Emerging roles of presynaptic proteins in Ca++-triggered exocytosis

Ca2+-regulated exocytosis and SNARE function

Secretory vesicles formed at the trans-Golgi network of neuroendocrine and endocrine cells must undergo several steps, such as translocation, docking and priming, before they are ready to fuse with the plasma membrane and deliver their cargo into the extracellular space. This process is called regulated exocytosis and is controlled by Ca(2+) (using synaptotagmin) and mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) proteins. Recent studies from three leading laboratories reveal novel details about the mechanism by which Ca(2+) and SNAREs regulate this complex process. These findings highlight the roles of both SNAP25 (synaptosome-associated protein of 25kD), one of the SNARE proteins, and CAPS (Ca(2+)-dependent activator protein for secretion), a Ca(2+)-sensor protein, in vesicle priming, depriming and fusion.

Effects of PKA-mediated phosphorylation of Snapin on synaptic transmission in cultured hippocampal neurons

Use-dependent activation of protein kinase A (PKA) modulates transmitter release, contributing to synaptic plasticity. Snapin, a PKA substrate in neurons, associates with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, and its phosphorylation leads to increased binding of synaptotagmin to the SNARE complex. We investigated the role of PKA-dependent phosphorylation of Snapin in hippocampal neurons. Overexpression of Snapin S50D, a mutant mimicking the phosphorylated state, resulted in a decreased number of readily releasable vesicles. In addition, both the release probability of individual vesicles and the depression rate during high-frequency stimulation were increased. Overexpression of Snapin S50A, a mutant that cannot be phosphorylated, did not alter the size of the pool or the probability of release. Furthermore, dialysis of Sp-cAMPS, a nonhydrolyzable analog of cAMP that will promote phosphorylation by PKA, also led to increased synaptic depression in cells overexpressing wild-type Snapin. These results establish Snapin as an important target of PKA in CNS synapses and indicate a role for Snapin in the plasticity of transmitter release.

Synaptic vesicle pools

Communication between cells reaches its highest degree of specialization at chemical synapses. Some synapses talk in a 'whisper'; others 'shout'. The 'louder' the synapse, the more synaptic vesicles are needed to maintain effective transmission, ranging from a few hundred (whisperers) to nearly a million (shouters). These vesicles reside in different 'pools', which have been given a bewildering array of names. In this review, we focus on five tissue preparations in which synaptic vesicle pools have been identified and thoroughly characterized. We argue that, in each preparation, each vesicle can be assigned to one of three distinct pools.

EBAG9 adds a new layer of control on large dense-core vesicle exocytosis via interaction with Snapin

Regulated exocytosis is subject to several modulatory steps that include phosphorylation events and transient protein-protein interactions. The estrogen receptor-binding fragment-associated gene9 (EBAG9) gene product was recently identified as a modulator of tumor-associated O-linked glycan expression in nonneuronal cells; however, this molecule is expressed physiologically in essentially all mammalian tissues. Particular interest has developed toward this molecule because in some human tumor entities high expression levels correlated with clinical prognosis. To gain insight into the cellular function of EBAG9, we scored for interaction partners by using the yeast two-hybrid system. Here, we demonstrate that EBAG9 interacts with Snapin, which is likely to be a modulator of Synaptotagmin-associated regulated exocytosis. Strengthening of this interaction inhibited regulated secretion of neuropeptide Y from PC12 cells, whereas evoked neurotransmitter release from hippocampal neurons remained unaltered. Mechanistically, EBAG9 decreased phosphorylation of Snapin; subsequently, association of Snapin with synaptosome-associated protein of 25 kDa (SNAP25) and SNAP23 was diminished. We suggest that the occurrence of SNAP23, Snapin, and EBAG9 also in nonneuronal cells might extend the modulatory role of EBAG9 to a broad range of secretory cells. The conjunction between EBAG9 and Snapin adds an additional layer of control on exocytosis processes; in addition, mechanistic evidence is provided that inhibition of phosphorylation has a regulatory function in exocytosis.

Coupling actin and membrane dynamics during calcium-regulated exocytosis: a role for Rho and ARF GTPases

Release of neurotransmitters and hormones occurs by calcium-regulated exocytosis, a process that shares many similarities in neurons and neuroendocrine cells. Exocytosis is confined to specific regions in the plasma membrane, where actin remodelling, lipid modifications and protein-protein interactions take place to mediate vesicle/granule docking, priming and fusion. The spatial and temporal coordination of the various players to form a "fast and furious" machinery for secretion remain poorly understood. ARF and Rho GTPases play a central role in coupling actin dynamics to membrane trafficking events in eukaryotic cells. Here, we review the role of Rho and ARF GTPases in supplying actin and lipid structures required for synaptic vesicle and secretory granule exocytosis. Their possible functional interplay may provide the molecular cues for efficient and localized exocytotic fusion.

Molecular regulation of membrane resealing in 3T3 fibroblasts

Membrane resealing in mammalian cells after injury depends on Ca(2+)-dependent fusion of intracellular vesicles with the plasma membrane. When cells are wounded twice, the subsequent resealing is generally faster. Physiological and biochemical studies have shown the initiation of two different repair signaling pathways, which are termed facilitated and potentiated responses. The facilitated response is dependent on the generation and recruitment of new vesicles, whereas the potentiated response is not. Here, we report that the two responses can be differentially defined molecularly. Using recombinant fragments of synaptobrevin-2 and synaptotagmin C2 domains we were able to dissociate the molecular requirements of vesicle exocytosis for initial membrane resealing and the facilitated and potentiated responses. The initial resealing response was blocked by fragments of synaptobrevin-2 and the C2B domain of synaptotagmin VII. Both the facilitated and potentiated responses were also blocked by the C2B domain of synaptotagmin VII. Although the initial resealing response was not blocked by the C2AB domain of synaptotagmin I or the C2A domain of synaptotagmin VII, recruitment of new vesicles for the facilitated response was inhibited. We also used Ca2+ binding mutant studies to show that the effects of synaptotagmins on membrane resealing are Ca(2+)-dependent. The pattern of inhibition by synaptotagmin C2 fragments that we observed cannot be used to specify a vesicle compartment, such as lysosomes, in membrane repair.

Mechnisms underlying different facilitation forms at the lobster neuromuscular synapse

At the crustacean neuromuscular junction, facilitation elicited by a repetitive stimulation reaches a plateau level that is proportional to the stimulation frequency. In the present study we demonstrated that plateau facilitation (F(plateau)) does not depend on Ca(2+) manipulations. We manipulated Ca(2+) concentration in the following ways: (1) applying cell permeable chelators BAPTA-AM or EGTA-AM; (2) decreasing Ca(2+) concentration in the extracellular media; (3) enhancing Ca(2+) influx by 4-aminipyridin. We found that neither F(plateau) is decreased by lowering Ca(2+) nor it is increased by enhancing Ca(2+) influx. In contrast, facilitation elicited by a short train of stimuli (F(growth)) was altered by Ca(2+) manipulations. These results suggested that F(plateau) does not result from accumulation of free intracellular Ca(2+). We hypothesized that F(plateau) results from the accumulation of synaptic vesicles properly activated for transmitter release, the readily releasable pool (RRP). To test this hypothesis, we measured the increase in RRP employing local applications of hypertonic solutions (HS). We found that the size of RRP was significantly increased after F(plateau) was induced. Our results suggest that facilitation is mediated by two mechanisms: the increase in the residual Ca(2+) and the increase in RRP. Frequency facilitation during continuous stimulation, F(plateau), is primarily controlled by the increase in RRP.

Single molecule observation of liposome-bilayer fusion thermally induced by soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)

A single molecule fluorescence assay is presented for studying the mechanism of soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)-mediated liposome fusion to supported lipid bilayers. The three neuronal SNAREs syntaxin-1A, synaptobrevin-II (VAMP), and SNAP-25A were expressed separately, and various dye-labeled combinations of the SNAREs were tested for their ability to dock liposomes and induce fusion. Syntaxin and synaptobrevin in opposing membranes were both necessary and sufficient to dock liposomes to supported bilayers and to induce thermally activated fusion. As little as one SNARE interaction was sufficient for liposome docking. Fusion of docked liposomes with the supported bilayer was monitored by the dequenching of soluble fluorophores entrapped within the liposomes. Fusion was stimulated by illumination with laser light, and the fusion probability was enhanced by raising the ambient temperature from 22 to 37 degrees C, suggesting a thermally activated process. Surprisingly, SNAP-25 had little effect on docking efficiency or the probability of thermally induced fusion. Interprotein fluorescence resonance energy transfer experiments suggest the presence of other conformational states of the syntaxin*synaptobrevin interaction in addition to those observed in the crystal structure of the SNARE complex. Furthermore, although SNARE complexes involved in liposome docking preferentially assemble into a parallel configuration, both parallel and antiparallel configurations were observed.

Structural Basis for the Inhibitory Role of Tomosyn in Exocytosis

Upon Ca2+ influx synaptic vesicles fuse with the plasma membrane and release their neurotransmitter cargo into the synaptic cleft. Key players during this process are the Q-SNAREs syntaxin 1a and SNAP-25 and the R-SNARE synaptobrevin 2. It is thought that these membrane proteins gradually assemble into a tight trans-SNARE complex between vesicular and plasma membrane, ultimately leading to membrane fusion. Tomosyn is a soluble protein of 130 kDa that contains a COOH-terminal R-SNARE motif but lacks a transmembrane anchor. Its R-SNARE motif forms a stable core SNARE complex with syntaxin 1a and SNAP-25. Here we present the crystal structure of this core tomosyn SNARE complex at 2.0-A resolution. It consists of a four-helical bundle very similar to that of the SNARE complex containing synaptobrevin. Most differences are found on the surface, where they prevented tight binding of complexin. Both complexes form with similar rates as assessed by CD spectroscopy. In addition, synaptobrevin cannot displace the tomosyn helix from the tight complex and vice versa, indicating that both SNARE complexes represent end products. Moreover, data bank searches revealed that the R-SNARE motif of tomosyn is highly conserved throughout all eukaryotic kingdoms. This suggests that the formation of a tight SNARE complex is important for the function of tomosyn.

Age-related changes in the levels of voltage-dependent calcium channels and other synaptic proteins in rat brain cortices

Neurotransmitter release from synapses is one of the most important interneuronal signaling in the nervous system. We previously reported that aging decreases depolarization-induced acetylcholine release in rat brain synaptosomes. To investigate the mechanisms underlying the age-related decrements of neurotransmission, we determined the levels of the alpha1 subunit proteins of voltage-dependent calcium channels (VDCCs) and three synaptic proteins that relate to exocytotic processes using synaptosomes prepared from cerebral cortices of young (6-month-old) and aged (27-month-old) rats. Immunoblotting analyses revealed that the protein levels of alpha1A (P/Q-type) and alpha1B (N-type) subunits in aged rats were 38% and 43% lower than the levels of young rats, respectively, but the levels of the alpha1C (L-type) subunit were not different between young and aged. On the contrary, the levels of synaptotagmin-1, synaptophysin and syntaxin were not significantly different between the two age groups in the synaptosomal preparations. These results suggest that synaptic density does not change much in the cerebral cortex in normal aging, and that the reduction of P/Q-type and N-type VDCCs, both of which participate in neurotransmitter release, is one of the causes for the decrease of neurotransmission at aged synapses.

Trans-SNARE interactions elicit Ca2+ efflux from the yeast vacuole lumen

Ca²⁺ transients trigger many SNARE-dependent membrane fusion events. The homotypic fusion of yeast vacuoles occurs after a release of lumenal Ca²⁺. Here, we show that trans-SNARE interactions promote the release of Ca²⁺ from the vacuole lumen. Ypt7p–GTP, the Sec1p/Munc18-protein Vps33p, and Rho GTPases, all of which function during docking, are required for Ca²⁺ release. Inhibitors of SNARE function prevent Ca²⁺ release. Recombinant Vam7p, a soluble Q-SNARE, stimulates Ca²⁺ release. Vacuoles lacking either of two complementary SNAREs, Vam3p or Nyv1p, fail to release Ca²⁺ upon tethering. Mixing these two vacuole populations together allows Vam3p and Nyv1p to interact in trans and rescues Ca²⁺ release. Sec17/18p promote sustained Ca²⁺ release by recycling SNAREs (and perhaps other limiting factors), but are not required at the release step itself. We conclude that trans-SNARE assembly events during docking promote Ca²⁺ release from the vacuole lumen.

A Role of VAMP8/Endobrevin in Regulated Exocytosis of Pancreatic Acinar Cells

Despite our general understanding that members of the SNARE superfamily participate in diverse intracellular docking/fusion events, the physiological role of the majority of SNAREs in the intact organism remains elusive. In this study, through targeted gene knockout in mice, we establish that VAMP8/endobrevin is a major player in regulated exocytosis of the exocrine pancreas. VAMP8 is enriched on the membrane of zymogen granules and exists in a complex with syntaxin 4 and SNAP-23. VAMP8-/- mice developed normally but showed severe defects in the pancreas. VAMP8 null acinar cells contained three times more zymogen granules than control acinar cells. Furthermore, secretagogue-stimulated secretion was abolished in pancreatic fragments derived from VAMP8-/- mice. In addition, VAMP8-/- mice were partially resistant to supramaximal caerulein-induced pancreatitis. These results suggest a major physiological role of VAMP8 in regulated exocytosis of pancreatic acinar cells by serving as a v-SNARE of zymogen granules.

Determination of exocytosis mechanisms of DOPA in rat striatum using in vivo microdialysis

To explore the exocytosis mechanism of striatal 3,4-dihydroxyphenylalanine (DOPA), this study determined the interaction between voltage-sensitive Ca2+-channel (VSCC) and SNARE on releases of DOPA and glutamate in rat striatum using microdialysis. Inhibitors of VSCCs and SNAREs did not affect basal glutamate release but decreased basal DOPA release, however, blocking effects of P-type-VSCC and synaptobrevin inhibitors were weaker than those of N-type-VSCC and syntaxin. The K+-evoked releases of DOPA and glutamate were reduced by inhibitors of P-type-VSCC and synaptobrevin predominantly and by inhibitors of N-type-VSCC and syntaxin weakly. However, interaction study between VSCC and SNARE on K+-evoked DOPA release indicates that DOPA release is regulated by different exocytosis mechanism from glutamate and monoamine during the depolarization stage (N-type-VSCC/P-type-VSCC/synaptobrevin and/or combination with N-type-VSCC/synaptobrevin and P-type-VSCC/synaptobrevin). Therefore we conclude that striatal DOPA release might be regulated by its specific exocytosis mechanism via different from dopaminergic presynaptic vesicle.

Pharmacological discrimination of protein kinase associated exocytosis mechanisms between dopamine and 3,4-dihydroxyphenylalanine in rat striatum using in vivo microdialysis

To explore the exocytosis mechanism of dopamine and its precursor, 3,4-dihydroxyphenylalanine (DOPA), we determined the effects of protein-kinase, cyclic-AMP-dependent protein-kinase (PKA), Ca(2+)-phospholipid-dependent protein-kinase (PKC) and Ca(2+)-calmodulin-dependent protein-kinase II (CaMK-II) on dopamine and DOPA releases in rat striatum using microdialysis. Basal DOPA and dopamine releases were reduced by PKC and CaMK-II inhibitors predominantly, and PKA inhibitor weakly. Ca(2+)-evoked releases were reduced by PKC and CaMK-II inhibitors, but not by PKA inhibitor. K(+)-evoked (20 min) releases were reduced by PKA and CaMK-II inhibitors predominantly, and PKC inhibitor weakly. Sustained K(+)-evoked (120 min) releases of DOPA and dopamine were reduced by CaMK-II inhibitor, but not by PKC or PKA. DOPA accumulation was reduced by PKA and CaMK-II inhibitors strongly, and PKC inhibitor weakly. Therefore, the present study demonstrates that striatal DOPA exocytosis is regulated by a similar protein kinase-associated exocytosis mechanism as that of dopamine.

CAPS Acts at a Prefusion Step in Dense-Core Vesicle Exocytosis as a PIP2 Binding Protein

CAPS-1 is required for Ca2+-triggered fusion of dense-core vesicles with the plasma membrane, but its site of action and mechanism are unknown. We analyzed the kinetics of Ca2+-triggered exocytosis reconstituted in permeable PC12 cells. CAPS-1 increased the initial rate of Ca2+-triggered vesicle exocytosis by acting at a rate-limiting, Ca2+-dependent prefusion step. CAPS-1 activity depended upon prior ATP-dependent priming during which PIP2 synthesis occurs. CAPS-1 activity and binding to the plasma membrane depended upon PIP2. Ca2+ was ineffective in triggering vesicle fusion in the absence of CAPS-1 but instead promoted desensitization to CAPS-1 resulting from decreased plasma membrane PIP2. We conclude that CAPS-1 functions following ATP-dependent priming as a PIP2 binding protein to enhance Ca2+-dependent DCV exocytosis. Essential prefusion steps in dense-core vesicle exocytosis involve sequential ATP-dependent synthesis of PIP2 and the subsequent PIP2-dependent action of CAPS-1. Regulation of PIP2 levels and CAPS-1 activity would control the secretion of neuropeptides and monoaminergic transmitters.

Visualization of regulated exocytosis with a granule-membrane probe using total internal reflection microscopy

Secretory granules labeled with Vamp-green fluorescent protein (GFP) showed distinct signatures upon exocytosis when viewed by total internal reflection fluorescence microscopy. In approximately 90% of fusion events, we observed a large increase in fluorescence intensity coupled with a transition from a small punctate appearance to a larger, spreading cloud with free diffusion of the Vamp-GFP into the plasma membrane. Quantitation suggests that these events reflect the progression of an initially fused and spherical granule flattening into the plane of the plasma membrane as the Vamp-GFP simultaneously diffuses through the fusion junction. Approximately 10% of the events showed a transition from puncta to ring-like structures coupled with little or no spreading. The ring-like images correspond quantitatively to granules fusing and retaining concavity (recess of approximately 200 nm). A majority of fusion events involved granules that were present in the evanescent field for at least 12 s. However, approximately 20% of the events involved granules that were present in the evanescent field for no more than 0.3 s, indicating that the interaction of the granule with the plasma membrane that leads to exocytosis can occur within that time. In addition, approximately 10% of the exocytotic sites were much more likely to occur within a granule diameter of a previous event than can be accounted for by chance, suggestive of sequential (piggy-back) exocytosis that has been observed in other cells. Overall granule behavior before and during fusion is strikingly similar to exocytosis previously described in the constitutive secretory pathway.

Both 3,4-dihydroxyphenylalanine and dopamine releases are regulated by Ca2+-induced Ca2+ releasing system in rat striatum

To clarify the striatal Ca2+-dependent monoaminergic exocytosis mechanisms, this study determined the effects of the Ca2+-induced Ca2+ releasing system (CICR), containing inositol-trisphosphate-receptor (IP3R) and ryanodine-receptor (RyR), on striatal releases of dopamine and its precursor, 3,4-dihydroxyphenylalanine (DOPA), using microdialysis. The basal dopamine release is regulated by IP3R but not by RyR, whereas basal DOPA release does not require CICR. The K+-evoked releases of DOPA and dopamine were enhanced by IP3R agonist, whereas RyR agonist reduced it. Additionally, inhibition of dopamine release induced by RyR hyperactivation was prevented by inhibition of L-type voltage-sensitive Ca2+-channel activity. These present results suggest that CICR-associated regulation of striatal releases of DOPA and dopamine is restrictive during the resting stage, whereas CICRs play an important role as a reserve mechanism of exocytosis of striatal DOPA and dopamine during the hyperexcitable stage.

Noc2 is essential in normal regulation of exocytosis in endocrine and exocrine cells

Rab3 is a subfamily of the small GTP-binding protein Rab family and plays an important role in exocytosis. Several potential effectors of Rab3, including rabphilin3 and Rims (Rim1 and Rim2), have been isolated and characterized. Noc2 was identified originally in endocrine pancreas as a molecule homologous to rabphilin3, but its role in exocytosis is unclear. To clarify the physiological function of Noc2 directly, we have generated Noc2 knockout (Noc2(-/-)) mice. Glucose intolerance with impaired insulin secretion was induced in vivo by acute stress in Noc2(-/-) mice, but not in wild-type (Noc2(+/+)) mice. Ca(2+)-triggered insulin secretion from pancreatic isles of Noc2(-/-) mice was markedly impaired, but was completely restored by treatment with pertussis toxin, which inhibits inhibitory G protein Gi/o signaling. In addition, the inhibitory effect of clonidine, an alpha(2)-adrenoreceptor agonist, on insulin secretion was significantly greater in Noc2(-/-) islets than in Noc2(+/+) islets. Impaired Ca(2+)-triggered insulin secretion was rescued by adenovirus gene transfer of wild-type Noc2 but not by that of mutant Noc2, which does not bind to Rab3. Accordingly, Noc2 positively regulates insulin secretion from endocrine pancreas by inhibiting Gi/o signaling, and the interaction of Noc2 and Rab3 is required for the effect. Interestingly, we also found a marked accumulation of secretory granules in various exocrine cells of Noc2(-/-) mice, especially in exocrine pancreas with no amylase response to stimuli. Thus, Noc2, a critical effector of Rab3, is essential in normal regulation of exocytosis in both endocrine and exocrine cells.

N-terminal myristoylation regulates calcium-induced conformational changes in neuronal calcium sensor-1

Neuronal calcium sensor-1 (NCS-1), a Ca(2+)-binding protein, plays an important role in the modulation of neurotransmitter release and phosphatidylinositol signaling pathway. It is known that the physiological activity of NCS-1 is governed by its myristoylation. Here, we present the role of myristoylation of NSC-1 in governing Ca(2+) binding and Ca(2+)-induced conformational changes in NCS-1 as compared with the role in the nonmyristoylated protein. The (45)Ca binding and isothermal titration calorimetric data show that myristoylation increases the degree of cooperativity; thus, the myristoylated NCS-1 binds Ca(2+) more strongly (with three Ca(2+) binding sites) than the non-myristoylated one (with two Ca(2+) binding sites). Both forms of protein show different conformational features in far-UV CD when titrated with Ca(2+). Large conformational changes were seen in the near-UV CD with more changes in the case of nonmyristoylated protein than the myristoylated one. Although the changes in the far-UV CD upon Ca(2+) binding were not seen in E120Q mutant (disabling EF-hand 3), the near-UV CD changes in conformation also were not influenced by this mutation. The difference in the binding affinity of myristoylated and non-myristoylated proteins to Ca(2+) also was reflected by Trp fluorescence. Collisional quenching by iodide showed more inaccessibility of the fluorophore in the myristoylated protein. Mg(2+)-induced changes in near-UV CD are different from Ca(2+)-induced changes, indicating ion selectivity. 8-Anilino-1-naphthalene sulfonic acid binding data showed solvation of the myristoyl group in the presence of Ca(2+), which could be attributed to the myristoyl-dependent conformational changes in NCS-1. These results suggest that myristoylation influences the protein conformation and Ca(2+) binding, which might be crucial for its physiological functions.

Tomosyn inhibits priming of large dense-core vesicles in a calcium-dependent manner

Neurotransmitter release is a multistep process that is coordinated by a large number of synaptic proteins and depends on proper protein-protein interactions. Using morphological, capacitance, and amperometric measurements, we investigated the effect of tomosyn, a Syntaxin-binding protein, on the different kinetic components of exocytosis in adrenal chromaffin cells. Overexpression of tomosyn decreased the release probability and led to a 50% reduction in the number of fusion-competent vesicles. The number of docked vesicles and the fusion kinetics of single vesicles were not altered suggesting that tomosyn inhibits the priming step. Interestingly, this inhibition is partially relieved at elevated calcium concentration. Calcium ramp experiments supported the latter finding and indicated that the reduction in secretion is caused by a shift in the calcium-dependence of release. These results indicate that secretion is not entirely blocked but occurs at higher calcium concentrations. We suggest that tomosyn inhibits the priming step and impairs the efficiency of vesicle pool refilling in a calcium-dependent manner.

SNAP-23 functions in docking/fusion of granules at low Ca2+

Ca(2+)-triggered exocytosis of secretory granules mediates the release of hormones from endocrine cells and neurons. The plasma membrane protein synaptosome-associated protein of 25 kDa (SNAP-25) is thought to be a key component of the membrane fusion apparatus that mediates exocytosis in neurons. Recently, homologues of SNAP-25 have been identified, including SNAP-23, which is expressed in many tissues, albeit at different levels. At present, little is known concerning functional differences among members of this family of proteins. Using an in vitro assay, we show here that SNAP-25 and SNAP-23 mediate the docking of secretory granules with the plasma membrane at high (1 microM) and low (100 nM) Ca(2+) levels, respectively, by interacting with different members of the synaptotagmin family. In intact endocrine cells, expression of exogenous SNAP-23 leads to high levels of hormone secretion under basal conditions. Thus, the relative expression levels of SNAP-25 and SNAP-23 might control the mode (regulated vs. basal) of granule release by forming docking complexes at different Ca(2+) thresholds.

Regulation of releasable vesicle pool sizes by protein kinase A-dependent phosphorylation of SNAP-25

Protein kinase A (PKA) is a key regulator of neurosecretion, but the molecular targets remain elusive. We combined pharmacological manipulations of kinase and phosphatase activities with mutational studies on the exocytotic machinery driving fusion of catecholamine-containing vesicles from chromaffin cells. We found that constitutive PKA activity was necessary to maintain a large number of vesicles in the release-ready, so-called primed, state, whereas calcineurin (protein phosphatase 2B) activity antagonized this effect. Overexpression of the SNARE protein SNAP-25a mutated in a PKA phosphorylation site (Thr-138) eliminated the effect of PKA inhibitors on the vesicle priming process. Another, unidentified, PKA target regulated the relative size of two different primed vesicle pools that are distinguished by their release kinetics. Overexpression of the SNAP-25b isoform increased the size of both primed vesicle pools by a factor of two, and mutations in the conserved Thr-138 site had similar effects as in the a isoform.

Crystal structure of the Habc domain of neuronal syntaxin from the squid Loligo pealei reveals conformational plasticity at its C-terminus

BACKGROUND

Intracellular membrane fusion processes are mediated by the spatial and temporal control of SNARE complex assembly that results in the formation of a four-helical bundle, composed of one vesicle SNARE and three target membrane SNARE polypeptide chains. Syntaxins are essential t-SNAREs and are characterized by an N-terminal Habc domain, a flexible linker region, a coiled-coil or SNARE motif and a membrane anchor. The N-terminal Habc domain fulfills important regulatory functions while the coiled-coil motif, present in all SNAREs, is sufficient for SNARE complex formation, which is thought to drive membrane fusion.

RESULTS

Here we report the crystal structure of the Habc domain of neuronal syntaxin from the squid Loligo pealei, s-syntaxin. Squid Habc crystallizes as a dimer and the monomer structure consists of a three-helical bundle. One molecule is strikingly similar to mammalian syntaxin 1A while the second one shows a structural deviation from the common fold in that the C-terminal part of helix C unwinds and adopts an extended conformation.

CONCLUSION

Conservation of surface residues indicates that the cytosolic part of s-syntaxin can adopt an auto-inhibitory closed conformation that may bind squid neuronal Sec1, s-Sec1, in the same manner as observed in structure of the rat nSec1/syntaxin 1A complex. Furthermore, despite the overall structural similarity, the observed changes at the C-terminus of one molecule indicate structural plasticity in neuronal syntaxin. Implications of the structural conservation and the changes are discussed with respect to potential Habc domain binding partners such as Munc13, which facilitates the transition from the closed to the open conformation.

Intracellular Ca2+ microdomain-triggered exocytosis in neuroendocrine cells

Colocalization of voltage-gated Ca2+ channels and exocytotic sites at the active zones of nerve terminals underlies 'synchronous' action potential discharge and synaptic vesicle exocytosis, thus allowing fast interneuronal signalling. Such a demand for a rapid release is not expected in neuroendocrine cells whose secretory products act throughout the entire organism. Nevertheless, by using evanescent field imaging of near-membrane Ca2+ concentrations and fluorescently labelled vesicles, Becherer et al. have recently reported exocytosis of individual large dense-core vesicles triggered by Ca2+ microdomains formed around clusters of open L-type Ca2+ channels in chromaffin cells from the adrenal medulla. This finding, besides illustrating the power of new microscopy imaging techniques, directly demonstrates in neuroendocrine cells a functional interaction between Ca2+ channels and secretory vesicles very much reminiscent of that in neurons.

Evidence for structural and functional diversity among SDS-resistant SNARE complexes in neuroendocrine cells

The core complex, formed by the SNARE proteins synaptobrevin 2, syntaxin 1 and SNAP-25, is an important component of the synaptic fusion machinery and shows remarkable in vitro stability, as exemplified by its SDS-resistance. In western blots, antibodies against one of these SNARE proteins reveal the existence of not only an SDS-resistant ternary complex but also as many as five bands between 60 and >200 kDa. Structural conformation as well as possible functions of these various complexes remained elusive. In western blots of protein extracts from PC12 cell membranes, an antibody against SNAP-25 detected two heat-sensitive SDS-resistant bands with apparent molecular weights of 100 and 230 kDa. A syntaxin antibody recognized only the 230 kDa band and required heat-treatment of the blotting membrane to detect the 100 kDa band. Various antibodies against synaptobrevin failed to detect SNARE complexes in conventional western blots and detected either the 100 kDa band or the 230 kDa band on heat-treated blotting membranes. When PC12 cells were exposed to various extracellular K(+)-concentrations (to evoke depolarization-induced Ca(2+) influx) or permeabilized in the presence of basal or elevated free Ca(2+), levels of these SNARE complexes were altered differentially: moderate Ca(2+) rises (</=1 microM) caused an increase, whereas Ca(2+) elevations of more than 1 microM led to a decrease in the 230 kDa band. Under both conditions the 100 kDa band was either increased or remained unchanged. Our data show that various SDS-resistant complexes occur in living cells and indicate that they represent SNARE complexes with different structures and diverging functions. The distinct behavior of these complexes under release-promoting conditions indicates that these SNARE structures have different roles in exocytosis.

The secretory granule-associated protein CAPS2 regulates neurotrophin release and cell survival

Neurotrophins are key modulators of various neuronal functions, including differentiation, survival, and synaptic plasticity, but the molecules that regulate their secretion are poorly understood. We isolated a clone that is predominantly expressed in granule cells of postnatally developing mouse cerebellum, which turned out to be a paralog of CAPS (Ca2+-dependent activator protein for secretion), and named CAPS2. CAPS2 is enriched on vesicular structures of presynaptic parallel fiber terminals of granule cells connecting postsynaptic spines of Purkinje cell dendrites. Vesicle factions affinity-purified by the CAPS2 antibody from mouse cerebella contained significant amounts of neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and chromogranin B but not marker proteins for synaptic vesicle synaptophysin and synaptotagmin. In cerebellar primary cultures, punctate CAPS2 immunoreactivities are primarily colocalized with those of NT-3 and BDNF and near those of a postsynaptic marker, postsynaptic density-95, around dendritic arborization of Purkinje cells. Exogenously expressed CAPS2 enhanced release of exogenous NT-3 and BDNF from PC12 cells and endogenous NT-3 from cultured granule cells in a depolarization-dependent manner. Moreover, the overexpression of CAPS2 in granule cells promotes the survival of Purkinje cells in cerebellar cultures. Thus, we suggest that CAPS2 mediates the depolarization-dependent release of NT-3 and BDNF from granule cells, leading to regulation in cell differentiation and survival during cerebellar development.

Hyperactivity of Endoplasmic Reticulum Associated Exocytosis Mechanism Contributes to Acute Phencyclidine Intoxication

Phencyclidine (PCP) produces schizophrenia-like psychosis and acute PCP-intoxications; however, whether glutamate/NMDA-receptor blockade by PCP modulates or not these mechanisms has remained to be clarified. To clarify this mechanism, we determined interaction among voltage-gated Na(+)-channel inhibitor, tetrodotoxin (TTX), Golgi-disturbing-agent, brefeldin-A (BFA), and PCP on releases of glutamate, GABA, and monoamine in prefrontal-cortex (pFC), using microdialysis. PCP increased basal monoamine release, whereas it decreased basal GABA release, without affecting glutamate release. PCP increased K(+)-evoked monoamine release, whereas it decreased K(+)-evoked glutamate and GABA releases. TTX reduced basal monoamine and GABA releases without affecting glutamate release, whereas BFA did not affect them. Interestingly, BFA and TTX inhibited PCP-associated basal monoamine release and abolished PCP-induced reduction of basal GABA release without affecting glutamate release. BFA and TTX reduced K(+)-evoked releases of all neurotransmitters. BFA inhibited PCP-associated K(+)-evoked monoamine release, but TTX did not affect them. PCP-induced reduction of K(+)-evoked GABA and glutamate releases was abolished by TTX and BFA. These results indicate that PCP reduces GABAergic transmission via NMDA-receptor blockade and activates intracellular endoplasmic-reticulum-associated signal-transduction, resulting in enhancement of monoaminergic transmission in pFC. Thus, these PCP properties support the hypothesis that mechanisms of the neurological symptoms of acute PCP-intoxication, convulsion, and rhabdomyolysis may be involved in both reduction of GABAergic-transmission and activation of endoplasmic-reticulum-associated signal-transduction induced by PCP.

Generation of a phenotypic array of hypothalamic neuronal cell models to study complex neuroendocrine disorders

Knowledge of how the brain achieves its diverse central control of basic physiology is severely limited by the virtual absence of appropriate cell models. Isolation of clonal populations of unique peptidergic neurons from the hypothalamus will facilitate these studies. Herein we describe the mass immortalization of mouse primary hypothalamic cells in monolayer culture, resulting in the generation of a vast representation of hypothalamic cell types. Subcloning of the heterogeneous cell populations resulted in the establishment of 38 representative clonal neuronal cell lines, of which 16 have been further characterized by analysis of 28 neuroendocrine markers. These cell lines represent the first available models to study the regulation of neuropeptides associated with the control of feeding behavior, including neuropeptide Y, ghrelin, urocortin, proopiomelanocortin, melanin-concentrating hormone, neurotensin, proglucagon, and GHRH. Importantly, a representative cell line responds appropriately to leptin stimulation and results in the repression of neuropeptide Y gene expression. These cell models can be used for detailed molecular analysis of neuropeptide gene regulation and signal transduction events involved in the direct hormonal control of unique hypothalamic neurons, not yet possible in the whole brain. Such studies may contribute information necessary for the strategic design of therapeutic interventions for complex neuroendocrine disorders, such as obesity.

A Family of Ca2+-Dependent Activator Proteins for Secretion

Ca2+-dependent activator protein for secretion (CAPS) 1 is an essential cytosolic component of the protein machinery involved in large dense-core vesicle (LDCV) exocytosis and in the secretion of a subset of neurotransmitters. In the present study, we report the identification, cloning, and comparative characterization of a second mammalian CAPS isoform, CAPS2. The structure of CAPS2 and its function in LDCV exocytosis from PC12 cells are very similar to those of CAPS1. Both isoforms are strongly expressed in neuroendocrine cells and in the brain. In subcellular fractions of the brain, both CAPS isoforms are enriched in synaptic cytosol fractions and also present on vesicular fractions. In contrast to CAPS1, which is expressed almost exclusively in brain and neuroendocrine tissues, CAPS2 is also expressed in lung, liver, and testis. Within the brain, CAPS2 expression seems to be restricted to certain brain regions and cell populations, whereas CAPS1 expression is strong in all neurons. During development, CAPS2 expression is constant between embryonic day 10 and postnatal day 60, whereas CAPS1 expression is very low before birth and increases after postnatal day 0 to reach a plateau at postnatal day 21. Light microscopic data indicate that both CAPS isoforms are specifically enriched in synaptic terminals. Ultrastructural analyses show that CAPS1 is specifically localized to glutamatergic nerve terminals. We conclude that at the functional level, CAPS2 is largely redundant with CAPS1. Differences in the spatial and temporal expression patterns of the two CAPS isoforms most likely reflect as yet unidentified subtle functional differences required in particular cell types or during a particular developmental period. The abundance of CAPS proteins in synaptic terminals indicates that they may also be important for neuronal functions that are not exclusively related to LDCV exocytosis.

Real-Time Measurement of Exocytosis and Endocytosis Using Interference of Light

We describe a new approach for making real-time measurements of exocytosis and endocytosis in neurons and neuroendocrine cells. The method utilizes interference reflection microscopy (IRM) to image surface membrane in close contact with a glass coverslip (the "footprint"). At the synaptic terminal of retinal bipolar cells, the footprint expands during exocytosis and retracts during endocytosis, paralleling changes in total surface area measured by capacitance. In chromaffin cells, IRM detects the fusion of individual granules as the appearance of bright spots within the footprint with spatial and temporal resolution similar to total internal reflection fluorescence microscopy. Advantages of IRM over capacitance are that it can monitor changes in surface area while cells are electrically active and it can be applied to mammalian neurons with relatively small synaptic terminals. IRM reveals that vesicles at the synapse of bipolar cells rapidly collapse into the surface membrane while secretory granules in chromaffin cells do not.

SNARE Protein Structure and Function

The SNARE superfamily has become, since its discovery approximately a decade ago, the most intensively studied element of the protein machinery involved in intracellular trafficking. Intracellular membrane fusion in eukaryotes requires SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor) proteins that form complexes bridging the two membranes. Although common themes have emerged from structural and functional studies of SNAREs and other components of the eukaryotic membrane fusion machinery, there is still much to learn about how the assembly and activity of this machinery is choreographed in living cells.

Kinetic simulation of signal transduction system in hippocampal long-term potentiation with dynamic modeling of protein phosphatase 2A

We modeled and analyzed a signal transduction system of long-term potentiation (LTP) in hippocampal post-synapse. Bhalla and Iyengar [Science 283(1999) 381] have developed a hippocampal LTP model. In the conventional model, the concentration of protein phosphatase 2A (PP2A) was fixed. However, it was reported that dynamic inactivation of PP2A was essential for LTP [J. Neurochem. 74 (2000) 807]. We introduced a dynamic modeling of PP2A; inactivation (phosphorylation) of PP2A by calcium/calmodulin-dependent protein kinase II (CaMKII) in the presence of calcium/calmodulin, self-activation (autodephosphorylation) of PP2A, and inactivation (dephosphorylation) of CaMKII by PP2A. This model includes complex feedback loops; both CaMKII and PP2A are autoactivated, while they inactivate each other. Moreover, we proposed an analysis strategy for model validation by applying the results of sensitivity analysis. In our system, calcineurin (CaN) played an essential role, rather than the activation of protein kinase C (PKC) as documented in the conventional model. From results of the analysis of our model, we found the following robustness as characteristics of bistability in our model: (1). PP2A reactions against calcium ion (Ca(2+)) perturbation; (2). PP2A inactivation against PP2A increase; (3). protein phosphatase 1 (PP1) activation against PF2A increase; and (4). PP2A reactions against PP2A initial concentration. These properties facilitated LTP induction in our system. We showed that another mechanism could introduce bistable behavior by adding dynamic reactions of PP2A.

Neurotransmitter Release at Central Synapses

Our understanding of synaptic transmission has grown dramatically during the 15 years since the first issue of Neuron was published, a growth rate expected from the rapid progress in modern biology. As in all of biology, new techniques have led to major advances in the cell and molecular biology of synapses, and the subject has evolved in ways (like the production of genetically engineered mice) that could not even be imagined 15 years ago. My plan for this review is to summarize what we knew about neurotransmitter release when Neuron first appeared and what we recognized we did not know, and then to describe how our views have changed in the intervening decade and a half. Some things we knew about synapses--"knew" in the sense that the field had reached a consensus--are no longer accepted, but for the most part, impressive advances have led to a new consensus on many issues. What I find fascinating is that in certain ways nothing has changed--many of the old arguments persist or recur in a different guise--but in other ways the field would be unrecognizable to a neurobiologist time-transported from 1988 to 2003.

Regulation of exocytosis in adrenal chromaffin cells: focus on ARF and Rho GTPases

Neurons and neuroendocrine cells release transmitters and hormones by exocytosis, a highly regulated process in which secretory vesicles or granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Several stages have been recognized in exocytosis. After recruitment and docking at the plasma membrane, vesicles/granules enter a priming step, which is then followed by the fusion process. Cortical actin remodelling accompanies the exocytotic reaction, but the links between actin dynamics and trafficking events remain poorly understood. Here, we review the action of Rho and ADP-ribosylation factor (ARF) GTPases within the exocytotic pathway in adrenal chromaffin cells. Rho proteins are well known for their pivotal role in regulating the actin cytoskeleton. ARFs were originally identified as regulators of vesicle transport within cells. The possible interplay between these two families of GTPases and their downstream effectors provides novel insights into the mechanisms that govern exocytosis.

Tuning exocytosis for speed: fast and slow modes

Exocytic fusion reactions triggered by Ca(2+) are widespread in neural, endocrine, exocrine, hemapoetic and perhaps all cell types. These processes exhibit tremendous variation in latencies to fusion following a Ca(2+) rise and in rates of fusion. We review reported differences for synaptic vesicle (SV) and dense-core vesicle (DCV) exocytosis and attempt to identify key features in the molecular mechanisms of docking, priming and fusion of SVs and DCVs that may account for differences in speed.

ARF6 regulates a plasma membrane pool of phosphatidylinositol(4,5)bisphosphate required for regulated exocytosis

ADP-ribosylation factor (ARF) 6 regulates endosomal plasma membrane trafficking in many cell types, but is also suggested to play a role in Ca2+-dependent dense-core vesicle (DCV) exocytosis in neuroendocrine cells. In the present work, expression of the constitutively active GTPase-defective ARF6Q67L mutant in PC12 cells was found to inhibit Ca2+-dependent DCV exocytosis. The inhibition of exocytosis was accompanied by accumulation of ARFQ67L, phosphatidylinositol 4,5-bisphosphate (PIP2), and the phosphatidylinositol 4-phosphate 5-kinase type I (PIP5KI) on endosomal membranes with their corresponding depletion from the plasma membrane. That the depletion of PIP2 and PIP5K from the plasma membrane caused the inhibition of DCV exocytosis was demonstrated directly in permeable cell reconstitution studies in which overexpression or addition of PIP5KIgamma restored Ca2+-dependent exocytosis. The restoration of exocytosis in ARF6Q67L-expressing permeable cells unexpectedly exhibited a Ca2+ dependence, which was attributed to the dephosphorylation and activation of PIP5K. Increased Ca2+ and dephosphorylation stimulated the association of PIP5KIgamma with ARF6. The results reveal a mechanism by which Ca2+ influx promotes increased ARF6-dependent synthesis of PIP2. We conclude that ARF6 plays a role in Ca2+-dependent DCV exocytosis by regulating the activity of PIP5K for the synthesis of an essential plasma membrane pool of PIP2.

Structural insights into the SNARE mechanism

Eukaryotic cells distribute materials among intracellular organelles and secrete into the extracellular space through cargo-loaded vesicles. A concluding step during vesicular transport is the fusion of a transport vesicle with a target membrane. SNARE proteins are essential for all vesicular fusion steps, thus they possibly comprise a conserved membrane fusion machinery. According to the "zipper" model, they assemble into stable membrane-bridging complexes that gradually bring membranes in juxtaposition. Hence, complex formation may provide the necessary energy for overcoming the repulsive forces between two membranes. During the last years, detailed structural and functional studies have extended the evidence that SNAREs are mostly in accord with the zipper model. Nevertheless, it remains unclear whether SNARE assembly between membranes directly leads to the merger of lipid bilayers.

The role of serine/threonine protein phosphatases in exocytosis

Modulation of exocytosis is integral to the regulation of cellular signalling, and a variety of disorders (such as epilepsy, hypertension, diabetes and asthma) are closely associated with pathological modulation of exocytosis. Emerging evidence points to protein phosphatases as key regulators of exocytosis in many cells and, therefore, as potential targets for the design of novel therapies to treat these diseases. Diverse yet exquisite regulatory mechanisms have evolved to direct the specificity of these enzymes in controlling particular cell processes, and functionally driven studies have demonstrated differential regulation of exocytosis by individual protein phosphatases. This Review discusses the evidence for the regulation of exocytosis by protein phosphatases in three major secretory systems, (1) mast cells, in which the regulation of exocytosis of inflammatory mediators plays a major role in the respiratory response to antigens, (2) insulin-secreting cells in which regulation of exocytosis is essential for metabolic control, and (3) neurons, in which regulation of exocytosis is perhaps the most complex and is essential for effective neurotransmission.

Calcium regulates exocytosis at the level of single vesicles

Ca2+ microdomains that form during the opening of voltage-gated Ca2+ channels have been implicated in regulating the kinetics of hormone and transmitter release. Direct assessment of the interaction between a single Ca2+ microdomain and a single secretory vesicle has been impossible because of technical limitations. Using evanescent field imaging of near-membrane micromolar Ca2+ concentration ([Ca2+]) and fluorescently labeled vesicles, we have observed exocytosis of individual chromaffin dense-core vesicles that was triggered by Ca2+ microdomains. Ca2+ microdomains selectively triggered the release of vesicles that were docked within 300 nm. Not all vesicles exposed to a Ca2+ microdomain were released, indicating that some vesicles are docked but are not ready for release. In addition to its established role as a trigger for release, elevated near-membrane [Ca2+] reduced the distance between docked vesicles and Ca2+ entry sites. Our results suggest a new mechanism for stimulation-dependent facilitation of exocytosis, whereby vesicles are moved closer to Ca2+ entry sites, thereby increasing a Ca2+ microdomain's efficacy to trigger vesicle fusion.

Insulin granule dynamics in pancreatic beta cells

Glucose-induced insulin secretion in response to a step increase in blood glucose concentrations follows a biphasic time course consisting of a rapid and transient first phase followed by a slowly developing and sustained second phase. Because Type 2 diabetes involves defects of insulin secretion, manifested as a loss of first phase and a reduction of second phase, it is important to understand the cellular mechanisms underlying biphasic insulin secretion. Insulin release involves the packaging of insulin in small (diameter approximately 0.3 micro m) secretory granules, the trafficking of these granules to the plasma membrane, the exocytotic fusion of the granules with the plasma membrane and eventually the retrieval of the secreted membranes by endocytosis. Until recently, studies on insulin secretion have been confined to the appearance of insulin in the extracellular space and the cellular events preceding exocytosis have been inaccessible to more detailed analysis. Evidence from a variety of secretory tissues, including pancreatic islet cells suggests, however, that the secretory granules can be functionally divided into distinct pools that are distinguished by their release competence and/or proximity to the plasma membrane. The introduction of fluorescent proteins that can be targeted to the secretory granules, in combination with the advent of new techniques that allow real-time imaging of granule trafficking in living cells (granule dynamics), has led to an explosion of our knowledge of the pre-exocytotic and post-exocytotic processes in the beta cell. Here we discuss these observations in relation to previous functional and ultra-structural data as well as the secretory defects of Type 2 diabetes.

Control of vesicular trafficking by Rho GTPases

Although vesicular trafficking is essential for a large variety of cellular processes, the regulation of vesicular trafficking is still poorly understood. Members of the Rho family of small GTPases have recently emerged as important control elements of many stages of vesicular trafficking, providing new insight into the regulation of these events. We will discuss the diverse roles played by Rho proteins in membrane trafficking and focus on the biological implications of these functions.

Syntaxin 1A binds to the cytoplasmic C terminus of Kv2.1 to regulate channel gating and trafficking

Voltage-gated K(+) (Kv) 2.1 is the dominant Kv channel that controls membrane repolarization in rat islet beta-cells and downstream insulin exocytosis. We recently showed that exocytotic SNARE protein SNAP-25 directly binds and modulates rat islet beta-cell Kv 2.1 channel protein at the cytoplasmic N terminus. We now show that SNARE protein syntaxin 1A (Syn-1A) binds and modulates rat islet beta-cell Kv2.1 at its cytoplasmic C terminus (Kv2.1C). In HEK293 cells overexpressing Kv2.1, we observed identical effects of channel inhibition by dialyzed GST-Syn-1A, which could be blocked by Kv2.1C domain proteins (C1: amino acids 412-633, C2: amino acids 634-853), but not the Kv2.1 cytoplasmic N terminus (amino acids 1-182). This was confirmed by direct binding of GST-Syn-1A to the Kv2.1C1 and C2 domains proteins. These findings are in contrast to our recent report showing that Syn-1A binds and modulates the cytoplasmic N terminus of neuronal Kv1.1 and not by its C terminus. Co-expression of Syn-1A in Kv2.1-expressing HEK293 cells inhibited Kv2.1 surfacing, which caused a reduction of Kv2.1 current density. In addition, Syn-1A caused a slowing of Kv2.1 current activation and reduction in the slope factor of steady-state inactivation, but had no affect on inactivation kinetics or voltage dependence of activation. Taken together, SNAP-25 and Syn-1A mediate secretion not only through its participation in the exocytotic SNARE complex, but also by regulating membrane potential and calcium entry through their interaction with Kv and Ca(2+) channels. In contrast to Ca(2+) channels, where these SNARE proteins act on a common synprint site, the SNARE proteins act not only on distinct sites within a Kv channel, but also on distinct sites between different Kv channel families.

The calcium-signalling saga: tap water and protein crystals

Of the approximately 1,400 grams of calcium that are in the human body, less than 10 grams manage to escape being trapped in the skeleton and teeth. These few grams might be an insignificant quantity, but they are extraordinarily significant qualitatively. They circulate in the blood and extracellular spaces, and penetrate cells to regulate their most important activities.