Synaptotagmin I functions as a calcium regulator of release probability

Calmodulin and lipid binding to synaptobrevin regulates calcium-dependent exocytosis

Neurotransmitter release involves the assembly of a heterotrimeric SNARE complex composed of the vesicle protein synaptobrevin (VAMP 2) and two plasma membrane partners, syntaxin 1 and SNAP-25. Calcium influx is thought to control this process via Ca(2+)-binding proteins that associate with components of the SNARE complex. Ca(2+)/calmodulin or phospholipids bind in a mutually exclusive fashion to a C-terminal domain of VAMP (VAMP(77-90)), and residues involved were identified by plasmon resonance spectroscopy. Microinjection of wild-type VAMP(77-90), but not mutant peptides, inhibited catecholamine release from chromaffin cells monitored by carbon fibre amperometry. Pre-incubation of PC12 pheochromocytoma cells with the irreversible calmodulin antagonist ophiobolin A inhibited Ca(2+)-dependent human growth hormone release in a permeabilized cell assay. Treatment of permeabilized cells with tetanus toxin light chain (TeNT) also suppressed secretion. In the presence of TeNT, exocytosis was restored by transfection of TeNT-resistant (Q(76)V, F(77)W) VAMP, but additional targeted mutations in VAMP(77-90) abolished its ability to rescue release. The calmodulin- and phospholipid-binding domain of VAMP 2 is thus required for Ca(2+)-dependent exocytosis, possibly to regulate SNARE complex assembly.

Non-polarized distribution of synaptotagmin IV in neurons: evidence that synaptotagmin IV is not a synaptic vesicle protein

Synaptotagmin IV (Syt IV) expression is regulated by neuronal development and by depolarization in the brain and in neuronal cell cultures. In cultures, immunocytochemical analysis has shown that Syt IV is localized at the Golgi and at the tips of growing neurites, but little was known about associations between Syt IV and vesicles or organelles [J. Neurochem. 74 (2000) 518]. In this study we performed an electron microscopic (EM) analysis of developing mouse neocortex to determine the exact localization of Syt IV in native mouse tissues. In neurons of layer II/III, Syt IV was found to be localized in the dendrites and axons, and at the Golgi in the cell body. Some Syt IV signals were clearly associated with vesicles and/or organelles, but EM and cell fractionation studies showed no Syt IV signals at synaptic vesicles. Detection of fluorescence protein-tagged Syt IV (Syt IV-EGFP) in hippocampal neurons also showed the presence of Syt IV-EGFP vesicles or organelles in the axons and dendrites. These results suggest that Syt IV regulates non-polarized membrane trafficking in neurons, which may be involved in synaptic plasticity or neuronal development.

Regulation of Ca(2+) signaling in rat bile duct epithelia by inositol 1,4,5-trisphosphate receptor isoforms

Cytosolic Ca(2+) (Ca(i)(2+)) regulates secretion of bicarbonate and other ions in the cholangiocyte. In other cell types, this second messenger acts through Ca(2+) waves, Ca(2+) oscillations, and other subcellular Ca(2+) signaling patterns, but little is known about the subcellular organization of Ca(2+) signaling in cholangiocytes. Therefore, we examined Ca(2+) signaling and the subcellular distribution of Ca(2+) release channels in cholangiocytes and in a model cholangiocyte cell line. The expression and subcellular distribution of inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) isoforms and the ryanodine receptor (RyR) were determined in cholangiocytes from normal rat liver and in the normal rat cholangiocyte (NRC) polarized bile duct cell line. Subcellular Ca(2+) signaling in cholangiocytes was examined by confocal microscopy. All 3 InsP(3)R isoforms were expressed in cholangiocytes, whereas RyR was not expressed. The type III InsP(3)R was the most heavily expressed isoform at the protein level and was concentrated apically, whereas the type I and type II isoforms were expressed more uniformly. The type III InsP(3)R was expressed even more heavily in NRC cells but was concentrated apically in these cells as well. Adenosine triphosphate (ATP), which increases Ca(2+) via InsP(3) in cholangiocytes, induced Ca(2+) oscillations in both cholangiocytes and NRC cells. Acetylcholine (ACh) induced apical-to-basal Ca(2+) waves. In conclusion, Ca(2+) signaling in cholangiocytes occurs as polarized Ca(2+) waves that begin in the region of the type III InsP(3)R. Differential subcellular localization of InsP(3)R isoforms may be an important molecular mechanism for the formation of Ca(2+) waves and oscillations in cholangiocytes. Because Ca(i)(2+) is in part responsible for regulating ductular secretion, these findings also may have implications for the molecular basis of cholestatic disorders.

Calcium-independent stimulation of membrane fusion and SNAREpin formation by synaptotagmin I

Neurotransmitter release requires the direct coupling of the calcium sensor with the machinery for membrane fusion. SNARE proteins comprise the minimal fusion machinery, and synaptotagmin I, a synaptic vesicle protein, is the primary candidate for the main neuronal calcium sensor. To test the effect of synaptotagmin I on membrane fusion, we incorporated it into a SNARE-mediated liposome fusion assay. Synaptotagmin I dramatically stimulated membrane fusion by facilitating SNAREpin zippering. This stimulatory effect was topologically restricted to v-SNARE vesicles (containing VAMP 2) and only occurred in trans to t-SNARE vesicles (containing syntaxin 1A and SNAP-25). Interestingly, calcium did not affect the overall fusion reaction. These results indicate that synaptotagmin I can directly accelerate SNARE-mediated membrane fusion and raise the possibility that additional components might be required to ensure tight calcium coupling.

The C(2)B Ca(2+)-binding motif of synaptotagmin is required for synaptic transmission in vivo

Synaptotagmin is a synaptic vesicle protein that is postulated to be the Ca(2+) sensor for fast, evoked neurotransmitter release. Deleting the gene for synaptotagmin (syt(null)) strongly suppresses synaptic transmission in every species examined, showing that synaptotagmin is central in the synaptic vesicle cycle. The cytoplasmic region of synaptotagmin contains two C(2) domains, C(2)A and C(2)B. Five, highly conserved, acidic residues in both the C(2)A and C(2)B domains of synaptotagmin coordinate the binding of Ca(2+) ions, and biochemical studies have characterized several in vitro Ca(2+)-dependent interactions between synaptotagmin and other nerve terminal molecules. But there has been no direct evidence that any of the Ca(2+)-binding sites within synaptotagmin are required in vivo. Here we show that mutating two of the Ca(2+)-binding aspartate residues in the C(2)B domain (D(416,418)N in Drosophila) decreased evoked transmitter release by >95%, and decreased the apparent Ca(2+) affinity of evoked transmitter release. These studies show that the Ca(2+)-binding motif of the C(2)B domain of synaptotagmin is essential for synaptic transmission.

Synaptotagmins I and IV promote transmitter release independently of Ca(2+) binding in the C(2)A domain

At nerve terminals, a focal and transient increase in intracellular Ca(2+) triggers the fusion of neurotransmitter-filled vesicles with the plasma membrane. The most extensively studied candidate for the Ca(2+)-sensing trigger is synaptotagmin I, whose Ca(2+)-dependent interactions with acidic phospholipids and syntaxin have largely been ascribed to its C(2)A domain, although the C(2)B domain also binds Ca(2+) (refs 7, 8). Genetic tests of synaptotagmin I have been equivocal as to whether it is the Ca(2+)-sensing trigger of fusion. Synaptotagmin IV, a related isoform that does not bind Ca(2+) in the C(2)A domain, might be an inhibitor of release. We mutated an essential aspartate of the Ca(2+)-binding site of the synaptotagmin I C(2)A domain and expressed it in Drosophila lacking synaptotagmin I. Here we show that, despite the disruption of the binding site, the Ca(2+)-dependent properties of transmission were not altered. Similarly, we found that synaptotagmin IV could substitute for synaptotagmin I. We conclude that the C(2)A domain of synaptotagmin is not required for Ca(2+)-dependent synaptic transmission, and that synaptotagmin IV promotes rather than inhibits transmission.

Additional binding sites for anionic phospholipids and calcium ions in the crystal structures of complexes of the C2 domain of protein kinase calpha

The C2 domain of protein kinase Calpha (PKCalpha) corresponds to the regulatory sequence motif, found in a large variety of membrane trafficking and signal transduction proteins, that mediates the recruitment of proteins by phospholipid membranes. In the PKCalpha isoenzyme, the Ca2+-dependent binding to membranes is highly specific to 1,2-sn-phosphatidyl-l-serine. Intrinsic Ca2+ binding tends to be of low affinity and non-cooperative, while phospholipid membranes enhance the overall affinity of Ca2+ and convert it into cooperative binding. The crystal structure of a ternary complex of the PKCalpha-C2 domain showed the binding of two calcium ions and of one 1,2-dicaproyl-sn-phosphatidyl-l-serine (DCPS) molecule that was coordinated directly to one of the calcium ions. The structures of the C2 domain of PKCalpha crystallised in the presence of Ca2+ with either 1,2-diacetyl-sn-phosphatidyl-l-serine (DAPS) or 1,2-dicaproyl-sn-phosphatidic acid (DCPA) have now been determined and refined at 1.9 A and at 2.0 A, respectively. DAPS, a phospholipid with short hydrocarbon chains, was expected to facilitate the accommodation of the phospholipid ligand inside the Ca2+-binding pocket. DCPA, with a phosphatidic acid (PA) head group, was used to investigate the preference for phospholipids with phosphatidyl-l-serine (PS) head groups. The two structures determined show the presence of an additional binding site for anionic phospholipids in the vicinity of the conserved lysine-rich cluster. Site-directed mutagenesis, on the lysine residues from this cluster that interact directly with the phospholipid, revealed a substantial decrease in C2 domain binding to vesicles when concentrations of either PS or PA were increased in the absence of Ca2+. In the complex of the C2 domain with DAPS a third Ca2+, which binds an extra phosphate group, was identified in the calcium-binding regions (CBRs). The interplay between calcium ions and phosphate groups or phospholipid molecules in the C2 domain of PKCalpha is supported by the specificity and spatial organisation of the binding sites in the domain and by the variable occupancies of ligands found in the different crystal structures. Implications for PKCalpha activity of these structural results, in particular at the level of the binding affinity of the C2 domain to membranes, are discussed.

Diversification of synaptic strength: presynaptic elements

Synapses are not static; their performance is modified adaptively in response to activity. Presynaptic mechanisms that affect the probability of transmitter release or the amount of transmitter that is released are important in synaptic diversification. Here, we address the diversity of presynaptic performance and its underlying mechanisms: how much of the variation can be accounted for by variation in synaptic morphology and how much by molecular differences? Significant progress has been made in defining presynaptic structural contributions to synaptic strength; by contrast, we know little about how presynaptic proteins produce normally observed functional differentiation, despite abundant information on presynaptic proteins and on the effects of their individual manipulation. Closing the gap between molecular and physiological synaptic diversification still represents a considerable challenge.

Synaptotagmin function in dense core vesicle exocytosis studied in cracked PC12 cells

Ca(2+)-triggered dense-core vesicle exocytosis in PC12 cells does not require vesicular synaptotagmins 1 and 2, but may use plasma membrane synaptotagmins 3 and 7 as Ca(2+) sensors. In support of this hypothesis, C(2) domains from the plasma membrane but not vesicular synaptotagmins inhibit PC12 cell exocytosis. Ca(2+) induces binding of both plasma membrane and vesicular synaptotagmins to phospholipids and SNAREs (soluble N-ethylmaleimide-sensitive attachment protein receptors), although with distinct apparent Ca(2+) affinities. Here we used gain-of-function C(2)-domain mutants of synaptotagmin 1 and loss-of-function C(2)-domain mutants of synaptotagmin 7 to examine how synaptotagmins function in dense-core vesicle exocytosis. Our data indicate that phospholipid- but not SNARE-binding by plasma membrane synaptotagmins is the primary determinant of Ca(2+)-triggered dense-core vesicle exocytosis. These results support a general lipid-based mechanism of action of synaptotagmins in exocytosis, with the specificity of various synaptotagmins for different types of fusion governed by their differential localizations and Ca(2+) affinities.

Calcium-sensitive phospholipid binding properties of normal and mutant ferlin C2 domains

Mutations in dysferlin, a novel membrane protein of unknown function, lead to muscular dystrophy. Myoferlin is highly homologous to dysferlin and like dysferlin is a plasma membrane protein with six C2 domains highly expressed in muscle. C2 domains are found in a variety of membrane-associated proteins where they have been implicated in calcium, phospholipid, and protein-binding. We investigated the pattern of dysferlin and myoferlin expression in a cell culture model of muscle development and found that dysferlin is expressed in mature myotubes. In contrast, myoferlin is highly expressed in elongated "prefusion" myoblasts and is decreased in mature myotubes where dysferlin expression is greatest. We tested ferlin C2 domains for their ability to bind phospholipid in a calcium-sensitive manner. We found that C2A, the first C2 domain of dysferlin and myoferlin, bound 50% phosphatidylserine and that phospholipid binding was regulated by calcium concentration. A dysferlin point mutation responsible for muscular dystrophy was engineered into the dysferlin C2A domain and demonstrated reduced calcium-sensitive phospholipid binding. Based on these data, we propose a mechanism for muscular dystrophy in which calcium-regulated phospholipid binding is abnormal, leading to defective maintenance and repair of muscle membranes.

Direct, Ca2+-dependent interaction between tubulin and synaptotagmin I: a possible mechanism for attaching synaptic vesicles to microtubules

The synaptic vesicle protein synaptotagmin I probably plays important roles in the synaptic vesicle cycle. However, the mechanisms of its action remain unclear. In this study, we have searched for cytoplasmic proteins that interact with synaptotagmin I. We found that the cytoskeletal protein tubulin directly and stoichiometrically bound to recombinant synaptotagmin I. The binding depended on mm Ca(2+), and 1 mol of tubulin dimer bound 2 mol of synaptotagmin I with half-maximal binding at 6.6 microm tubulin. The Ca(2+) dependence mainly resulted from Ca(2+) binding to the Ca(2+) ligands of synaptotagmin I. The C-terminal region of beta-tubulin and both C2 domains of synaptotagmin I were involved in the binding. The YVK motif in the C2 domains of synaptotagmin I was essential for tubulin binding. Tubulin and synaptotagmin I were co-precipitated from the synaptosome extract with monoclonal antibodies to tubulin and SNAP-25 (synaptosome-associated protein of 25 kDa), indicating the presence of tubulin/synaptotagmin I complex and tubulin binding to synaptotagmin I in SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes. Synaptotagmin I promoted tubulin polymerization and bundled microtubules in the presence of Ca(2+). These results suggest that direct interaction between synaptotagmin I and tubulin provides a mechanism for attaching synaptic vesicles to microtubules in high Ca(2+) concentrations.

Phosphorylation-dependent interaction of the synaptic vesicle proteins cysteine string protein and synaptotagmin I

The secretory vesicle cysteine string proteins (CSPs) are members of the DnaJ family of chaperones, and function at late stages of Ca2+-regulated exocytosis by an unknown mechanism. To determine novel binding partners of CSPs, we employed a pull-down strategy from purified rat brain membrane or cytosolic proteins using recombinant hexahistidine-tagged (His(6)-)CSP. Western blotting of the CSP-binding proteins identified synaptotagmin I to be a putative binding partner. Furthermore, pull-down assays using cAMP-dependent protein kinase (PKA)-phosphorylated CSP recovered significantly less synaptotagmin. Complexes containing CSP and synaptotagmin were immunoprecipitated from rat brain membranes, further suggesting that these proteins interact in vivo. Binding assays in vitro using recombinant proteins confirmed a direct interaction between the two proteins and demonstrated that the PKA-phosphorylated form of CSP binds synaptotagmin with approximately an order of magnitude lower affinity than the non-phosphorylated form. Genetic studies have implicated each of these proteins in the Ca2+-dependency of exocytosis and, since CSP does not bind Ca2+, this novel interaction might explain the Ca2+-dependent actions of CSP.

Ca2+-dependent synaptotagmin binding to SNAP-25 is essential for Ca2+-triggered exocytosis

Synaptotagmin is a proposed Ca2+ sensor on the vesicle for regulated exocytosis and exhibits Ca2+-dependent binding to phospholipids, syntaxin, and SNAP-25 in vitro, but the mechanism by which Ca2+ triggers membrane fusion is uncertain. Previous studies suggested that SNAP-25 plays a role in the Ca2+ regulation of secretion. We found that synaptotagmins I and IX associate with SNAP-25 during Ca2+-dependent exocytosis in PC12 cells, and we identified C-terminal amino acids in SNAP-25 (Asp179, Asp186, Asp193) that are required for Ca2+-dependent synaptotagmin binding. Replacement of SNAP-25 in PC12 cells with SNAP-25 containing C-terminal Asp mutations led to a loss-of-function in regulated exocytosis at the Ca2+-dependent fusion step. These results indicate that the Ca2+-dependent interaction of synaptotagmin with SNAP-25 is essential for the Ca2+-dependent triggering of membrane fusion.

Synaptotagmin-like protein 5: a novel Rab27A effector with C-terminal tandem C2 domains

Synaptotagmin-like proteins 1-4 (Slp1-4) are new members of the carboxyl-terminal-type (C-type) tandem C2 proteins and are classified as a subfamily distinct from the synaptotagmin and the Doc2 families, because the Slp family contains a unique homology domain at the amino terminus, referred to as the Slp homology domain (SHD). We previously showed that the SHD functions as a binding site for Rab27A, which is associated with human hemophagocytic syndrome (Griscelli syndrome) [J. Biol. Chem. 277 (2002) 9212; J. Biol. Chem. 277 (2002) 12432]. In the present study, we identified a novel member of the Slp family, Slp5. The same as other Slp family members, the SHD of Slp5 preferentially interacted with the GTP-bound form of Rab27A and marginally with Rab3A and Rab6A, both in vitro and in intact cells, but not with other Rabs tested (Rab1, Rab2, Rab4A, Rab5A, Rab7, Rab8, Rab9, Rab10, Rab11A, Rab17, Rab18, Rab20, Rab22, Rab23, Rab25, Rab28, and Rab37). However, unlike other members of the Slp family, expression of Slp5 mRNA was highly restricted to human placenta and liver. Expression of Slp5 protein and in vivo association of Slp5 with Rab27A in the mouse liver were further confirmed by immunoprecipitation. The results suggest that Slp5 might be involved in Rab27A-dependent membrane trafficking in specific tissues.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Regulation of synaptotagmin gene expression during ascidian embryogenesis

The ascidian embryo, a model for the primitive mode of chordate development, rapidly forms a dorsal nervous system which consists of a small number of neurons. Here, we have characterized the transcriptional regulation of an ascidian synaptotagmin (syt) gene to explore the molecular mechanisms underlying development of synaptic transmission. In situ hybridization showed that syt is expressed in all neurons described in previous studies and transiently in the embryonic epidermis. Neuronal expression of syt requires induction from the vegetal side of the embryo, whereas epidermal expression occurs autonomously in isolated ectodermal blastomeres. Introduction of green fluorescent protein reporter gene constructs into the ascidian embryos indicates that a genomic fragment of the 3.4-kb 5' upstream region contains promoter elements of syt gene. Deletion analysis of the promoter suggests that syt expression in neurons and in the embryonic epidermis depends on distinct cis-regulatory regions.

NMR measurement of the off rate from the first calcium-binding site of the synaptotagmin I C2A domain

The off rate from the first calcium-binding site of the C2A domain of synaptotagmin I, a putative calcium receptor in neurotransmitter release, has been determined by 15N-nuclear magnetic resonance relaxation dispersion measurements. The exchange rate was obtained by fitting the dependence of the transverse relaxation rates on the interval between 180 degrees pulses in relaxation-compensated CPMG experiments at 3.2 microM calcium concentration. The measured k(ex) is 2.0x10(3) x s(-1). The calcium on rate of 3.5+/-1x10(7) x s(-1), determined from the measured off rate and the dissociation constant (5.3x10(-5) M), is close to the diffusion limit. These results are consistent with the proposed role of synaptotagmin I as a calcium sensor in release, but suggest that additional factors may help to accelerate the diffusion of Ca2+ to the sensor.

The type II inositol 1,4,5-trisphosphate receptor can trigger Ca2+ waves in rat hepatocytes

BACKGROUND & AIMS

Ca2+ regulates cell functions through signaling patterns such as Ca2+ oscillations and Ca2+ waves. The type I inositol 1,4,5-trisphosphate receptor is thought to support Ca2+ oscillations, whereas the type III inositol 1,4,5-trisphosphate receptor is thought to initiate Ca2+ waves. The role of the type II inositol 1,4,5-trisphosphate receptor is less clear, because it behaves like the type III inositol 1,4,5-trisphosphate receptor at the single-channel level but can support Ca2+ oscillations in intact cells. Because the type II inositol 1,4,5-trisphosphate receptor is the predominant isoform in liver, we examined whether this isoform can trigger Ca2+ waves in hepatocytes.

METHODS

The expression and distribution of inositol 1,4,5-trisphosphate receptor isoforms was examined in rat liver by immunoblot and confocal immunofluorescence. The effects of inositol 1,4,5-trisphosphate on Ca2+ signaling were examined in isolated rat hepatocyte couplets by using flash photolysis and time-lapse confocal microscopy.

RESULTS

The type II inositol 1,4,5-trisphosphate receptor was concentrated near the canalicular pole in hepatocytes, whereas the type I inositol 1,4,5-trisphosphate receptor was found elsewhere. Stimulation of hepatocytes with vasopressin or directly with inositol 1,4,5-trisphosphate induced Ca2+ waves that began in the canalicular region and then spread to the rest of the cell. Inositol 1,4,5-Trisphosphate-induced Ca2+ signals also increased more rapidly in the canalicular region. Hepatocytes did not express the ryanodine receptor, and cyclic adenosine diphosphate-ribose had no effect on Ca2+ signaling in these cells.

CONCLUSIONS

The type II inositol 1,4,5-trisphosphate receptor establishes a pericanalicular trigger zone from which Ca2+ waves originate in hepatocytes.

Synaptotagmin I-DeltaC2B. A novel synaptotagmin isoform with a single C2 domain in the bovine adrenal medulla

Synaptotagmin I is a 65 kDa type 1 membrane glycoprotein found in secretory organelles that plays a key role in regulated exocytosis. We have characterised two forms (long and short) of synaptotagmin I that are present in the bovine adrenal medulla. The long form is a type I integral membrane protein which has two cytoplasmic C2 domains and corresponds to the previously characterised full-length synaptotagmin I isoform. The short-form synaptotagmin I-DeltaC2B has the same structure in the lumenal and transmembrane sequences, but synaptotagmin I-DeltaC2B is truncated such that it only has a single cytoplasmic C2 domain. Analysis of synaptotagmin I-DeltaC2B expression indicates that synaptotagmin I-DeltaC2B is preferentially expressed in the bovine adrenal medulla. However, it is absent from the dense core chromaffin granules. Furthermore, when expressed in the rat pheochromocytoma cell line PC12 bovine synaptotagmin I-DeltaC2B is largely absent from dense core granules and synaptic-like microvesicles. Instead, indirect immunofluorescence microscopy reveals the intracellular location of synaptotagmin I-DeltaC2B to be the plasma membrane.

Activation of protein kinase Cη triggers cortical granule exocytosis in Xenopus oocytes

Previous work has shown that phorbol esters or diacylglycerol trigger cortical granule exocytosis in Xenopus oocytes. We sought to identify the isoform(s) of protein kinase C (PKC) that mediate(s) this regulated secretory event. Because this process is initiated by lipid activators of PKC but is independent of calcium ions, we focused on the family of novel(calcium-independent) PKCs. Pharmacological investigations using Gö6976 and Gö6983 tended to exclude PKCδ, ϵ and μ as secretory triggers. Subcellular fractionation and immunoblot data revealed that these oocytes expressed all five members of the novel PKC family, but it was only PKCη that colocalized with cortical granules. Finally, expression of wild type or constitutively active forms of PKCδ and η strongly supported the conclusion that it is PKCη that initiates cortical granule exocytosis in these cells. These observations represent an important step in identifying the mechanism of secretory triggering in this system.

The regulation of exocytosis in the pancreatic acinar cell

The pancreatic acinar cell synthesises a variety of digestive enzymes. In transit through the secretory pathway, these enzymes are separated from constitutively secreted proteins and packaged into zymogen granules, which are localised in the apical pole of the cell. Stimulation of the cell by secretagogues such as acetylcholine and cholecystokinin, acting at receptors on the basolateral plasma membrane, causes the generation of an intracellular Ca(2+) signal. This signal, in turn, triggers the fusion of the zymogen granules with the apical plasma membrane, leading to the polarised secretion of the enzymes. This review describes recent advances in our understanding of the control of secretion in the acinar cell. In particular, we discuss the mechanisms underlying the sorting of digestive enzymes into the zymogen granules, the molecular components of the exocytotic "membrane fusion machine," the generation and propagation of the Ca(2+ signal and the development of new techniques for the visualisation of single granule fusion events.

Synchronous and asynchronous exocytosis induced by subthreshold high K+ at Cs(+)-loaded terminals of rat hippocampal neurons

Transmitter release at Cs(+)-loaded autaptic terminals was selectively activated by the subthreshold concentration of external K+, and Ca(2+) channel types and transmitter pools involved in synchronous and asynchronous exocytosis were studied. When a neuron was depolarized to +30 mV by applying a current through a pipette containing Cs(+) for >30 s, a rapid external K+ jump to 3.75-10 mM, otherwise ineffective, produced an outward current (K10 response). K10 responses were initially graded (type-1) and then became a spike and plateau-shape with (type-2) or without a latency (type-3). On repolarization to -60 mV, a high K+ jump induced inward currents (called also K10 response) similar to those at +30 mV, whose shape changed from that of type-3, then type-2 and finally type-1 over 30 min. During a period favorable for inducing a type-3 response, a current similar to this response was generated by a voltage pulse (+ 80 or 90 mV, 20 or 30 ms) to the cell soma. Currents similar to K10 responses were rarely induced by a high K+ jump without a conditioning depolarization except for some cells, but consistently produced when 3 mM Cs(+) and 50 microM 4-aminopyridine were externally applied for tens of minutes. Picrotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione with 3-[(RS)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid or Cd(2+) in, or Ca(2+) removal from, a high-K+ solution blocked all the K10 responses, while a plateau remaining after a high K+ jump was not blocked by Ca(2+) removal immediately after the K+ jump. Thus Cs(+) loading and decreased K+ concentration in autaptic terminals by a conditioning depolarizing current selectively sensitize the terminals to a subthreshold high K+ jump for depolarization to activate synchronous or asynchronous transmitter release. Nicardipine (5-10 microM) blocked type-1 and -2 responses but not type-3 responses, while omega-conotoxin (10 microM) blocked all the types of K10 response in the presence of nicardipine. Increasing the interval of high K+ jumps biphasically increased the magnitude of K10 response, preferentially in the postjump fraction reflecting purely the asynchronous activation of exocytotic machinery, and decreased the reduction of miniature postsynaptic current frequency after a K10 response. These results suggest the roles of N(P/Q)-type Ca(2+) channels in synchronous exocytosis at the terminals, L-type Ca(2+) channels in initiating a Ca(2+) action potential at the parent axon and both types in asynchronous exocytosis and also suggest the different releasable pools of transmitter for two modes of exocytosis in cultured hippocampal neurons.

Synaptotagmin IX regulates Ca2+-dependent secretion in PC12 cells

Synaptotagmin (Syt) I-deficient phaeochromocytoma (PC12) cell lines show normal Ca(2+)-dependent norepinephrine (NE) release (Shoji-Kasai, Y., Yoshida, A., Sato, K., Hoshino, T., Ogura, A., Kondo, S., Fujimoto, Y., Kuwahara, R., Kato, R., and Takahashi, M. (1992) Science 256, 1821-1823). To identify an alternative Ca(2+) sensor, we searched for other Syt isoforms in Syt I-deficient PC12 cells and identified Syt IX, an isoform closely related to Syt I, as an abundantly expressed dense-core vesicle protein. Here we show that Syt IX is required for the Ca(2+)-dependent release of NE from PC12 cells. Antibodies directed against the C2A domain of either Syt IX or Syt I inhibited Ca(2+)-dependent NE release in permeable PC12 cells indicating that both Syt proteins function in dense-core vesicle exocytosis. Our results support the idea that Syt family proteins that co-reside on secretory vesicles may function cooperatively and redundantly as potential Ca(2+) sensors for exocytosis.

Vesicular restriction of synaptobrevin suggests a role for calcium in membrane fusion

Release of neurotransmitter occurs when synaptic vesicles fuse with the plasma membrane. This neuronal exocytosis is triggered by calcium and requires three SNARE (soluble-N-ethylmaleimide-sensitive factor attachment protein receptors) proteins: synaptobrevin (also known as VAMP) on the synaptic vesicle, and syntaxin and SNAP-25 on the plasma membrane. Neuronal SNARE proteins form a parallel four-helix bundle that is thought to drive the fusion of opposing membranes. As formation of this SNARE complex in solution does not require calcium, it is not clear what function calcium has in triggering SNARE-mediated membrane fusion. We now demonstrate that whereas syntaxin and SNAP-25 in target membranes are freely available for SNARE complex formation, availability of synaptobrevin on synaptic vesicles is very limited. Calcium at micromolar concentrations triggers SNARE complex formation and fusion between synaptic vesicles and reconstituted target membranes. Although calcium does promote interaction of SNARE proteins between opposing membranes, it does not act by releasing synaptobrevin from synaptic vesicle restriction. Rather, our data suggest a mechanism in which calcium-triggered membrane apposition enables syntaxin and SNAP-25 to engage synaptobrevin, leading to membrane fusion.

C2A activates a cryptic Ca(2+)-triggered membrane penetration activity within the C2B domain of synaptotagmin I

Synaptotagmin (syt) I, an integral membrane protein localized to secretory vesicles, is a putative Ca(2+) sensor for exocytosis. Its N terminus spans the membrane once, and its cytoplasmic domain contains two conserved C2 domains, designated C2A and C2B. The isolated C2A domain penetrates membranes in response to Ca(2+); isolated C2B does not. Here, we have addressed the function of each C2 domain, but in the context of the intact cytoplasmic domain (C2A-C2B), by using fluorescent reporters placed in the Ca(2+)-binding loops of either C2A or C2B. Surprisingly, these reporters revealed that, analogous to C2A, a Ca(2+)-binding loop in C2B directly penetrates into lipid bilayers. Penetration of each C2 domain was very rapid (k(on) approximately 10(10) M(-1) x s(-1)) and resulted in high affinity C2A-C2B-liposome complexes (K(d) approximately 13-14 nM). C2B-bilayer penetration strictly depended on the presence, but not the membrane binding activity, of an adjacent C2A domain, severing C2A from C2B after protein synthesis abolished the ability of C2B to dip into bilayers in response to Ca(2+). The activation of C2B by C2A was also displayed by the C2 domains of syt III but not the C2 domains of syt IV. A number of proteins contain more than one C2 domain; the findings reported here suggest these domains may harbor cryptic activities that are not detected when they are studied in isolation.

The SNARE protein SNAP-25 is linked to fast calcium triggering of exocytosis

Synchronous neurotransmission depends on the tight coupling between Ca(2+) influx and fusion of neurotransmitter-filled vesicles with the plasma membrane. The vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein synaptobrevin 2 and the plasma membrane SNAREs syntaxin 1 and synaptosomal protein of 25 kDa (SNAP-25) are essential for calcium-triggered exocytosis. However, the link between calcium triggering and SNARE function remains elusive. Here we describe mutations in two sites on the surface of the SNARE complex formed by acidic and hydrophilic residues of SNAP-25 and synaptobrevin 2, which were found to coordinate divalent cations in the neuronal SNARE complex crystal structure. By reducing the net charge of the site in SNAP-25 we identify a mutation that interferes with calcium triggering of exocytosis when overexpressed in chromaffin cells. Exocytosis was elicited by photorelease of calcium from a calcium cage and evaluated by using patch-clamp capacitance measurements at millisecond time resolution. We present a method for monitoring the dependence of exocytotic rate upon calcium concentration at the release site and demonstrate that the mutation decreased the steepness of this relationship, indicating that the number of sequential calcium-binding steps preceding exocytosis is reduced by one. We conclude that the SNARE complex is linked directly to calcium triggering of exocytosis, most likely in a complex with auxiliary proteins.

Molecular determinants of regulated exocytosis

Regulated exocytosis forms the basis for many intercellular signaling processes, for example, in hormone secretion or neurotransmitter release. During regulated exocytosis, the membrane of a secretory vesicle fuses with the plasma membrane in a tightly controlled reaction that is most often triggered by calcium. Recent advances have allowed major insights into the molecular mechanisms that mediate regulated exocytosis. In the present review, we will briefly discuss two key features of regulated exocytosis that have been particularly well studied recently. First, we will examine the current understanding of the membrane fusion reaction that underlies regulated exocytosis and that is effected by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) and munc18-like proteins similar to other membrane fusion reactions. Second, we will describe the role of the major candidates for the calcium sensors that trigger exocytosis, a protein family called synaptotagmins. Although our understanding of regulated exocytosis is as yet incomplete, the results from the studies of SNAREs, munc18s, and synaptotagmins have provided a molecular anchor for a more complete future description.

Protein kinase A cascade regulates quantal release dispersion at frog muscle endplate

Uniquantal endplate currents (EPCs) were recorded simultaneously at the proximal, central and distal parts of the frog neuromuscular synapse, and their minimal synaptic latencies, latency dispersions and sensitivity to noradrenaline, cAMP and protein kinase A inhibition were measured. The latency dispersion was highest in the proximal part (P90 = 1.25 ms); it decreased to P90 = 0.95 ms in the central part and to P90 = 0.75 ms (60 % of the proximal part) in the distal part. In the proximal parts of the long neuromuscular synapse, stimulation-evoked EPCs with long release latencies were eliminated when the intracellular cAMP was increased by beta1 activation by noradrenaline, by the permeable analogue db-cAMP, by activation of adenylyl cyclase or by inhibition of cAMP hydrolysis. This makes the evoked release more compact, and the amplitude of the reconstructed multiquantal currents increases. Protein kinase A is a target of this regulation, since a specific inhibitor, Rp-cAMP, prevents the action of cAMP in the proximal parts and increases the occurrence of long-latency events in the distal parts of the synapse. Our results show that protein kinase A is involved in the timing of quantal release and can be regulated by presynaptic adrenergic receptors.

Role of electrostatic and hydrophobic interactions in Ca(2+)-dependent phospholipid binding by the C(2)A-domain from synaptotagmin I

Most C(2)-domains bind to phospholipid bilayers as a function of Ca(2+). Although phospholipid binding is central for the normal functions of C(2)-domain proteins, the precise mechanism of phospholipid binding is unclear. One of the key questions is whether phospholipid binding by C(2)-domains is primarily governed by electrostatic or hydrophobic interactions. We have now examined this question for the C(2)A-domain of synaptotagmin I, a membrane protein of secretory vesicles with an essential function in Ca(2+)-triggered exocytosis. Our results confirm previous data showing that Ca(2+)-dependent phospholipid binding by the synaptotagmin C(2)A-domain is exquisitely sensitive to ionic strength, suggesting an essential role for electrostatic interactions. However, we find that hydrophobic interactions mediated by exposed residues in the Ca(2+)-binding loops of the C(2)A-domain, in particular methionine 173, are also essential for tight phospholipid binding. Furthermore, we demonstrate that the apparent Ca(2+) affinity of the C(2)A-domain is determined not only by electrostatic interactions as shown previously, but also by hydrophobic interactions. Together these data indicate that phospholipid binding by the C(2)A-domain, although triggered by an electrostatic Ca(2+)-dependent switch, is stabilized by a hydrophobic mechanism. As a result, Ca(2+)-dependent phospholipid binding proceeds by a multimodal mechanism that mirrors the amphipathic nature of the phospholipid bilayer. The complex phospholipid binding mode of synaptotagmins may be important for its role in regulated exocytosis of secretory granules and synaptic vesicles.

Synaptotagmins form a hierarchy of exocytotic Ca(2+) sensors with distinct Ca(2+) affinities

Synaptotagmins constitute a large family of membrane proteins implicated in Ca(2+)-triggered exocytosis. Structurally similar synaptotagmins are differentially localized either to secretory vesicles or to plasma membranes, suggesting distinct functions. Using measurements of the Ca(2+) affinities of synaptotagmin C2-domains in a complex with phospholipids, we now show that different synaptotagmins exhibit distinct Ca(2+) affinities, with plasma membrane synaptotagmins binding Ca(2+) with a 5- to 10-fold higher affinity than vesicular synaptotagmins. To test whether these differences in Ca(2+) affinities are functionally important, we examined the effects of synaptotagmin C2-domains on Ca(2+)-triggered exocytosis in permeabilized PC12 cells. A precise correlation was observed between the apparent Ca(2+) affinities of synaptotagmins in the presence of phospholipids and their action in PC12 cell exocytosis. This was extended to PC12 cell exocytosis triggered by Sr(2+), which was also selectively affected by high-affinity C2-domains of synaptotagmins. Together, our results suggest that Ca(2+) triggering of exocytosis involves tandem Ca(2+) sensors provided by distinct plasma membrane and vesicular synaptotagmins. According to this hypothesis, plasma membrane synaptotagmins represent high-affinity Ca(2+) sensors involved in slow Ca(2+)-dependent exocytosis, whereas vesicular synaptotagmins function as low-affinity Ca(2+) sensors specialized for fast Ca(2+)-dependent exocytosis.

RIM1alpha forms a protein scaffold for regulating neurotransmitter release at the active zone

Neurotransmitters are released by synaptic vesicle fusion at the active zone. The active zone of a synapse mediates Ca2+-triggered neurotransmitter release, and integrates presynaptic signals in regulating this release. Much is known about the structure of active zones and synaptic vesicles, but the functional relation between their components is poorly understood. Here we show that RIM1alpha, an active zone protein that was identified as a putative effector for the synaptic vesicle protein Rab3A, interacts with several active zone molecules, including Munc13-1 (ref. 6) and alpha-liprins, to form a protein scaffold in the presynaptic nerve terminal. Abolishing the expression of RIM1alpha in mice shows that RIM1alpha is essential for maintaining normal probability of neurotransmitter release, and for regulating release during short-term synaptic plasticity. These data indicate that RIM1alpha has a central function in integrating active zone proteins and synaptic vesicles into a molecular scaffold that controls neurotransmitter release.

Rabphilin potentiates soluble N-ethylmaleimide sensitive factor attachment protein receptor function independently of rab3

Rabphilin, a putative rab effector, interacts specifically with the GTP-bound form of the synaptic vesicle-associated protein rab3a. In this study, we define in vivo functions for rabphilin through the characterization of mutants that disrupt the Caenorhabditis elegans rabphilin homolog. The mutants do not display the general synaptic defects associated with rab3 lesions, as assayed at the pharmacological, physiological, and ultrastructural level. However, rabphilin mutants exhibit severe lethargy in the absence of mechanical stimulation. Furthermore, rabphilin mutations display strong synergistic interactions with hypomorphic lesions in the syntaxin, synaptosomal-associated protein of 25 kDa, and synaptobrevin soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) genes; double mutants were nonresponsive to mechanical stimulation. These synergistic interactions were independent of rab3 function and were not observed in rab3-SNARE double mutants. Our data reveal rab3-independent functions for rabphilin in the potentiation of SNARE function.

Electrostatic control of the membrane targeting of C2 domains

Many proteins involved in signal transduction and vesicle trafficking contain C2 domains whose membrane association is often regulated by calcium. Here, finite-difference Poisson-Boltzmann calculations are used to describe the electrostatic interactions between C2 domains of known structure and phospholipid membranes. The results explain how calcium binding can drive the association of some C2 domains to negatively charged membranes and others to neutral, zwitterionic membranes. Nonspecific electrostatic interactions are shown to be a general feature of many C2 domains of known structure, including the calcium-independent C2 domain of the PTEN tumor suppressor.

Three-dimensional structure of the synaptotagmin 1 C2B-domain: synaptotagmin 1 as a phospholipid binding machine

Synaptotagmin 1 probably functions as a Ca2+ sensor in neurotransmitter release via its two C2-domains, but no common Ca2+-dependent activity that could underlie a cooperative action between them has been described. The NMR structure of the C2B-domain now reveals a beta sandwich that exhibits striking similarities and differences with the C2A-domain. Whereas the bottom face of the C2B-domain has two additional alpha helices that may be involved in specialized Ca2+-independent functions, the top face binds two Ca2+ ions and is remarkably similar to the C2A-domain. Consistent with these results, but in contrast to previous studies, we find that the C2B-domain binds phospholipids in a Ca2+-dependent manner similarly to the C2A-domain. These results suggest a novel view of synaptotagmin function whereby the two C2-domains cooperate in a common activity, Ca2+-dependent phospholipid binding, to trigger neurotransmitter release.

Formation of crystalloid endoplasmic reticulum induced by expression of synaptotagmin lacking the conserved WHXL motif in the C terminus. Structural importance of the WHXL motif in the C2B domain

Synaptotagmin (Syt) is a family of type I membrane proteins that consists of a single transmembrane domain, a spacer domain, two Ca(2+)-binding C2 domains, and a short C terminus. We recently showed that deletion of the short C terminus (17 amino acids) of Syt IV prevented the Golgi localization of Syt IV proteins in PC12 cells and induced granular structures of various sizes in the cell body by an unknown mechanism (Fukuda, M., Ibata, K., and Mikoshiba, K. (2001) J. Neurochem. 77, 730-740). In this study we showed by electron microscopy that these structures are crystalloid endoplasmic reticulum (ER), analyzed the mechanism of its induction, and demonstrated that: (a) mutation or deletion of the evolutionarily conserved WHXL motif in the C terminus of the synaptotagmin family (Syt DeltaC) destabilizes the C2B domain structure (i.e. causes misfolding of the protein), probably by disrupting the formation of stable anti-parallel beta-sheets between the beta-1 and beta-8 strands of the C2B domain; (b) the resulting malfolded proteins accumulate in the ER rather than being transported to other membrane structures (e.g. the Golgi apparatus), with the malfolded proteins also inducing the expression of BiP (immunoglobulin binding protein), one of the ER stress proteins; and (c) the ERs in which the Syt DeltaC proteins have accumulated associate with each other as a result of oligomerization capacity of the synaptotagmin family, because the Syt IDeltaC mutant, which lacks oligomerization activity, cannot induce crystalloid ER. Our findings indicate that the conserved WHXL motif is important not only for protein interaction site but for proper folding of the C2B domain.

Characterization of the nucleotide-binding capacity and the ATPase activity of the PIP3-binding protein JFC1

In this work, we demonstrate that the phosphatidylinositol 3,4,5-trisphosphate-binding protein JFC1 is an ATP-binding protein with magnesium-dependent ATPase activity. We show that JFC1 specifically binds to the ATP analog 8-azido-[alpha-(32)P]ATP. The affinity of JFC1 for [alpha-(32)P]ATP was 10x greater than its affinity for [alpha-(32)P]ADP; the protein did not appear to bind to [alpha-(32)P]GTP. JFC1 hydrolyzed [alpha-(32)P]ATP in a Mg(2+)-dependent manner. JFC1, which also hydrolyzed dATP, has a relatively high affinity for ATP, with a K(M) value of 58 microM, and a k(cat) value of 2.27 per min. The predicted amino acid sequence of JFC1 denotes a putative nucleotide-binding site similar to those in the GHKL ATPase/kinase superfamily. However, a truncation of JFC1 that contains boxes G2 and G3 but not boxes N and G1 of the Bergerat-binding site showed residual ATPase activity. Secondly, the antitumor ATP-mimetic agent geldanamycin, which inhibits the ATPase activity of Hsp-90, did not affect JFC1 ATPase. Therefore, the characteristics of the ATP-binding site of JFC1 are unique. Phosphatidylinositol 3,4,5-trisphosphate, a high-affinity ligand of JFC1 did not affect its ATPase kinetics parameters, suggesting that the phosphoinositide have a different role in JFC1 function.

Intracellular calcium dependence of large dense-core vesicle exocytosis in the absence of synaptotagmin I

Synaptotagmin I is a synaptic vesicle-associated protein essential for synchronous neurotransmission. We investigated its impact on the intracellular Ca(2+)-dependence of large dense-core vesicle (LDCV) exocytosis by combining Ca(2+)-uncaging and membrane capacitance measurements in adrenal slices from mouse synaptotagmin I null mutants. Synaptotagmin I-deficient chromaffin cells displayed prolonged exocytic delays and slow, yet Ca(2+)-dependent fusion rates, resulting in strongly reduced LDCV release in response to short depolarizations. Vesicle recruitment, the shape of individual amperometric events, and endocytosis appeared unaffected. These findings demonstrate that synaptotagmin I is required for rapid, highly Ca(2+)-sensitive LDCV exocytosis and indicate that it regulates the equilibrium between a slowly releasable and a readily releasable state of the fusion machinery. Alternatively, synaptotagmin I could function as calcium sensor for the readily releasable pool, leading to the destabilization of the pool in its absence.

The tandem C2 domains of synaptotagmin contain redundant Ca2+ binding sites that cooperate to engage t-SNAREs and trigger exocytosis

Real-time voltammetry measurements from cracked PC12 cells were used to analyze the role of synaptotagmin-SNARE interactions during Ca2+-triggered exocytosis. The isolated C2A domain of synaptotagmin I neither binds SNAREs nor inhibits norepinephrine secretion. In contrast, two C2 domains in tandem (either C2A-C2B or C2A-C2A) bind strongly to SNAREs, displace native synaptotagmin from SNARE complexes, and rapidly inhibit exocytosis. The tandem C2 domains of synaptotagmin cooperate via a novel mechanism in which the disruptive effects of Ca2+ ligand mutations in one C2 domain can be partially alleviated by the presence of an adjacent C2 domain. Complete disruption of Ca2+-triggered membrane and target membrane SNARE interactions required simultaneous neutralization of Ca2+ ligands in both C2 domains of the protein. We conclude that synaptotagmin-SNARE interactions regulate membrane fusion and that cooperation between synaptotagmin's C2 domains is crucial to its function.

The actions of barium and strontium on exocytosis and endocytosis in the synaptic terminal of goldfish bipolar cells

1. We investigated the properties of Ca2+-sensitive steps in the cycling of synaptic vesicles by comparing the actions of Ca2+, Ba2+ and Sr2+ in the synaptic terminal of depolarizing bipolar cells isolated from the retina of goldfish. FM1-43 fluorescence and capacitance measurements demonstrated that exocytosis, endocytosis and vesicle mobilization were maintained when external Ca2+ was replaced by either Ba2+ or Sr2+. 2. The rapidly releasable pool of vesicles (RRP) was equivalent to 1.5 % of the membrane surface area when measured in the presence of 2.5 mM Ca2+, but only 0.4 % in 2.5 mM Sr2+. The relative sizes of the RRP in Ca2+, Sr2+ and Ba2+ were 1.0, 0.28 and 0.1, respectively. We conclude that a smaller proportion of docked vesicles are available for fast exocytosis triggered by the influx of Sr2+ or Ba2+ compared to Ca2+. 3. The slow phase of exocytosis was not altered when Ca2+ was replaced by Ba2+, but it was accelerated 1.6-fold in Sr2+. The peak concentrations of Ca2+, Sr2+ and Ba2+ (measured using Mag-fura-5) were approximately 4, approximately 14 and approximately 60 microM, respectively. The order of efficiency for the stimulation of slow exocytosis was Ca2+ approximately Sr2+ > Ba2+. 4. Exocytosis was prolonged after the influx of Sr2+ and Ba2+. Sr2+ was cleared from the synaptic terminal with the same time constant as Ca2+ (1.3 s), but Ba2+ was cleared 10-100 times more slowly. Although Ba(2+) stimulates the slow release of a large number of vesicles, it did so less efficiently than Ca2+ or Sr2+. 5. The recovery of the membrane capacitance was equally rapid in Sr2+ and Ca2+, demonstrating that the fast mode of endocytosis could be triggered by either cation.

Clathrin-mediated endocytosis: membrane factors pull the trigger

Clathrin-mediated endocytosis is a vesicular transport event involved in the internalization and recycling of receptors participating in signal transduction events and nutrient import as well as in the reformation of synaptic vesicles. Recent studies in vitro and in living cells have provided a number of new insights into the initial steps of clathrin-coated vesicle formation and the membrane factors involved in this process. The unexpected complexity of these interactions at the cytosol-membrane interface suggests that clathrin-coated vesicle assembly is a highly cooperative process occurring under tight regulatory control. In this review, we focus on the role of membrane proteins and lipids in the nucleation of clathrin-coated pits and provide a hypothetical model for the early steps in clathrin-mediated endocytosis.

Drosophila synaptotagmin I null mutants survive to early adulthood

Synaptotagmin is a synaptic vesicle protein required for efficient neurotransmitter release, yet its exact role in the synaptic vesicle cycle is unclear. Drosophila presents an ideal organism for studies aimed at determining the in vivo functions of proteins. However, synaptotagmin studies have been limited by the early (embryonic or first instar) lethality previously reported for Drosophila synaptotagmin I null (syt(null)) mutants. Here we report a new culturing technique that enhances survival of severely uncoordinated mutants thereby permitting Drosophila syt(null) mutants to survive through early adulthood. We examined synapses in syt(null) third instar larvae by electrophysiology and found that they exhibit severely decreased and asynchronous evoked neurotransmitter release, as well as an increased rate of spontaneous neurotransmitter release, as previously seen in first instar syt(null) larvae. The ability to examine severe synaptotagmin mutants as third instar larvae, a stage where electrophysiological and morphological analyses are more easily accomplished, will facilitate structure/function studies.

The top loops of the C(2) domains from synaptotagmin and phospholipase A(2) control functional specificity

The phospholipid-binding specificities of C(2) domains, widely distributed Ca(2+)-binding modules, differ greatly despite similar three-dimensional structures. To understand the molecular basis for this specificity, we have examined the synaptotagmin 1 C(2)A domain, which interacts in a primarily electrostatic, Ca(2+)-dependent reaction with negatively charged phospholipids, and the cytosolic phospholipase A(2) (cPLA(2)) C(2) domain, which interacts by a primarily hydrophobic Ca(2+)-dependent mechanism with neutral phospholipids. We show that grafting the short Ca(2+)-binding loops from the tip of the cPLA(2) C(2) domain onto the top of the synaptotagmin 1 C(2)A domain confers onto the synaptotagmin 1 C(2)A domain the phospholipid binding specificity of the cPLA(2) C(2) domain, indicating that the functional specificity of C(2) domains is determined by their short top loops.

The AP2 binding site of synaptotagmin 1 is not an internalization signal but a regulator of endocytosis

One characteristic linking members of the synaptotagmin family to endocytosis is their ability to bind the heterotetrameric AP2 complex via their C2B domain. By using CD4/synaptotagmin 1 chimeras, we found that the internalization signal of synaptotagmin 1 lies at the extreme COOH-terminus of the protein and can function in the absence of the C2B domain that contains the AP2 binding site. However, although not essential for internalization, the C2B domain of synaptotagmin 1 appeared to control the recognition of the internalization motif. By mutagenesis, two sites have been identified that modify regulation by the C2B domain in the neuroendocrine PC12 cell line. Mutation of a dilysine motif in the beta sandwich core of the domain eliminates endocytosis. This site is known to be a site of protein-protein interaction. Mutations in the calcium binding region, or in its close proximity, also affect internalization in PC12 cells. In fibroblasts, the C2B domain inhibits the COOH-terminal internalization signal, resulting in an absence of internalization in those cells. Thus, internalization of synaptotagmin 1 is controlled by the presence of a latent internalization signal in the COOH-terminal region and a regulatory region in the C2B domain. We propose that internalization of synaptotagmin 1 is regulated in this way to allow it to couple the processes of endocytosis and calcium-mediated exocytosis in cells of the neuroendocrine lineage.

The C2B domain of synaptotagmin I is a Ca2+-binding module

Synaptotagmin I is a synaptic vesicle protein that contains two C(2) domains and acts as a Ca(2+) sensor in neurotransmitter release. The Ca(2+)-binding properties of the synaptotagmin I C(2)A domain have been well characterized, but those of the C(2)B domain are unclear. The C(2)B domain was previously found to pull down synaptotagmin I from brain homogenates in a Ca(2+)-dependent manner, leading to an attractive model whereby Ca(2+)-dependent multimerization of synaptotagmin I via the C(2)B domain participates in fusion pore formation. However, contradictory results have been described in studies of Ca(2+)-dependent C(2)B domain dimerization, as well as in analyses of other C(2)B domain interactions. To shed light on these issues, the C(2)B domain has now been studied using biophysical techniques. The recombinant C(2)B domain expressed as a GST fusion protein and isolated by affinity chromatography contains tightly bound bacterial contaminants despite being electrophoretically pure. The contaminants bind to a polybasic sequence that has been previously implicated in several C(2)B domain interactions, including Ca(2+)-dependent dimerization. NMR experiments show that the pure recombinant C(2)B domain binds Ca(2+) directly but does not dimerize upon Ca(2+) binding. In contrast, a cytoplasmic fragment of native synaptotagmin I from brain homogenates, which includes the C(2)A and C(2)B domains, participates in a high molecular weight complex as a function of Ca(2+). These results show that the recombinant C(2)B domain of synaptotagmin I is a monomeric, autonomously folded Ca(2+)-binding module and suggest that a potential function of synaptotagmin I multimerization in fusion pore formation does not involve a direct interaction between C(2)B domains or requires a posttranslational modification.

Brain glucose utilization in mice with a targeted mutation in the thyroid hormone alpha or beta receptor gene

Brain glucose utilization is markedly depressed in adult rats made cretinous after birth. To ascertain which subtype of thyroid hormone (TH) receptors, TRalpha1 or TRbeta, is involved in the regulation of glucose utilization during brain development, we used the 2-[(14)C]deoxyglucose method in mice with a mutation in either their TRalpha or TRbeta gene. A C insertion produced a frameshift mutation in their carboxyl terminus. These mutants lacked TH binding and transactivation activities and exhibited potent dominant negative activity. Glucose utilization in the homozygous TRbetaPV mutant mice and their wild-type siblings was almost identical in 19 brain regions, whereas it was markedly reduced in all brain regions of the heterozygous TRalpha1PV mice. These suggest that the alpha1 receptor mediates the TH effects in brain. Inasmuch as local cerebral glucose utilization is closely related to local synaptic activity, we also examined which thyroid hormone receptor is involved in the expression of synaptotagmin-related gene 1 (Srg1), a TH-positively regulated gene involved in the formation and function of synapses [Thompson, C. C. (1996) J. Neurosci. 16, 7832-7840]. Northern analysis showed that Srg1 expression was markedly reduced in the cerebellum of TRalpha(PV/+) mice but not TRbeta(PV/PV) mice. These results show that the same receptor, TRalpha1, is involved in the regulation by TH of both glucose utilization and Srg1 expression.

Calcium Regulation of Exocytosis in PC12 Cells

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

A Discontinuous SNAP-25 C-terminal Coil Supports Exocytosis

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