time-specific translocation to its targets. Hippocalcin also possesses an ability to produce local signalling at the single synaptic level providing a molecular mechanism for homosynaptic plasticity.

Regulation of exocytosis by protein kinase C

PKC (protein kinase C) has been known for many years to modulate regulated exocytosis in a wide variety of cell types. In neurons and neuroendocrine cells, PKC regulates several different stages of the exocytotic process, suggesting that these multiple actions of PKC are mediated by phosphorylation of distinct protein targets. In recent years, a variety of exocytotic proteins have been identified as PKC substrates, the best characterized of which are SNAP-25 (25 kDa synaptosome-associated protein) and Munc18. In the present study, we review recent evidence suggesting that site-specific phosphorylation of SNAP-25 and Munc18 by PKC regulates distinct stages of exocytosis.

Role of myristoylation in the intracellular targeting of neuronal calcium sensor (NCS) proteins

The control of the intracellular localization of NCS (neuronal calcium sensor) proteins is of importance for their ability to respond appropriately to differing calcium signals. We examine the localization of three NCS proteins: NCS-1, KChIP-1 (potassium-channel-interacting protein 1) and hippocalcin. Additionally, the [Ca2+] dependency of the calcium-induced translocation of hippocalcin is investigated. The implications of the differential targeting of these proteins on calcium signal interpretation are considered.

Relationship Between Intracellular Free Calcium Concentration and NMDA-induced Cerebellar Granule Cell Survival In Vitro

The survival of cerebellar granule cells in culture is stimulated by activation of the N-methyl-d-aspartate (NMDA) class of glutamate receptors. Activation of these receptors at the key period for cell survival in vitro (3 days; 3DIV) resulted in a sustained elevation of intracellular free calcium concentration [Ca2+]i over the same concentration range of NMDA that led to granule cell survival. Agents that release Ca2+ from intracellular stores led to only small, transient elevations of [Ca2+]i and were unable to stimulate granule cell survival. Addition of the Ca2+ ionophore ionomycin to granule cell cultures at 3DIV resulted in increased granule cell number at 7DIV. The ability of ionomycin to stimulate granule cell survival was related to the [Ca2+]i elicited, indicating that a rise in [Ca2+]i is sufficient to activate the processes leading to granule cell survival and that the extent of the elevation in [Ca2+]i is crucially important in determining granule cell fate.

Phosphorylation of cysteine string protein by protein kinase A. Implications for the modulation of exocytosis

Cyclic AMP-dependent protein kinase (PKA) enhances regulated exocytosis in neurons and most other secretory cells. To explore the molecular basis of this effect, known exocytotic proteins were screened for PKA substrates. Both cysteine string protein (CSP) and soluble NSF attachment protein-alpha (alpha-SNAP) were phosphorylated by PKA in vitro, but immunoprecipitation of cellular alpha-SNAP failed to detect (32)P incorporation. In contrast, endogenous CSP was phosphorylated in synaptosomes, PC12 cells, and chromaffin cells. In-gel kinase assays confirmed PKA to be a cellular CSP kinase, with phosphorylation occurring on Ser(10). PKA phosphorylation of CSP reduced its binding to syntaxin by 10-fold but had little effect on its interaction with HSC70 or G-protein subunits. Furthermore, an in vivo role for Ser(10) phosphorylation at a late stage of exocytosis is suggested by analysis of chromaffin cells transfected with wild type or non-phosphorylatable mutant CSP. We propose that PKA phosphorylation of CSP could modulate the exocytotic machinery, by selectively altering its availability for protein-protein interactions.

SNAP-25 with mutations in the zero layer supports normal membrane fusion kinetics

Considerable data support the idea that intracellular membrane fusion involves a conserved machinery containing the SNARE proteins. SNAREs assembled in vitro form a stable 4-helix bundle and it has been suggested that formation of this complex provides the driving force for bilayer fusion. We have tested this possibility in assays of exocytosis in cells expressing a botulinum neurotoxin E (BoNT/E)-resistant mutant of SNAP-25 in which additional disruptive mutations have been introduced. Single or double mutations of glutamine to glutamate or to arginine in the central zero layer residues of SNAP-25 did not impair the extent, time course or Ca2+-dependency of exocytosis in PC12 cells. Using adrenal chromaffin cells, we found that exocytosis could be reconstituted in cells transfected to express BoNT/E. A double Q→E mutation did not prevent reconstitution and the kinetics of single granule release events were indistinguishable from control cells. This shows a high level of tolerance of changes in the zero layer indicating that the conservation of these residues is not due to an essential requirement in vesicle docking or fusion and suggests that formation of a fully stable SNARE complex may not be required to drive membrane fusion.

Voltage-independent inhibition of P/Q-type Ca2+ channels in adrenal chromaffin cells via a neuronal Ca2+ sensor-1-dependent pathway involves Src family tyrosine kinase

In common with many neurons, adrenal chromaffin cells possess distinct voltage-dependent and voltage-independent pathways for Ca(2+) channel regulation. In this study, the voltage-independent pathway was revealed by addition of naloxone and suramin to remove tonic blockade of Ca(2+) currents via opioid and purinergic receptors due to autocrine feedback inhibition. This pathway requires the Ca(2+)-binding protein neuronal calcium sensor-1 (NCS-1). The voltage-dependent pathway was pertussis toxin-sensitive, whereas the voltage-independent pathway was largely pertussis toxin-insensitive. Characterization of the voltage-independent inhibition of Ca(2+) currents revealed that it did not involve protein kinase C-dependent signaling pathways but did require the activity of a Src family tyrosine kinase. Two structurally distinct Src kinase inhibitors, 4-amino-5-(4-methylphenyl)7-(t-butyl)pyrazolo[3,4-d] pyrimidine (PP1) and a Src inhibitory peptide, increased the Ca(2+) currents, and no further increase in Ca(2+) currents was elicited by addition of naloxone and suramin. In addition, the Src-like kinase appeared to act in the same pathway as NCS-1. In contrast, addition of PP1 did not prevent a voltage-dependent facilitation elicited by a strong pre-pulse depolarization indicating that this pathway was independent of Src kinase activity. PPI no longer increased Ca(2+) currents after addition of the P/Q-type channel blocker omega-agatoxin TK. The alpha(1A) subunit of P/Q-type Ca(2+) channels was immunoprecipitated from chromaffin cell extracts and found to be phosphorylated in a PP1-sensitive manner by endogenous kinases in the immunoprecipitate. A high molecular mass (around 220 kDa) form of the alpha(1A) subunit was detected by anti-phosphotyrosine, suggesting a possible target for Src family kinase action. These data demonstrate a voltage-independent mechanism for autocrine inhibition of P/Q-type Ca(2+) channel currents in chromaffin cells that requires Src family kinase activity and suggests that this may be a widely distributed pathway for Ca(2+) channel regulation.

Cysteine residues of SNAP-25 are required for SNARE disassembly and exocytosis, but not for membrane targeting

The release of neurotransmitter at a synapse occurs via the regulated fusion of synaptic vesicles with the plasma membrane. The fusion of the two lipid bilayers is mediated by a protein complex that includes the plasma membrane target soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (t-SNAREs), syntaxin 1A and synaptosome-associated protein of 25 kDa (SNAP-25), and the vesicle SNARE (v-SNARE), vesicle-associated membrane protein (VAMP). Whereas syntaxin 1A and VAMP are tethered to the membrane by a C-terminal transmembrane domain, SNAP-25 has been suggested to be anchored to the membrane via four palmitoylated cysteine residues. We demonstrate that the cysteine residues of SNAP-25 are not required for membrane localization when syntaxin 1A is present. Analysis of the 7 S and 20 S complexes formed by mutants that lack cysteine residues demonstrates that the cysteines are required for efficient SNARE complex dissociation. Furthermore, these mutants are unable to support exocytosis, as demonstrated by a PC12 cell secretion assay. We hypothesize that syntaxin 1A serves to direct newly synthesized SNAP-25 through the Golgi transport pathway to the axons and synapses, and that palmitoylation of cysteine residues is not required for targeting, but to optimize interactions required for SNARE complex dissociation.

Control of membrane fusion dynamics during regulated exocytosis

The study of regulated exocytosis uniquely allows the direct measurement of intracellular membrane fusion events in real time. We have exploited this to examine factors that regulate not only the extent but also the dynamics of single fusion/release events. The general strategy used has been to assess exocytosis in transiently transfected PC12 or adrenal chromaffin cells. We aimed to design mutant constructs based on in vitro biochemistry, in some cases informed by knowledge of protein structure. Using this approach we have demonstrated an inhibitory role for the putative Rab3 effector Noc2 that requires interaction with Rab3. Using carbon-fibre amperometry on adrenal chromaffin cells, we have demonstrated regulation of the kinetics of single granule release events consistent with changes in fusion pore dynamics and switches between full fusion and 'kiss-and-run' fusion. These studies have demonstrated a late role for cysteine string protein in exocytosis. In addition, they have focused attention on a key role for Munc18 in the regulation of post-fusion events that affect fusion pore dynamics.

SNARE proteins are highly enriched in lipid rafts in PC12 cells: implications for the spatial control of exocytosis

Lipid rafts are microdomains present within membranes of most cell types. These membrane microdomains, which are enriched in cholesterol and glycosphingolipids, have been implicated in the regulation of certain signal transduction and membrane traffic pathways. To investigate the possibility that lipid rafts organize exocytotic pathways in neuroendocrine cells, we examined the association of proteins of the exocytotic machinery with rafts purified from PC12 cells. The target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (tSNARE) proteins syntaxin 1A and synaptosomal-associated protein of 25 kDa (SNAP-25) were both found to be highly enriched in lipid rafts ( approximately 25-fold). The vesicle SNARE vesicle-associated membrane protein (VAMP)2 was also present in raft fractions, but the extent of this recovery was variable. However, further analysis revealed that the majority of VAMP2 was associated with a distinct class of raft with different detergent solubility characteristics to the rafts containing syntaxin 1A and SNAP-25. Interestingly, no other studied secretory proteins were significantly associated with lipid rafts, including SNARE effector proteins such as nSec1. Chemical crosslinking experiments showed that syntaxin1A/SNAP-25 heterodimers were equally present in raft and nonraft fractions, whereas syntaxin1A/nSec1 complexes were detected only in nonraft fractions. SDS-resistance assays revealed that raft-associated syntaxin1A/SNAP-25 heterodimers were able to interact with VAMP2. Finally, reduction of cellular cholesterol levels decreased the extent of regulated exocytosis of dopamine from PC12 cells. The results described suggest that the interaction of SNARE proteins with lipid rafts is important for exocytosis and may allow structural and spatial organization of the secretory machinery.

A direct inhibitory role for the Rab3-specific effector, Noc2, in Ca2+-regulated exocytosis in neuroendocrine cells

Rab proteins comprise a family of GTPases, conserved from yeast to mammals, which are integral components of membrane trafficking pathways. Rab3A is a neural/neuroendocrine-specific member of the Rab family involved in Ca(2+) -regulated exocytosis, where it functions in an inhibitory capacity controlling recruitment of secretory vesicles into a releasable pool at the plasma membrane. The effector by which Rab3A exerts its inhibitory effect is unclear as the Rab3A effectors Rabphilin and RIM have been excluded from for this role. One putative Rab3A effector in dense-core granule exocytosis is the cytosolic zinc finger protein, Noc2. We have established that overexpression of Noc2 in PC12 cells has a direct inhibitory effect upon Ca(2+)-triggered exocytosis in permeabilized cells. We demonstrate specific nucleotide-dependent binding of Noc2 to Rab3A and show that the inhibition of exocytosis is dependent upon this interaction since Rab3A binding-deficient mutants of Noc2 do not inhibit exocytosis. We propose that Noc2 may be a negative effector for Rab3A in regulated exocytosis of dense-core granules from endocrine cells.

Control of fusion pore dynamics during exocytosis by Munc18

Intracellular membrane fusion is mediated by the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. All vesicle transport steps also have an essential requirement for a member of the Sec1 protein family, including the neuronal Munc18-1 (also known as nSec1) in regulated exocytosis. Here, in adrenal chromaffin cells, we expressed a Munc18 mutant with reduced affinity for syntaxin, which specifically modified the kinetics of single-granule exocytotic release events, consistent with an acceleration of fusion pore expansion. Thus, Munc18 functions in a late stage in the fusion process, where its dissociation from syntaxin determines the kinetics of postfusion events.

Cysteine string protein expression in mammary epithelial cells

Cysteine string protein (Csp) is a secretory vesicle protein previously demonstrated to be required for Ca2+-regulated exocytosis in neurons and endocrine cells. It has been suggested to function by regulating voltage-gated Ca2+ channels or, alternatively, to have a more direct effect on the regulated exocytotic machinery. Here we demonstrate the expression of Csp in mammary epithelial cells and in the KIM-2 mammary cell line. In KIM-2 cells, Csp was found to be associated with a population of small vesicles and showed partial co-distribution with the vesicle protein cellubrevin. KIM-2 cells do not express detectable levels of voltage-gated Ca2+ channels, ruling these out as a site of action. Using the release of transfected growth hormone (GH) as an assay of secretion, we found that GH is secreted in an exclusively constitutive manner from KIM-2 cells. Overexpression of Csp1 inhibits regulated exocytosis in other cell types but has no effect on constitutive GH release by KIM-2 cells. These results suggest that Csp does not have a major function in constitutive exocytosis.

The synaptic vesicle protein, cysteine-string protein, is associated with the plasma membrane in 3T3-L1 adipocytes and interacts with syntaxin 4

Adipocytes and muscle cells play a major role in blood glucose homeostasis. This is dependent upon the expression of Glut4, an insulin-responsive facilitative glucose transporter. Glut4 is localised to specialised intracellular vesicles that fuse with the plasma membrane in response to insulin stimulation. The insulin-induced translocation of Glut4 to the cell surface is essential for the maintenance of optimal blood glucose levels, and defects in this system are associated with insulin resistance and type II diabetes. Therefore, a major focus of recent research has been to identify and characterise proteins that regulate Glut4 translocation. Cysteine-string protein (Csp) is a secretory vesicle protein that functions in presynaptic neurotransmission and also in regulated exocytosis from non-neuronal cells. We show that Csp1 is expressed in 3T3-L1 adipocytes and that cellular levels of this protein are increased following cell differentiation. Combined fractionation and immunofluorescence analyses reveal that Csp1 is not a component of intracellular Glut4-storage vesicles (GSVs), but is associated with the adipocyte plasma membrane. This association is stable, and not affected by either insulin stimulation or chemical depalmitoylation of Csp1. We also demonstrate that Csp1 interacts with the t-SNARE syntaxin 4. As syntaxin 4 is an important mediator of insulin-stimulated GSV fusion with the plasma membrane, this suggests that Csp1 may play a regulatory role in this process. Syntaxin 4 interacts specifically with Csp1, but not with Csp2. In contrast, syntaxin 1A binds to both Csp isoforms, and actually exhibits a higher affinity for the Csp2 protein. The results described raise a number of interesting questions concerning the intracellular targeting of Csp in different cell types, and suggest that the composition and synthesis of GSVs may be different from synaptic and other secretory vesicles. In addition, the interaction of Csp1 with syntaxin 4 suggests that this Csp isoform may play a role in insulin-stimulated fusion of GSVs with the plasma membrane.

The neuronal calcium sensor family of Ca2+-binding proteins

Ca(2+) plays a central role in the function of neurons as the trigger for neurotransmitter release, and many aspects of neuronal activity, from rapid modulation to changes in gene expression, are controlled by Ca(2+). These actions of Ca(2+) must be mediated by Ca(2+)-binding proteins, including calmodulin, which is involved in Ca(2+) regulation, not only in neurons, but in most other cell types. A large number of other EF-hand-containing Ca(2+)-binding proteins are known. One family of these, the neuronal calcium sensor (NCS) proteins, has a restricted expression in retinal photoreceptors or neurons and neuroendocrine cells, suggesting that they have specialized roles in these cell types. Two members of the family (recoverin and guanylate cyclase-activating protein) have established roles in the regulation of phototransduction. Despite close sequence similarities, the NCS proteins have distinct neuronal distributions, suggesting that they have different functions. Recent work has begun to demonstrate the physiological roles of members of this protein family. These include roles in the modulation of neurotransmitter release, control of cyclic nucleotide metabolism, biosynthesis of polyphosphoinositides, regulation of gene expression and in the direct regulation of ion channels. In the present review we describe the known sequences and structures of the NCS proteins, information on their interactions with target proteins and current knowledge about their cellular and physiological functions.

Neuronal Ca2+ sensor-1/frequenin functions in an autocrine pathway regulating Ca2+ channels in bovine adrenal chromaffin cells

NCS-1/frequenin belongs to a family of EF-hand-containing Ca(2+) sensors expressed mainly in neurons. Overexpression of NCS-1/frequenin has been shown to stimulate neurotransmitter release but little else is known of its cellular roles. We have constructed an EF-hand mutant, NCS-1(E120Q), as a likely dominant inhibitor of cellular NCS-1 function. Recombinant NCS-1(E120Q) showed an impaired Ca(2+)-dependent conformational change but could still bind to cellular proteins. Transient expression of this mutant, but not NCS-1, in bovine adrenal chromaffin cells increased non-L-type Ca(2+) channel currents. Cells expressing NCS-1(E120Q) no longer responded effectively to the removal of autocrine purinergic/opioid inhibition of Ca(2+) currents but still showed voltage-dependent facilitation. These data are consistent with the existence of both voltage-dependent and voltage-independent pathways for Ca(2+) channel inhibition in chromaffin cells. Our results suggest a novel function for NCS-1 specific for the voltage-independent autocrine pathway that negatively regulates non-L-type Ca(2+) channels in chromaffin cells.

Purification of Golgi casein kinase from bovine milk

Caseins and many other secretory proteins are phosphorylated during their transport through the secretory pathway by a protein kinase present within Golgi compartments. Molecular analysis of the Golgi casein kinase (GCK) has not been possible since it has not been purified to homogeneity or been cloned. Previous attempts have been made to purify GCK activity from mammary gland Golgi fractions, but these have not resulted in extensive purification of the enzyme. In the present study, we have demonstrated that substantial amounts of GCK activity, assayed using a specific peptide substrate, can be detected as a soluble form in bovine milk, and we have used milk as a source for purification. A purification protocol was established that allowed>80000-fold purification to a specific activity of GCK (approx. 700 nmoles/min per mg of protein) far higher than previously achieved. These findings cast doubts on previous claims for purification of GCK activity. In addition, ion-exchange chromatography resolved two closely eluting peaks of activity, suggesting the existence of two related, but distinct, GCK activities.

Cysteine-string protein: the chaperone at the synapse

Cysteine-string protein (Csp) is a major synaptic vesicle and secretory granule protein first discovered in Drosophila and Torpedo. Csps were subsequently identified from Xenopus, Caenorhabditis elegans, and mammalian species. It is clear from the study of a null mutant in Drosophila that Csp is required for viability of the organism and that it has a key role in neurotransmitter release. In addition, other studies have directly implicated Csp in regulated exocytosis in mammalian neuroendocrine and endocrine cell types, and its distribution suggests a general role in regulated exocytosis. An early hypothesis was that Csp functioned in the control of voltage-gated Ca2+ channels. Csp, however, must have an additional function as a direct regulator of the exocytotic machinery as changes in Csp expression modify the extent of exocytosis triggered directly by Ca2+ in permeabilised cells. Csps possess a cysteine-string domain that is highly palmitoylated and confers membrane targeting. In addition, Csps have a conserved "J" domain that mediates binding to an activation of the Hsp70/ Hsc70 chaperone ATPases. This and other evidence implicate Csps as molecular chaperones in the synapse that are likely to control the correct conformational folding of one or more components of the vesicular exocytotic machinery. Targets for Csp include the vesicle protein VAMP/synaptobrevin and the plasma membrane protein syntaxin 1, the significance of which is discussed in possible models to account for current knowledge of Csp function.

Measurement of exocytosis by amperometry in adrenal chromaffin cells: effects of clostridial neurotoxins and activation of protein kinase C on fusion pore kinetics

We have used carbon-fibre amperometry to examine the kinetics of individual secretory granule fusion/release events in bovine adrenal chromaffin cells. Transfection with plasmids encoding the light chains of botulinum neurotoxins (BoNTs) was used to investigate the effects of cleavage of syntaxin or SNAP-25 on exocytosis. Expression of BoNT/C1 or BoNT/E inhibited the extent of exocytosis that was evoked by application of digitonin/Ca(2+) to permeabilise and stimulate single chromaffin cells. Following neurotoxin expression, the residual release events were no different from those of control cells in their magnitude and kinetics from analysis of the amperometric spikes. In contrast, activation of protein kinase C (PKC) resulted in a modification of the kinetics of single granule release events. Following phorbol ester treatment, the amperometric spikes showed a significant decrease in their total charge due to a decrease in their mean half-width with increases in the rate of the initial rise and also the fall to baseline of the spikes. These changes were prevented by pre-treatment with the PKC inhibitor bisindolylmaleimide. These results suggest that PKC regulates the rate of fusion pore expansion and also subsequent pore closure or granule retrieval. A PKC-mediated regulation of kiss-and-run fusion may, therefore, control the extent of catecholamine release from single secretory granules. The experimental approach used here may provide further information on the protein constituents and regulation of the fusion pore machinery.

Comparison of cysteine string protein (Csp) and mutant alpha-SNAP overexpression reveals a role for csp in late steps of membrane fusion in dense-core granule exocytosis in adrenal chromaffin cells

Assembly of the SNARE complex and its disassembly caused by the action of soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) and NSF is crucial for the maintenance of vesicular traffic, including fusion of regulated exocytotic vesicles. Various other proteins may also have important roles in the processes leading to membrane fusion via interaction with the SNARE proteins, including the secretory vesicle cysteine string protein (Csp). Here we have examined the effect of overexpression of a dominant negative alpha-SNAP mutant or Csp on exocytosis of dense-core granules in single chromaffin cells monitored using amperometry to detect released catecholamine. Exocytosis of trans-Golgi network (TGN)-derived dense-core granules was substantially inhibited by expression of alpha-SNAP(L294A). The amplitude and characteristics of the individual release events were unaffected by expression of alpha-SNAP(L294A), consistent with an essential role for alpha-SNAP in early steps of priming but not in the fusion process. In contrast, Csp overexpression, which also inhibited the extent of exocytosis, also modified the kinetics of the individual release events seen as an increase in the rise time and a broadening of the residual amperometric spikes in Csp-transfected cells. These results suggest that unlike alpha-SNAP, Csp plays a key role in the protein interactions close to the fusion process or fusion pore opening during Ca(2+)-regulated exocytosis.

Botulinum neurotoxin E-insensitive mutants of SNAP-25 fail to bind VAMP but support exocytosis

Neurotransmitter release from synaptic vesicles is mediated by complex machinery, which includes the v- and t-SNAP receptors (SNAREs), vesicle-associated membrane protein (VAMP), synaptotagmin, syntaxin, and synaptosome-associated protein of 25 kDa (SNAP-25). They are essential for neurotransmitter exocytosis because they are the proteolytic substrates of the clostridial neurotoxins tetanus neurotoxin and botulinum neurotoxins (BoNTs), which cause tetanus and botulism, respectively. Specifically, SNAP-25 is cleaved by both BoNT/A and E at separate sites within the COOH-terminus. We now demonstrate, using toxin-insensitive mutants of SNAP-25, that these two toxins differ in their specificity for the cleavage site. Following modification within the COOH-terminus, the mutants completely resistant to BoNT/E do not bind VAMP but were still able to form a sodium dodecyl sulfate-resistant complex with VAMP and syntaxin. Furthermore, these mutants retain function in vivo, conferring BoNT/E-resistant exocytosis to transfected PC12 cells. These data provide information on structural requirements within the C-terminal domain of SNAP-25 for its function in exocytosis and raise doubts about the significance of in vitro binary interactions for the in vivo functions of synaptic protein complexes.

Neuronal Ca(2+) sensor 1. Characterization of the myristoylated protein, its cellular effects in permeabilized adrenal chromaffin cells, Ca(2+)-independent membrane association, and interaction with binding proteins, suggesting a role in rapid Ca(2+) signal transduction

Overexpression of frequenin and its orthologue neuronal Ca(2+) sensor 1 (NCS-1) has been shown to increase evoked exocytosis in neurons and neuroendocrine cells. The site of action of NCS-1 and its biochemical targets that affect exocytosis are unknown. To allow further investigation of NCS-1 function, we have demonstrated that NCS-1 is a substrate for N-myristoyltransferase and generated recombinant myristoylated NCS-1. The bacterially expressed NCS-1 shows Ca(2+)-induced conformational changes. The possibility that NCS-1 directly interacts with the exocytotic machinery to enhance exocytosis was tested using digitonin-permeabilized chromaffin cells. Exogenous NCS-1 was retained in permeabilized cells but had no effect on Ca(2+)-dependent release of catecholamine. In addition, exogenous NCS-1 did not regulate cyclic nucleotide levels in this system. These data suggest that the effects of NCS-1 seen in intact cells are likely to be due to an action on the early steps of stimulus-secretion coupling or on Ca(2+) homeostasis. Myristoylated NCS-1 bound to membranes in the absence of Ca(2+) and endogenous NCS-1 was tightly membrane-associated. Using biotinylated NCS-1, a series of specific binding proteins were detected in cytosol, chromaffin granule membrane, and microsome fractions of adrenal medulla. These included proteins distinct from those detected by biotinylated calmodulin, demonstrating the presence of multiple specific Ca(2+)-independent and Ca(2+)-dependent binding proteins as putative targets for NCS-1 action. A model for NCS-1 function, from these data, indicates a constitutive membrane association independent of Ca(2+). This differs from the Ca(2+) myristoyl switch model for the closely related recoverin and suggests a possible action in rapid Ca(2+) signal transduction in response to local Ca(2+) signals.

Protein phosphorylation and the regulation of synaptic membrane traffic

It is well established that protein phosphorylation has an important role in synaptic plasticity. This is achieved, in part, via the presynaptic modulation of neurotransmitter release by protein kinases and protein phosphatases. In recent years, the increase in information available about proteins that are involved in synaptic exocytosis and endocytosis has been exploited in order to study the effects of protein phosphorylation on synaptic-vesicle cycling at the molecular level. The best-characterized protein in this respect is synapsin, whose function in the release of synaptic vesicles from the reserve pool is regulated by phosphorylation. More recently, it has emerged that proteins that function at other stages of the synaptic-vesicle cycle, which include priming of vesicles for docking-fusion and endocytic recycling, are also controlled by phosphorylation. Furthermore, recent work suggests that this regulation of membrane traffic by phosphorylation also occurs postsynaptically, where it contributes to synaptic plasticity.

nSec-1 (munc-18) interacts with both primed and unprimed syntaxin 1A and associates in a dimeric complex on adrenal chromaffin granules

The target-SNARE syntaxin 1A is an essential component of the core machinery required for regulated exocytosis (where SNARE is the soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor). Syntaxin 1A interacts with a variety of other proteins, two of which, N-ethylmaleimide-sensitive fusion protein (NSF) and alpha-soluble NSF attachment protein (alpha-SNAP) have been suggested to impart a conformational rearrangement on this protein during a reaction referred to as priming. We have studied the effect of the primed state on the binding properties of syntaxin 1A and we have confirmed that primed syntaxin 1A no longer associated with alpha-SNAP or its cognate vesicle-SNARE, vesicle-associated membrane protein (VAMP). Under such conditions, however, it retained the ability to bind to nSec-1. It has been demonstrated that nSec-1, a regulatory protein also involved in neuronal exocytosis, binds syntaxin 1A with high affinity in vitro, although evidence for this physical interaction occurring in vivo has proven elusive. We analysed the subcellular distribution of these two proteins in fractions from bovine adrenal medulla and detected syntaxin 1A and nSec-1 in both plasma membrane and chromaffin-granule fractions. Using a cross-linking approach with chromaffin-granule membranes we detected a putative dimeric complex composed of approx. 54% total granule membrane nSec-1 and approx. 30% total syntaxin 1A. The results of this study therefore suggest the possibility of nSec-1 interactions with primed syntaxin 1A and demonstrate a potentially significant interaction of syntaxin 1A and nSec-1 on the membranes of chromaffin granules.

Doc2 is not associated with known regulated exocytotic or endosomal compartments in adrenal chromaffin cells

Doc2 is a C2-domain-containing protein that is highly expressed in the nervous system and has a constitutively expressed isoform. It has been implicated as a potential Ca2+ sensor in regulated exocytosis, and has been suggested to be associated with synaptic vesicles. To examine whether Doc2 is associated with synaptic-like microvesicles (SLMVs) or dense-core granules in neuroendocrine cells, we examined the distribution of Doc2 in subcellular fractionation of chromaffin cells of the adrenal medulla and in PC12 cells. Doc2 did not co-distribute with SLMVs from either cell type, but did appear to co-distribute with dense-core granules from PC12 cells. In contrast, it was not associated with the dense-core granules during subcellular fractionation of the adrenal medulla, and nor did it appear to be associated with endosomes, cis-Golgi or the trans-Golgi network. In contrast, Doc2 co-distributed under all conditions with a mitochondrial marker. We conclude that Doc2 is not a general component of regulated secretory vesicles, but may instead be associated with mitochondria.

Early requirement for alpha-SNAP and NSF in the secretory cascade in chromaffin cells

NSF and alpha-SNAP have been shown to be required for SNARE complex disassembly and exocytosis. However, the exact requirement for NSF and alpha-SNAP in vesicular traffic through the secretory pathway remains controversial. We performed a study on the kinetics of exocytosis from bovine chromaffin cells using high time resolution capacitance measurement and electrochemical amperometry, combined with flash photolysis of caged Ca2+ as a fast stimulus. alpha-SNAP, a C-terminal mutant of alpha-SNAP, and NEM were assayed for their effects on secretion kinetics. Two kinetically distinct components of catecholamine release can be observed upon fast step-like elevation of [Ca2+]i. One is the exocytotic burst, thought to represent the readily releasable pool of vesicles. Following the exocytotic burst, secretion proceeds slowly at maintained high [Ca2+]i, which may represent vesicle maturation/recruitment, i.e. some priming steps after docking. alpha-SNAP increased the amplitude of both the exocytotic burst and the slow component but did not change their kinetics, which we examined with millisecond time resolution. In addition, NEM only partially inhibited the slow component without altering the exocytotic burst, fusion kinetics and the rate of endocytosis. These results suggest a role for alpha-SNAP/NSF in priming granules for release at an early step, but not modifying the fusion of readily releasable granules.

Mutational analysis of cysteine-string protein function in insulin exocytosis

Cysteine-string proteins (Csps) are vesicle proteins involved in neurotransmission. They contain at least four domains: an N-terminal J-domain which can interact with the chaperone Hsc70, an adjacent linker region, the defining cysteine rich domain and a variable C terminus. As the relevance of these domains for the function of Csps in exocytosis is unknown, we have performed a mutational analysis of Csp domains using insulin release by large dense core vesicles (LDCVs) as a model of regulated exocytosis. All mutants were apparently palmitoylated and their subcellular distribution was similar to endogenous Csp. Point mutations within the highly conserved HPD motif of the J-domain abolished activation of Hsc70. However, these mutations altered the effect of Csp on exocytosis only after additional truncation of the extreme C terminus as found in the Csp splice variant Csp2. Furthermore, the strikingly conserved linker region adjacent to the J-domain was important for Csp function in exocytosis, but not for the activation of Hsc70 ATPase. The effects of Csp wild-type or mutants were preserved in permeabilized cells excluding an effect on transmembrane ion fluxes. These observations demonstrate a functional difference between the two isoforms and suggest a role for the J-domain co-chaperone function as well as for the newly defined linker region in LDCV exocytosis.

The effect of transfection with Botulinum neurotoxin C1 light chain on exocytosis measured in cell populations and by single-cell amperometry in PC12 cells

We examined the effect on exocytosis in PC12 neuroendocrine cells of transient transfection with the specific endoprotease Botulinum neurotoxin C1 light chain (BoNT/C1), which cleaves syntaxin and SNAP-25. The effects of toxin expression on basal and evoked exocytosis were determined in cell population measurements and also in a single-cell transfection-amperometry assay. Co-expression of BoNT/C1 with human growth hormone (hGH) as a marker of secretory granules in transfected cells resulted in a 95% inhibition of hGH release evoked either by the purinergic agonist ATP or by depolarization with 55 mM K+. In addition, basal hGH release was also inhibited to the same extent. The high level of co-transfection efficiency revealed by this extent of inhibition was exploited in a high-resolution single-cell assay based on cell detection by expression of enhanced green fluorescent protein (EGFP) and analysis of evoked dopamine release by amperometry using a carbon fibre microelectrode. Cells expressing EGFP alone showed population responses and single-cell amperometric responses indistinguishable from those of control non-transfected cells. In contrast, co-expression of BoNT/C1 with EGFP resulted in an almost complete inhibition of current transients due to exocytosis evoked by ATP. These results establish and validate a single-cell assay of transfection-amperometry for analysing the effects of specific proteins on exocytosis.

The Rab5 effector EEA1 is a core component of endosome docking

Intracellular membrane docking and fusion requires the interplay between soluble factors and SNAREs. The SNARE hypothesis postulates that pairing between a vesicular v-SNARE and a target membrane z-SNARE is the primary molecular interaction underlying the specificity of vesicle targeting as well as lipid bilayer fusion. This proposal is supported by recent studies using a minimal artificial system. However, several observations demonstrate that SNAREs function at multiple transport steps and can pair promiscuously, questioning the role of SNAREs in conveying vesicle targeting. Moreover, other proteins have been shown to be important in membrane docking or tethering. Therefore, if the minimal machinery is defined as the set of proteins sufficient to reproduce in vitro the fidelity of vesicle targeting, docking and fusion as in vivo, then SNAREs are not sufficient to specify vesicle targeting. Endosome fusion also requires cytosolic factors and is regulated by the small GTPase Rab5. Here we show that Rab5-interacting soluble proteins can completely substitute for cytosol in an in vivo endosome-fusion assay, and that the Rab5 effector EEA1 is the only factor necessary to confer minimal fusion activity. Rab5 and other associated proteins seem to act upstream of EEA1, implying that Rab5 effectors comprise both regulatory molecules and mechanical components of the membrane transport machinery. We further show that EEA1 mediates endosome docking and, together with SNAREs, leads to membrane fusion.

Calcium sensors in regulated exocytosis

Neurotransmitter release, hormone secretion and a variety of other secretory process are tightly regulated with exocytotic fusion of secretory vesicles being triggered by a rise in cytosolic Ca2+ concentration. A series of proteins that act as part of a conserved core machinery for vesicle docking and fusion throughout the cell have been identified. In regulated exocytosis this core machinery must be controlled by Ca(2+)-sensor proteins that allow rapid activation of the fusion process following elevation of cytosolic Ca2+ concentration. The properties of such Ca2+ sensors are known from physiological studies but their molecular identity remains to be unequivocally established. The multiple Ca(2+)-dependent steps in the exocytotic pathway suggest the likely involvement of several Ca(2+)-binding proteins with distinct properties. Functional evidence for the role of various Ca(2+)-binding proteins and their possible sites of action is accumulating but a definitive identification of the major Ca(2+)-sensor in the final step of Ca(2+)-triggered membrane fusion in different cell types awaits further analysis.

Examination of the role of ADP-ribosylation factor and phospholipase D activation in regulated exocytosis in chromaffin and PC12 cells

The possible role of ADP-ribosylation factor (ARF)-activated and constitutive phospholipase D (PLD) activity in regulated exocytosis of preformed secretory granules in adrenal chromaffin and PC12 cells was examined. With use of digitonin-permeabilised cells, the effect of GTP analogues and exogenous ARF1 on PLD activity was determined. No evidence was seen for ARF-stimulated PLD activity in these cell types. Exocytosis from cytosol-depleted permeabilised chromaffin cells was not increased by adding recombinant nonmyristoylated or myristoylated ARF1, and exocytosis from both cell types was resistant to brefeldin A (BFA). Addition of bacterial PLD with demonstrably high activity in permeabilised chromaffin cells did not increase exocytosis in cytosol-depleted chromaffin cells. Diversion of PLD activity from production of phosphatidic acid (PA) due to the presence of 4% ethanol did not inhibit exocytosis triggered by Ca2+ or poorly hydrolysable GTP analogues in permeabilised chromaffin or PC12 cells. These results indicate that exocytosis in these cell types does not appear to require a BFA-sensitive ARF and the triggering of exocytosis does not require PLD activity and formation of PA. These findings rule out a general requirement for PLD activity during regulated exocytosis.

Cysteine-string proteins regulate exocytosis of insulin independent from transmembrane ion fluxes

Cysteine-string proteins (Csps) are vesicle proteins involved in exocytosis of synaptic vesicles in Drosophila and modulation of presynaptic calcium influx. As both the contribution of calcium channel regulation to the role of Csp in exocytosis and a function of Csp outside the nervous system are unknown, we studied its function in endocrine exocytosis from large dense core vesicles (LDCVs) using insulin-secreting pancreatic beta-cells. Csps were expressed in primary and derived beta-cell lines on insulin-containing LDCVs. Suppression of Csp expression reduced not only depolarisation induced insulin release but also exocytosis in permeabilised cells directly stimulated by Ca2+. Thus, Csp is a secretory granule protein and is required for endocrine exocytosis independent of the modulation of transmembrane calcium fluxes.

The cysteine-string domain of the secretory vesicle cysteine-string protein is required for membrane targeting

The post-translational addition of palmitic acid residues to cysteine-string protein (Csp) was originally thought to form the basis for membrane association of this secretory-vesicle protein. However, subsequent work showed that chemical depalmitoylation of Csp does not result in its release from membranes. We have confirmed these findings and employed [3H]palmitate labelling of PC12 cells to demonstrate that Csp1 remains associated with membranes following the complete removal of palmitic acid residues. Although palmitoylation is not essential for the stable membrane association of Csp, its role in membrane targeting has not been assessed. To examine this, we constructed a Csp mutant protein with seven cysteines replaced by serines in the cysteine-string domain. In contrast to wild-type Csps, this mutant protein was not targeted to membranes when expressed in PC12 or HeLa cells. We conclude that although a palmitoylated cysteine-string domain is not required for stable membrane association of Csp, it is essential for initial membrane targeting.

Stimulation of NSF ATPase activity during t-SNARE priming

N-Ethylmaleimide-sensitive factor (NSF) plays a key role in vesicular traffic by disassembling and priming SNARE proteins for their function in docking and fusion. We demonstrate that the ATPase activity of NSF is activated by alpha-soluble NSF attachment protein (alpha-SNAP) in a complex with syntaxin 1A. In addition, we show that a construct consisting of the H3 domain of syntaxin IA (GST-synt(195-263), which does not support NSF disassembly in the presence of MgATP gave a larger stimulation. NSF ATPase activation was specific and did not occur using mutant alpha-SNAPs unable to bind GST-synt or with mutated C-termini. We suggest that activation of NSF ATPase activity in the SNARE complex may be essential to allow SNARE priming.

Neuronal Ca2+ sensor 1, the mammalian homologue of frequenin, is expressed in chromaffin and PC12 cells and regulates neurosecretion from dense-core granules

Neuronal Ca2+ sensor 1 (NCS-1) is the mammalian homologue of the Ca2+-binding protein frequenin previously implicated in regulation of neurotransmission in Drosophila (Pongs, O., Lindemeier, J., Zhu, X. R., Theil, T., Endelkamp, D., Krah-Jentgens, I., Lambrecht, H.-G., Koch, K. W., Schwemer, J., Rivosecchi, R., Mallart, A., Galceran, J. , Canal, I., Barbas, J. A., and Ferrus, A. (1993) Neuron 11, 15-28). NCS-1 has been considered to be expressed only in neurons, but we show that NCS-1 expression can be detected in bovine adrenal chromaffin and PC12 cells, two widely studied model neuroendocrine cells. NCS-1 was present in both cytosolic and membrane fractions including purified chromaffin granules, and in immunofluorescence, its distribution overlapped with peripheral punctate staining seen with the synaptic-like microvesicle marker synaptophysin in PC12 cells. The possible functional role of NCS-1 in exocytosis of dense-core granules was tested using transient transfection in PC12 cells and assay of co-transfected growth hormone (GH) release. Overexpression of NCS-1 increased evoked GH release in intact cells in response to ATP. No effect of overexpression was seen on GH release because of Ca2+ in permeabilized cells suggesting that NCS-1 may have a regulatory but not direct role in neurosecretion.

Cysteine string protein functions directly in regulated exocytosis

Cysteine string protein (Csp) is essential for neurotransmitter release in Drosophila. It has been suggested that Csp functions by regulating the activity of presynaptic Ca2+ channels, thus controlling exocytosis. We have examined the effect of overexpressing Csp1 in PC12 cells, a neuroendocrine cell line. PC12 cell clones overexpressing Csp1 did not show any changes in morphology, granule number or distribution, or in the levels of other key exocytotic proteins. This overexpression did not affect intracellular Ca2+ signals after depolarization, suggesting that Csp1 has no gross effect on Ca2+ channel activity in PC12 cells. In contrast, we show that Csp1 overexpression enhances the extent of exocytosis from permeabilized cells in response to Ca2+ or GTPgammaS in the absence of Ca2+. Because secretion from permeabilized cells is not influenced by Ca2+ channel activity, this represents the first demonstration that Csp has a direct role in regulated exocytosis.

Secretion of milk proteins

Mammary epithelial cells secrete milk proteins in a polarized manner from their apical surface during lactation. These secreted proteins are either synthesized by the mammary cells or are transported by transcytosis from blood plasma. The intracellular trafficking pathways by which milk proteins are secreted are known in general outline. In this review the basic cell biology of the mammary epithelial cell secretory pathway is considered in relation to what is known in more detail for other cell types. In addition, potential points of control of protein secretion are examined. The secretory biology of mammary epithelial cells has not been characterized extensively in recent years and, while some aspects are well understood, other key issues, which still remain to be resolved, have been highlighted.

Analysis of regulated exocytosis in adrenal chromaffin cells: insights into NSF/SNAP/SNARE function

Many of the proteins that function in regulated exocytosis have now been identified. Several proteins form part of a conserved core machinery that acts in many intracellular vesicular fusion steps and their essential roles confirmed by molecular genetic analysis. In addition, studies with adrenal chromaffin and PC12 cells have demonstrated the function of various proteins in regulated exocytosis and have permitted dissection of the stages of exocytosis in which they act. N-Ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are key proteins in exocytosis. Examination of their function has indicated that they have a predocking role most likely as molecular chaperones to prepare the docking/fusion machinery. The exact site and time of action in exocytosis of many of the other identified proteins are unknown. A major emphasis for the future will be analysis of the molecular physiology of regulated exocytosis to permit the assignment of functions to identified proteins in particular stages of the regulated exocytotic pathway.

Role of calcium in the pathway for milk protein secretion and possible relevance for mammary gland physiology

In an attempt to define control points within the secretory pathway for casein synthesis and secretion, we have examined the role of both cytosolic and intra-organelle Ca2+ in the control of casein synthesis, phosphorylation and secretion. In addition, the possible role of cell volume changes in stretch-activation of Ca2+ signals was examined. Examination of the kinetics of casein secretion from freshly isolated lactating mouse mammary acini showed that a portion of the newly synthesized casein was secreted in a constitutive manner. A further portion remained within the cells, and this was released following elevation of the intracellular free calcium concentration ([Ca2+]i) using ionomycin, indicating the presence of a Ca(2+)-regulated pathway for casein release. An increase in [Ca2+]i occurred in response to hypotonic challenge to induce cell swelling, and this involved both Ca2+ entry and Ca2+ mobilization from intracellular stores. Experiments examining the effects of depletion of intra-organelle Ca2+ indicated that intra-organelle Ca2+ was required for maintained casein phosphorylation, but not its secretion. Depletion of Ca2+ from the endoplasmic reticulum led to a marked inhibition of casein synthesis. The possible significance of these control mechanisms for the physiology of the mammary gland is discussed.

The molecular chaperone function of the secretory vesicle cysteine string proteins

The "J" domains of eukaryotic DnaJ-like proteins specify interaction with various Hsp70s. The conserved tripeptide, HPD, present in all J domains has been shown to be important for the interaction between yeast and bacterial DnaJ/Hsp70 protein pairs. We have characterized mutations in the HPD motif of the synaptic vesicle protein cysteine-string protein (Csp). Mutation of the histidine (H43Q) or aspartic acid (D45A) residues of this motif reduced the ability of Csp to stimulate the ATPase activity of mammalian Hsc70. The H43Q and D45A mutant proteins were not able to stimulate the ATPase activity of Hsc70 to any significant extent. The mutant proteins were characterized by competition assays, tryptic digestion analysis, and direct binding analysis from which it was seen that these proteins were defective in binding to Hsc70. Thus, the HPD motif of Csp is required for binding to Hsc70. We also analyzed the interaction between Csp and a model substrate protein, denatured firefly luciferase. Both Csp1 and the C-terminally truncated isoform Csp2 were able to prevent aggregation of heat-denatured luciferase, and they also cooperated with Hsc70 to prevent aggregation. In addition, complexes of Csp1 or Csp2 with Hsc70 and luciferase were isolated, confirming that these proteins interact and that Csps can bind directly to denatured proteins. Csp1 and Csp2 isoforms must differ in some aspect other than interaction with Hsc70 and substrate protein. These results show that both Csp1 and Csp2 can bind a partially unfolded protein and act as chaperones. This suggests that Csps may have a general chaperone function in regulated exocytosis.

Evidence against an acute inhibitory role of nSec-1 (munc-18) in late steps of regulated exocytosis in chromaffin and PC12 cells

nSec-1 (munc-18) is a mammalian homologue of proteins implicated in constitutive exocytosis in yeast and neurotransmission in Caenorhabditis elegans and Drosophila. Mutant phenotypes in these species suggest that nSec-1 is likely to be required for neurotransmission. Various other data have been interpreted as suggesting that nSec-1 could also be a negative regulator of Ca2+-dependent exocytosis. We have tested this possibility by introducing exogenous nSec-1 into permeabilised chromaffin or PC12 cells and examining its effects on Ca2+-induced and alpha-soluble N-ethylmaleimide-sensitive fusion protein attachment protein-stimulated exocytosis. No effects of exogenous nSec-1 were observed in these assays. In addition, the effect of nSec-1 overexpression in transiently transfected PC12 cells on reporter growth hormone (GH) secretion was examined. Overexpression of nSec-1 resulted in a marked increase in GH production, reflected in an increase in both cell-associated and medium GH levels. The relative amounts retained in the cells were unaffected by nSec-1 overexpression, indicating that GH storage was unaffected and that the major effect was on its synthesis. In contrast, nSec-1 overexpression did not affect the proportion of GH that was released following stimulation in intact or permeabilised cells. These results suggest either that nSec-1 is already expressed at sufficient levels and remains so following permeabilisation or that nSec-1 may not be an acute inhibitory regulator of Ca2+-dependent exocytosis in chromaffin or PC12 cells.

Stimulation of NSF ATPase activity by alpha-SNAP is required for SNARE complex disassembly and exocytosis

N-ethylmaleimide-sensitive fusion protein (NSF) and alpha-SNAP play key roles in vesicular traffic through the secretory pathway. In this study, NH2- and COOH-terminal truncation mutants of alpha-SNAP were assayed for ability to bind NSF and stimulate its ATPase activity. Deletion of up to 160 NH2-terminal amino acids had little effect on the ability of alpha-SNAP to stimulate the ATPase activity of NSF. However, deletion of as few as 10 COOH-terminal amino acids resulted in a marked decrease. Both NH2-terminal (1-160) and COOH-terminal (160-295) fragments of alpha-SNAP were able to bind to NSF, suggesting that alpha-SNAP contains distinct NH2- and COOH-terminal binding sites for NSF. Sequence alignment of known SNAPs revealed only leucine 294 to be conserved in the final 10 amino acids of alpha-SNAP. Mutation of leucine 294 to alanine (alpha-SNAP(L294A)) resulted in a decrease in the ability to stimulate NSF ATPase activity but had no effect on the ability of this mutant to bind NSF. alpha-SNAP (1-285) and alpha-SNAP (L294A) were unable to stimulate Ca2+-dependent exocytosis in permeabilized chromaffin cells. In addition, alpha-SNAP (1-285), and alpha-SNAP (L294A) were able to inhibit the stimulation of exocytosis by exogenous alpha-SNAP. alpha-SNAP, alpha-SNAP (1-285), and alpha-SNAP (L294A) were all able to become incorporated into a 20S complex and recruit NSF. In the presence of MgATP, alpha-SNAP (1-285) and alpha-SNAP (L294A) were unable to fully disassemble the 20S complex and did not allow vesicle-associated membrane protein dissociation to any greater level than seen in control incubations. These findings imply that alpha-SNAP stimulation of NSF ATPase activity may be required for 20S complex disassembly and for the alpha-SNAP stimulation of exocytosis.

NSF and SNAP are present on adrenal chromaffin granules

N-ethylmaleimide sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are involved in many vesicular transport steps. It has been proposed that SNAPs and NSF associate with their membrane receptors only when vesicles dock on the target membrane. Analysis of NSF and alpha-SNAP distribution in fractionation of organelles from adrenal medulla indicated that a substantial amount of both proteins distributed with chromaffin granules. Further fractionation of intact granules and lysed granule membranes showed exact overlap of NSF and alpha-SNAP distribution with chromaffin granules. These results suggest that NSF and alpha-SNAP are associated with chromaffin granules and support the idea that they function prior to docking of the granules on the plasma membrane.

Common mechanisms for regulated exocytosis in the chromaffin cell and the synapse

Chromaffin granule exocytosis differs in many physiological respects from neuronal synaptic vesicle exocytosis, which has led to the assumption that the two processes occur by distinct mechanisms. While different mechanisms are certainly in operation for the biogenesis of granules and synaptic vesicles, it is now becoming clear that similar mechanisms are used by both beyond this stage. The similarities extend to various aspects of regulated exocytosis, including regulation of the number of vesicles released in response to cell stimulation. Most strikingly, it now appears that the same proteins mediate the docking and fusion of both chromaffin granules and synaptic vesicles, and that homologues of these proteins act similarly in constitutive membrane traffic throughout evolution.