Synaptic vesicle recycling in synapsin I knock-out mice

Identification of synapsin I peptides that insert into lipid membranes

The synapsins constitute a family of synaptic vesicle-associated phosphoproteins essential for regulating neurotransmitter release and synaptogenesis. The molecular mechanisms underlying the selective targeting of synapsin I to synaptic vesicles are thought to involve specific protein-protein interactions, while the high-affinity binding to the synaptic vesicle membrane may involve both protein-protein and protein-lipid interactions. The highly hydrophobic N-terminal region of the protein has been shown to bind with high affinity to the acidic phospholipids phosphatidylserine and phosphatidylinositol and to penetrate the hydrophobic core of the lipid bilayer. To precisely identify the domains of synapsin I which mediate the interaction with lipids, synapsin I was bound to liposomes containing the membrane-directed carbene-generating reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine and subjected to photolysis. Isolation and N-terminal amino acid sequencing of 125I-labelled synapsin I peptides derived from CNBr cleavage indicated that three distinct regions in the highly conserved domain C of synapsin I insert into the hydrophobic core of the phospholipid bilayer. The boundaries of the regions encompass residues 166-192, 233-258 and 278-327 of bovine synapsin I. These regions are surface-exposed in the crystal structure of domain C of bovine synapsin I and are evolutionarily conserved among isoforms across species. The present data offer a molecular explanation for the high-affinity binding of synapsin I to phospholipid bilayers and synaptic vesicles.

Amphiphysin 1 binds the cyclin-dependent kinase (cdk) 5 regulatory subunit p35 and is phosphorylated by cdk5 and cdc2

Amphiphysin 1 is a phosphoprotein expressed at high levels in neurons, where it participates in synaptic vesicle endocytosis and neurite outgrowth. It is a substrate for cyclin-dependent kinase (cdk) 5, a member of the cyclin-dependent protein kinase family, which has been functionally linked to neuronal migration and neurite outgrowth via its action on the actin cytoskeleton. The yeast homologue of amphiphysin, Rvs167, functions in endocytosis and actin dynamics, is phosphorylated by the cdk5 homologue Pho85, and binds the Pho85 regulatory subunit Pcl2. We show here that amphiphysin 1 interacts with the cdk5-activating subunit p35 and that this interaction is mediated by the conserved NH2-terminal region of amphiphysin. Amphiphysin 1 colocalizes with p35 in the growth cones of neurons and at actin-rich peripheral lamellipodia in transfected fibroblasts. Amphiphysin is phosphorylated by cdk5 in a region including serines 272, 276, and 285. Amphiphysin 1 is also phosphorylated by the cdc2/cyclin B kinase complex in the same region and undergoes mitotic phosphorylation in dividing cells. These data indicate that phosphorylation by members of the cyclin-dependent kinase family is a conserved property of amphiphysin and suggest that this phosphorylation may play an important physiological role both in mitosis and in differentiated cells.

Identification of synapsin I peptides that insert into lipid membranes

The synapsins constitute a family of synaptic vesicle-associated phosphoproteins essential for regulating neurotransmitter release and synaptogenesis. The molecular mechanisms underlying the selective targeting of synapsin I to synaptic vesicles are thought to involve specific protein-protein interactions, while the high-affinity binding to the synaptic vesicle membrane may involve both protein-protein and protein-lipid interactions. The highly hydrophobic N-terminal region of the protein has been shown to bind with high affinity to the acidic phospholipids phosphatidylserine and phosphatidylinositol and to penetrate the hydrophobic core of the lipid bilayer. To precisely identify the domains of synapsin I which mediate the interaction with lipids, synapsin I was bound to liposomes containing the membrane-directed carbene-generating reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine and subjected to photolysis. Isolation and N-terminal amino acid sequencing of 125I-labelled synapsin I peptides derived from CNBr cleavage indicated that three distinct regions in the highly conserved domain C of synapsin I insert into the hydrophobic core of the phospholipid bilayer. The boundaries of the regions encompass residues 166-192, 233-258 and 278-327 of bovine synapsin I. These regions are surface-exposed in the crystal structure of domain C of bovine synapsin I and are evolutionarily conserved among isoforms across species. The present data offer a molecular explanation for the high-affinity binding of synapsin I to phospholipid bilayers and synaptic vesicles.

Neurotransmitter release at ribbon synapses in the retina

The synapses of photoreceptors and bipolar cells in the retina are easily identified ultrastructurally by the presence of synaptic ribbons, electron-dense bars perpendicular to the plasma membrane at the active zones, extending about 0.5 microm into the cytoplasm. The neurotransmitter, glutamate, is released continuously (tonically) from these 'ribbon synapses' and the rate of release is modulated in response to graded changes in the membrane potential. This contrasts with action potential-driven bursts of release at conventional synapses. Similar to other synapses, neurotransmitter is released at ribbon synapses by the calcium-dependent exocytosis of synaptic vesicles. Most components of the molecular machinery governing transmitter release are conserved between ribbon and conventional synapses, but a few differences have been identified that may be important determinants of tonic transmitter release. For example, the presynaptic calcium channels of bipolar cells and photoreceptors are different from those elsewhere in the brain. Differences have also been found in the proteins involved in synaptic vesicle recruitment to the active zone and in synaptic vesicle fusion. These differences and others are discussed in terms of their implications for neurotransmitter release from photoreceptors and bipolar cells in the retina.

Synaptic vesicle endocytosis: calcium works overtime in the nerve terminal

The functions of Ca2+ are many and varied within cells, but in the nerve terminals of neurons it has had a very defined role. That is, the influx of extracellular Ca2+ through voltage-dependent Ca2+ channels stimulates neurotransmitter release by exocytosis. For years this was assumed to be the main role for Ca2+ in this specialized subcellular region. However recent studies have shown that Ca2+ also has multiple roles in synaptic-vesicle endocytosis. This review will present evidence for three Ca2+-dependent and -independent steps; a high-affinity Ca2+-dependent triggering step, a Ca2+-independent maintenance phase, and a low-affinity Ca2+-dependent inhibition step. How the control of endocytosis by Ca2+ might impact on different neuronal functions such as synaptic transmission, the nucleation of SV endocytosis, and the repair of damaged membrane is then discussed.

Presynaptic proteins of ribbon synapses in the retina

The synapses of photoreceptors and bipolar cells in the retina are characterized ultrastructurally by the presence of an electron-dense bar, the synaptic ribbon, lying perpendicular to the plasma membrane at the active zone and extending about 0.5 microm into the cytoplasm. Hence, these synapses are known as ribbon synapses. All neurons that make ribbon synapses release neurotransmitter tonically. That is, neurotransmitter is released continuously from these neurons and the rate of release is modulated in response to graded changes in the membrane potential. This contrasts with action potential-driven, phasic release from other neurons. Similar to other synapses, neurotransmitter is released at ribbon synapses by the calcium-dependent exocytosis of synaptic vesicles. Most components of the molecular machinery governing transmitter release are conserved between ribbon and conventional synapses, but several differences that may be important determinants of tonic transmitter release have been identified in the retina by immunohistochemistry. For example, the presynaptic calcium channels of bipolar cells and photoreceptors are different from those elsewhere in the brain. Differences have also been found in the proteins involved in synaptic vesicle recruitment to the active zone and in synaptic vesicle fusion. These differences and others are discussed in terms of their implications for neurotransmitter release from photoreceptors and bipolar cells in the retina.

Modulation of calcium-evoked [3H]noradrenaline release from permeabilized cerebrocortical synaptosomes by the MARCKS protein, calmodulin and the actin cytoskeleton

In order to examine intracellular modulation of CNS catecholamine release, cerebrocortical synaptosomes were prelabeled with [3H]noradrenaline and permeabilized with streptolysin-O in the absence or presence of Ca(2+). Plasma membrane permeabilization allowed efflux of cytosol and left a compartmentalized pool of [3H]noradrenaline intact, approximately 10% of which was released by addition of 10(-5) M Ca(2+). Addition of activators or inhibitors of protein kinase C, as well as inhibitors of Ca(2+)-calmodulin kinase II or calcineurin, failed to change Ca(2+)-induced noradrenaline release. Evoked release from permeabilized synaptosomes deficient in the vesicle-associated phosphoprotein synapsin I was also unchanged. In contrast, addition of a synthetic 'active domain' peptide from the myristoylated, alanine-rich C-kinase substrate (MARCKS) protein increased, while addition of calmodulin decreased Ca(2+)-induced release from the permeabilized synaptosomes, the latter effect being reversed by a peptide inhibitor of calcineurin. Moreover, addition of the actin-destabilizing agent DNase I, as well as antibodies to MARCKS, appeared to increase spontaneous, Ca(2+)-independent release from noradrenergic vesicles. These results indicate that the MARCKS protein may modulate release from permeabilized noradrenergic synaptosomes, possibly by modulating calmodulin levels and/or the actin cytoskeleton.

Decrease in phorbol ester-induced potentiation of noradrenaline release in synapsin I-deficient mice

Synapsin I is involved in regulating amino acid neurotransmitter release, but has a less clear role in noradrenergic nerve terminals. To better understand the role of synapsin I in the function of noradrenergic nerve terminals, we compared noradrenaline release in wild-type and synapsin I-deficient mice. No difference was found in the accumulation or in the Ca(2+)-independent release of [(3)H]noradrenaline in cerebrocortical synaptosomes from wild-type and synapsin I-deficient mice. Synaptosomes lacking synapsin I also displayed no gross alterations in either the time course or the Ca(2+)-dependency of [(3)H]noradrenaline release when stimulated by depolarizing secretagogues or ionophore treatment. In wild-type synaptosomes, activation of protein kinase C by phorbol ester treatment resulted in a Ca(2+)-dependent increase in [(3)H]noradrenaline release evoked by depolarizing secretagogues and ionophore treatment. The phorbol ester-mediated enhancement of [(3)H]noradrenaline release evoked by depolarizing secretagogues, but not by ionophore treatment, was greatly reduced in synapsin I-deficient synaptosomes. These results indicate that synapsin I plays a role in regulating noradrenaline release.

Synapsins as mediators of BDNF-enhanced neurotransmitter release

We examined enhancement of synaptic transmission by neurotrophins at the presynaptic level. In a synaptosomal preparation, brain-derived neurotrophic factor (BDNF) increased mitogen-activated protein (MAP) kinase-dependent synapsin I phosphorylation and acutely facilitated evoked glutamate release. PD98059, used to inhibit MAP kinase activity, markedly decreased synapsin I phosphorylation and concomitantly reduced neurotransmitter release. The stimulation of glutamate release by BDNF was strongly attenuated in mice lacking synapsin I and/or synapsin II. These results indicate a causal link of synapsin phosphorylation via BDNF, TrkB receptors and MAP kinase with downstream facilitation of neurotransmitter release.

Synaptic vesicle biogenesis

Synaptic vesicles, which have been a paradigm for the fusion of a vesicle with its target membrane, also serve as a model for understanding the formation of a vesicle from its donor membrane. Synaptic vesicles, which are formed and recycled at the periphery of the neuron, contain a highly restricted set of neuronal proteins. Insight into the trafficking of synaptic vesicle proteins has come from studying not only neurons but also neuroendocrine cells, which form synaptic-like microvesicles (SLMVs). Formation and recycling of synaptic vesicles/SLMVs takes place from the early endosome and the plasma membrane. The cytoplasmic machinery of synaptic vesicle/SLMV formation and recycling has been studied by a variety of experimental approaches, in particular using cell-free systems. This has revealed distinct machineries for membrane budding and fission. Budding is mediated by clathrin and clathrin adaptors, whereas fission is mediated by dynamin and its interacting protein SH3p4, a lysophosphatidic acid acyl transferase.

Mechanisms of synaptic vesicle recycling illuminated by fluorescent dyes

The recycling of synaptic vesicles in nerve terminals involves multiple steps, underlies all aspects of synaptic transmission, and is a key to understanding the basis of synaptic plasticity. The development of styryl dyes as fluorescent molecules that label recycling synaptic vesicles has revolutionized the way in which synaptic vesicle recycling can be investigated, by allowing an examination of processes in neurons that have long been inaccessible. In this review, we evaluate the major aspects of synaptic vesicle recycling that have been revealed and advanced by studies with styryl dyes, focussing upon synaptic vesicle fusion, retrieval, and trafficking. The greatest impact of styryl dyes has been to allow the routine visualization of endocytosis in central nerve terminals for the first time. This has revealed the kinetics of endocytosis, its underlying sequential steps, and its regulation by Ca2+. In studies of exocytosis, styryl dyes have helped distinguish between different modes of vesicle fusion, provided direct support for the quantal nature of exocytosis and endocytosis, and revealed how the probability of exocytosis varies enormously from one nerve terminal to another. Synaptic vesicle labelling with styryl dyes has helped our understanding of vesicle trafficking by allowing better understanding of different synaptic vesicle pools within the nerve terminal, vesicle intermixing, and vesicle clustering at release sites. Finally, the dyes are now being used in innovative ways to reveal further insights into synaptic plasticity.

Essential role of phosphoinositide metabolism in synaptic vesicle recycling

Growing evidence suggests that phosphoinositides play an important role in membrane traffic. A polyphosphoinositide phosphatase, synaptojanin 1, was identified as a major presynaptic protein associated with endocytic coated intermediates. We report here that synaptojanin 1-deficient mice exhibit neurological defects and die shortly after birth. In neurons of mutant animals, PI(4,5)P2 levels are increased, and clathrin-coated vesicles accumulate in the cytomatrix-rich area that surrounds the synaptic vesicle cluster in nerve endings. In cell-free assays, reduced phosphoinositide phosphatase activity correlated with increased association of clathrin coats with liposomes. Intracellular recording in hippocampal slices revealed enhanced synaptic depression during prolonged high-frequency stimulation followed by delayed recovery. These results provide genetic evidence for a crucial role of phosphoinositide metabolism in synaptic vesicle recycling.

Glycosylation sites flank phosphorylation sites on synapsin I: O-linked N-acetylglucosamine residues are localized within domains mediating synapsin I interactions

Synapsin I is concentrated in nerve terminals, where it appears to anchor synaptic vesicles to the cytoskeleton and thereby ensures a steady supply of fusion-competent synaptic vesicles. Although phosphorylation-dependent binding of synapsin I to cytoskeletal elements and synaptic vesicles is well characterized, little is known about synapsin I's O-linked N-acetylglucosamine (O-GlcNAc) modifications. Here, we identified seven in vivo O-GlcNAcylation sites on synapsin I by analysis of HPLC-purified digests of rat brain synapsin I. The seven O-GlcNAcylation sites (Ser55, Thr56, Thr87, Ser516, Thr524, Thr562, and Ser576) in synapsin I are clustered around its five phosphorylation sites in domains B and D. The proximity of phosphorylation sites to O-GlcNAcylation sites in the regulatory domains of synapsin I suggests that O-GlcNAcylation may modulate phosphorylation and indirectly affect synapsin I interactions. With use of synthetic peptides, however, the presence of an O-GlcNAc at sites Thr562 and Ser576 resulted in only a 66% increase in the Km of calcium/calmodulin-dependent protein kinase II phosphorylation of site Ser566 with no effect on its Vmax. We conclude that O-GlcNAcylation likely plays a more direct role in synapsin I interactions than simply modulating the protein's phosphorylation.

Homo- and heterodimerization of synapsins

In vertebrates, synapsins constitute a family of synaptic vesicle proteins encoded by three genes. Synapsins contain a central ATP-binding domain, the C-domain, that is highly homologous between synapsins and evolutionarily conserved in invertebrates. The crystal structure of the C-domain from synapsin I revealed that it constitutes a large (>300 amino acids), independently folded domain that forms a tight dimer with or without bound ATP. We now show that the C-domains of all synapsins form homodimers, and that in addition, C-domains from different synapsins associate into heterodimers. This conclusion is based on four findings: 1) in yeast two-hybrid screens with full-length synapsin IIa as a bait, the most frequently isolated prey cDNAs encoded the C-domain of synapsins; 2) quantitative yeast two-hybrid protein-protein binding assays demonstrated pairwise strong interactions between all synapsins; 3) immunoprecipitations from transfected COS cells confirmed that synapsin II heteromultimerizes with synapsins I and III in intact cells, and similar results were obtained with bacterial expression systems; and 4) quantification of the synapsin III level in synapsin I/II double knockout mice showed that the level of synapsin III is decreased by 50%, indicating that heteromultimerization of synapsin III with synapsins I or II occurs in vivo and is required for protein stabilization. These data suggest that synapsins coat the surface of synaptic vesicles as homo- and heterodimers in which the C-domains of the various subunits have distinct regulatory properties and are flanked by variable C-terminal sequences. The data also imply that synapsin III does not compensate for the loss of synapsins I and II in the double knockout mice.

Reversal of synaptic vesicle docking at central synapses

We used quantitative fluorescence imaging of vesicles labeled with membrane-soluble dyes to determine rates of undocking and spontaneous exocytosis of vesicles docked to the active zone of hippocampal synapses in culture. Individual vesicles undock about once per two minutes and spontaneously exocytose about once per eight minutes. Thus, not only does undocking occur, but it is over threefold faster than spontaneous fusion.

Impairment of inhibitory synaptic transmission in mice lacking synapsin I

Deletion of the synapsin I genes, encoding one of the major groups of proteins on synaptic vesicles, in mice causes late onset epileptic seizures and enhanced experimental temporal lobe epilepsy. However, mice lacking synapsin I maintain normal excitatory synaptic transmission and modulation but for an enhancement of paired-pulse facilitation. To elucidate the cellular basis for epilepsy in mutants, we examined whether the inhibitory synapses in the hippocampus from mutant mice are intact by electrophysiological and morphological means. In the cultured hippocampal synapses from mutant mice, repeated application of a hypertonic solution significantly suppressed the subsequent transmitter release, associated with an accelerated vesicle replenishing time at the inhibitory synapses, compared with the excitatory synapses. In the mutants, morphologically identifiable synaptic vesicles failed to accumulate after application of a hypertonic solution at the inhibitory preterminals but not at the excitatory preterminals. In the CA3 pyramidal cells in hippocampal slices from mutant mice, inhibitory postsynaptic currents evoked by direct electrical stimulation of the interneuron in the striatum oriens were characterized by reduced quantal content compared with those in wild type. We conclude that synapsin I contributes to the anchoring of synaptic vesicles, thereby minimizing transmitter depletion at the inhibitory synapses. This may explain, at least in part, the epileptic seizures occurring in the synapsin I mutant mice.

The optically determined size of exo/endo cycling vesicle pool correlates with the quantal content at the neuromuscular junction of Drosophila larvae

According to the current theory of synaptic transmission, the amplitude of evoked synaptic potentials correlates with the number of synaptic vesicles released at the presynaptic terminals. Synaptic vesicles in presynaptic boutons constitute two distinct pools, namely, exo/endo cycling and reserve pools (). We defined the vesicles that were endocytosed and exocytosed during high K+ stimulation as the exo/endo cycling vesicle pool. To determine the role of exo/endo cycling vesicle pool in synaptic transmission, we estimated the quantal content electrophysiologically, whereas the pool size was determined optically using fluorescent dye FM1-43. We then manipulated the size of the pool with following treatments. First, to change the state of boutons of nerve terminals, motoneuronal axons were severed. With this treatment, the size of exo/endo cycling vesicle pool decreased together with the quantal content. Second, we promoted the FM1-43 uptake using cyclosporin A, which inhibits calcineurin activities and enhances endocytosis. Cyclosporin A increased the total uptake of FM1-43, but neither the size of exo/endo cycling vesicle pool nor the quantal content changed. Third, we increased the size of exo/endo cycling vesicle pool by forskolin, which enhances synaptic transmission. The forskolin treatment increased both the size of exo/endo cycling vesicle pool and the quantal content. Thus, we found that the quantal content was closely correlated with the size of exo/endo cycling vesicle pool but not necessarily with the total uptake of FM1-43 fluorescence by boutons. The results suggest that vesicles in the exo/endo cycling pool primarily participate in evoked exocytosis of vesicles.

Monitoring secretory membrane with FM1-43 fluorescence

FM1-43 and similar styryl dyes have proven useful as probes for membrane trafficking because they reversibly stain membranes, are impermeable to membranes, and are more fluorescent when bound to membranes than when in solution. Because these dyes stain membranes in an activity-dependent manner, they are ideal for studies of neurotransmitter release mechanisms such as synaptic vesicle recycling, exocytosis, and endocytosis. FM dyes have been used in conjunction with other techniques such as fluorescent calcium indicator dyes and electrophysiological techniques to elucidate mechanisms of presynaptic calcium homeostasis and modulation of neurotransmitter release. Presynaptic membranes have been marked by FM dyes in studies of synaptogenesis and reinnervation. As a probe for endocytosed membranes, these dyes have been used to examine vacuole formation in yeast. These versatile membrane dyes are useful in a variety of applications.

Genetics of synaptic vesicle function: toward the complete functional anatomy of an organelle

Synaptic transmission starts with the release of neurotransmitters by exocytosis of synaptic vesicles. As a relatively simple organelle with a limited number of components, synaptic vesicles are in principle accessible to complete structural and functional genetic analysis. At present, the majority of synaptic vesicle proteins has been characterized, and many have been genetically analyzed in mice, Drosophila, and Caenorhabditis elegans. These studies have shown that synaptic vesicles contain proteins with diverse structures and functions. Although the genetic studies are as yet unfinished, they promise to lead to a full description of synaptic vesicles as macromolecular machines involved in all aspects of presynaptic neurotransmitter release.

The optically determined size of exo/endo cycling vesicle pool correlates with the quantal content at the neuromuscular junction of Drosophila larvae

According to the current theory of synaptic transmission, the amplitude of evoked synaptic potentials correlates with the number of synaptic vesicles released at the presynaptic terminals. Synaptic vesicles in presynaptic boutons constitute two distinct pools, namely, exo/endo cycling and reserve pools (). We defined the vesicles that were endocytosed and exocytosed during high K+ stimulation as the exo/endo cycling vesicle pool. To determine the role of exo/endo cycling vesicle pool in synaptic transmission, we estimated the quantal content electrophysiologically, whereas the pool size was determined optically using fluorescent dye FM1-43. We then manipulated the size of the pool with following treatments. First, to change the state of boutons of nerve terminals, motoneuronal axons were severed. With this treatment, the size of exo/endo cycling vesicle pool decreased together with the quantal content. Second, we promoted the FM1-43 uptake using cyclosporin A, which inhibits calcineurin activities and enhances endocytosis. Cyclosporin A increased the total uptake of FM1-43, but neither the size of exo/endo cycling vesicle pool nor the quantal content changed. Third, we increased the size of exo/endo cycling vesicle pool by forskolin, which enhances synaptic transmission. The forskolin treatment increased both the size of exo/endo cycling vesicle pool and the quantal content. Thus, we found that the quantal content was closely correlated with the size of exo/endo cycling vesicle pool but not necessarily with the total uptake of FM1-43 fluorescence by boutons. The results suggest that vesicles in the exo/endo cycling pool primarily participate in evoked exocytosis of vesicles.

Protein-protein interactions and protein modules in the control of neurotransmitter release

Information transfer among neurons is operated by neurotransmitters stored in synaptic vesicles and released to the extracellular space by an efficient process of regulated exocytosis. Synaptic vesicles are organized into two distinct functional pools, a large reserve pool in which vesicles are restrained by the actin-based cytoskeleton, and a quantitatively smaller releasable pool in which vesicles approach the presynaptic membrane and eventually fuse with it on stimulation. Both synaptic vesicle trafficking and neurotransmitter release depend on a precise sequence of events that include release from the reserve pool, targeting to the active zone, docking, priming, fusion and endocytotic retrieval of synaptic vesicles. These steps are mediated by a series of specific interactions among cytoskeletal, synaptic vesicle, presynaptic membrane and cytosolic proteins that, by acting in concert, promote the spatial and temporal regulation of the exocytotic machinery. The majority of these interactions are mediated by specific protein modules and domains that are found in many proteins and are involved in numerous intracellular processes. In this paper, the possible physiological role of these multiple protein-protein interactions is analysed, with ensuing updating and clarification of the present molecular model of the process of neurotransmitter release.

Synapsins as regulators of neurotransmitter release

One of the crucial issues in understanding neuronal transmission is to define the role(s) of the numerous proteins that are localized within presynaptic terminals and are thought to participate in the regulation of the synaptic vesicle life cycle. Synapsins are a multigene family of neuron-specific phosphoproteins and are the most abundant proteins on synaptic vesicles. Synapsins are able to interact in vitro with lipid and protein components of synaptic vesicles and with various cytoskeletal proteins, including actin. These and other studies have led to a model in which synapsins, by tethering synaptic vesicles to each other and to an actin-based cytoskeletal meshwork, maintain a reserve pool of vesicles in the vicinity of the active zone. Perturbation of synapsin function in a variety of preparations led to a selective disruption of this reserve pool and to an increase in synaptic depression, suggesting that the synapsin-dependent cluster of vesicles is required to sustain release of neurotransmitter in response to high levels of neuronal activity. In a recent study performed at the squid giant synapse, perturbation of synapsin function resulted in a selective disruption of the reserve pool of vesicles and in addition, led to an inhibition and slowing of the kinetics of neurotransmitter release, indicating a second role for synapsins downstream from vesicle docking. These data suggest that synapsins are involved in two distinct reactions which are crucial for exocytosis in presynaptic nerve terminals. This review describes our current understanding of the molecular mechanisms by which synapsins modulate synaptic transmission, while the increasingly well-documented role of the synapsins in synapse formation and stabilization lies beyond the scope of this review.

Inhibitors of myosin light chain kinase block synaptic vesicle pool mobilization during action potential firing

During repetitive action potential firing the maintenance of synaptic transmission relies on a continued supply of synaptic vesicles for fusion with the presynaptic plasma membrane. The mechanism of transport by which vesicles are delivered to the site of fusion from a reserve pool is unknown, as are the biochemical pathways linking intracellular Ca2+ elevation with vesicle mobilization. Here, using the fluorescent tracer FM1-43 in hippocampal synaptic terminals, I show that inhibitors of myosin light chain kinase can block mobilization of the reserve pool and not the immediately releasable pool.

Inhibitors of myosin light chain kinase block synaptic vesicle pool mobilization during action potential firing

During repetitive action potential firing the maintenance of synaptic transmission relies on a continued supply of synaptic vesicles for fusion with the presynaptic plasma membrane. The mechanism of transport by which vesicles are delivered to the site of fusion from a reserve pool is unknown, as are the biochemical pathways linking intracellular Ca2+ elevation with vesicle mobilization. Here, using the fluorescent tracer FM1-43 in hippocampal synaptic terminals, I show that inhibitors of myosin light chain kinase can block mobilization of the reserve pool and not the immediately releasable pool.

Measurement of synaptic vesicle exocytosis in aortic baroreceptor neurons

The purpose of this study was to evaluate the use of the fluorescent membrane label FM1-43 as a measure of synaptic terminal exocytosis during stimulation of labeled aortic baroreceptor and unlabeled nodose ganglia neurons. Activation of the nerve terminals with electrical stimulation or depolarization with 90 mM KCl in the presence of 2.0 microM FM1-43 resulted in bright, punctate staining of synaptic boutons. Additional depolarization in the absence of dye resulted in destaining with a time course that was consistent and repeatable in multiple boutons within a given terminal. Destaining was dependent on calcium influx and was blocked by bath application of 100 microM CdCl2. Whole cell patch-clamp studies have reported that depolarization-induced calcium influx in aortic baroreceptor cell bodies is predominantly caused by the activation of omega-conotoxin GVIA (omega-CgTx)-sensitive N-type calcium channels. In addition, these N-type channels have been shown to be inhibited by activation of metabotropic glutamate receptors. In the present study, exocytosis in aortic baroreceptor terminals was not affected by bath application of 5 microM nifedipine and only partially inhibited by bath application of 2.0 microM omega-CgTx. However, depolarization-induced exocytosis was significantly inhibited by bath application of 200 microM L-AP4, a type III metabotropic glutamate receptor agonist. Results from this study suggest that 1) FM1-43 can be used to measure synaptic vesicle exocytosis in baroreceptor neurons; 2) the N-type calcium channel may not be involved in the initial phase of vesicle exocytosis; and 3) activation of L-AP4-sensitive metabotropic glutamate receptors inhibits 90 mM KCl-induced vesicle release.

Kinetics of synaptic depression and vesicle recycling after tetanic stimulation of frog motor nerve terminals

We measured the time courses of two key components of the synaptic vesicle cycle during recovery from synaptic depression under different conditions, and used this and other information to create a kinetic model of the vesicle cycle. End plate potential (EPP) amplitudes were used to follow recovery from synaptic depression after different amounts of tetanic stimulation. This provided an estimate of the time course of vesicle mobilization from the reserve pool to the docked (readily releasable) pool. In addition, FM1-43 was used to measure the rate of membrane retrieval after tetanic stimulation, and the amount of membrane transferred to the surface membrane. This provided a measure of the rate of refilling of the reserve pool with recycled vesicles. The time courses of both synaptic depression and endocytosis were slowed by prolonged tetanic stimulation. This behavior could be fitted by a simple model, assuming a first-order kinetics for both vesicle endocytosis and mobilization. The results show that a nearly 20-fold decrease in the rate constant of endocytosis greatly delays refilling of the depleted reserve pool. However, to fully account for the slower recovery of depression, a decrease in the rate constant of vesicle mobilization from the reserve pool of about sixfold is also required.

Synapsin III, a novel synapsin with an unusual regulation by Ca2+

Synapsins I and II are synaptic vesicle proteins essential for normal Ca2+ regulation of neurotransmitter release. Synapsins are composed of combinations of common and variable sequences, with the central C-domain as the largest conserved domain. The C-domain is structurally homologous to ATPases, suggesting that synapsins function as ATP-dependent phosphotransfer enzymes. We have now identified an unanticipated third synapsin gene that is also expressed in brain. The product of this gene, synapsin IIIa, shares with synapsins Ia and IIa three conserved domains that are connected by variable sequences: the phosphorylated A-domain at the amino terminus, the large ATP-binding C-domain in the center, and the E-domain at the carboxyl terminus. Like other synapsins, synapsin IIIa binds ATP with high affinity and ADP with a lower affinity, consistent with a cycle of ATP binding and hydrolysis. ATP binding to the different synapsins is directly regulated by Ca2+ in a dramatically different fashion: Ca2+ activates ATP binding to synapsin I, has no effect on synapsin II, and inhibits synapsin III. Thus vertebrates express three distinct synapsins that utilize ATP but are specialized for different modes of direct Ca2+ regulation in synaptic function.

Two sites of action for synapsin domain E in regulating neurotransmitter release

Synapsins, a family of synaptic vesicle proteins, have been shown to regulate neurotransmitter release; the mechanism(s) by which they act are not fully understood. Here we have studied the role of domain E of synapsins in neurotransmitter release at the squid giant synapse. Two squid synapsin isoforms were cloned and found to contain a carboxy (C)-terminal domain homologous to domain E of the vertebrate a-type synapsin isoforms. Presynaptic injection of a peptide fragment of domain E greatly reduced the number of synaptic vesicles in the periphery of the active zone, and increased the rate and extent of synaptic depression, suggesting that domain E is essential for synapsins to regulate a reserve pool of synaptic vesicles. Domain E peptide had no effect on the number of docked synaptic vesicles, yet reversibly inhibited and slowed the kinetics of neurotransmitter release, indicating a second role for synapsins that is more intimately associated with the release process itself. Thus, synapsin domain E is involved in at least two distinct reactions that are crucial for exocytosis in presynaptic terminals.

The synaptic vesicle cycle

The ins and outs of the synaptic vesicle cycle are being examined in increasing detail with diverse investigative tools in a variety of cell types, particularly those with large granules. The cycle begins with the opening of a fusion pore that connects the vesicle lumen to the extracellular fluid. Sensitive electrophysiological techniques reveal the often-stuttering behavior of single pores in non-neuronal cells, through which small molecules trickle until the fusion pore expands and the remaining contents erupt from the vesicle. The granule membranes are then retrieved by multiple processes that appear to act in parallel and that are distinguished from each other kinetically and ultrastructurally. Following endocytosis, synaptic vesicles are then shuttled back into the vesicle pool, where they briefly mix with other vesicles, become immobilized, and remain gelled with their neighbors, even while moving en masse again to the presynaptic membrane as a prelude for another round of exocytosis.

A third member of the synapsin gene family

Synapsins are a family of neuron-specific synaptic vesicle-associated phosphoproteins that have been implicated in synaptogenesis and in the modulation of neurotransmitter release. In mammals, distinct genes for synapsins I and II have been identified, each of which gives rise to two alternatively spliced isoforms. We have now cloned and characterized a third member of the synapsin gene family, synapsin III, from human DNA. Synapsin III gives rise to at least one protein isoform, designated synapsin IIIa, in several mammalian species. Synapsin IIIa is associated with synaptic vesicles, and its expression appears to be neuron-specific. The primary structure of synapsin IIIa conforms to the domain model previously described for the synapsin family, with domains A, C, and E exhibiting the highest degree of conservation. Synapsin IIIa contains a novel domain, termed domain J, located between domains C and E. The similarities among synapsins I, II, and III in domain organization, neuron-specific expression, and subcellular localization suggest a possible role for synapsin III in the regulation of neurotransmitter release and synaptogenesis. The human synapsin III gene is located on chromosome 22q12-13, which has been identified as a possible schizophrenia susceptibility locus. On the basis of this localization and the well established neurobiological roles of the synapsins, synapsin III represents a candidate gene for schizophrenia.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Synapsins I and II are ATP-binding proteins with differential Ca2+ regulation

Synapsins I and II are abundant phosphoproteins that are localized to synaptic vesicles and have essential functions in regulating synaptic vesicle exocytosis. Synapsins contain a single evolutionarily conserved, large central domain, the C-domain, that accounts for the majority of their sequences. Unexpectedly, the crystal structure of the C-domain from synapsin I revealed that it is structurally closely related to several ATPases despite the absence of sequence similarities (Esser, L., Wang, C.-R., Hosaka, M., Smagula, C. S., Südhof, T. C., and Deisenhofer, J. (1998) EMBO J., in press). We now show that the C-domains of both synapsin I and synapsin II constitute high affinity ATP-binding modules. The two C-domains exhibit similar ATP affinities but are differentially regulated; ATP binding to synapsin I is Ca(2+)-dependent whereas ATP binding to synapsin II is Ca(2+)-independent. In synapsin I, the Ca2+ requirement for ATP binding is mediated by a single, evolutionarily conserved glutamate residue (Glu373) at a position where synapsin II contains a lysine residue. Exchange of Glu373 for lysine converts synapsin I from a Ca(2+)-dependent protein into a Ca(2+)-independent ATP-binding protein. Our studies suggest that synapsins I and II function on synaptic vesicles as ATP-binding proteins that are differentially regulated by Ca2+.

Sustained neurotransmitter release: new molecular clues

Chemical synapses convey impulses at high frequency by exocytosis of synaptic vesicles. To avoid failure of synaptic transmission, rapid replenishment of synaptic vesicles must occur. Recent molecular perturbation studies have confirmed that the recycling of synaptic vesicles involves clathrin-mediated endocytosis. The rate of exocytosis would thus be limited by the capacity of the synaptic clathrin machinery unless vesicles could be drawn from existing pools. The mobilization of vesicles from the pool clustered at the release sites appears to provide a mechanism by which the rate of exocytosis can intermittently exceed the rate of recycling. Perturbation of synapsins causes disruption of vesicle clusters and impairment of synaptic transmission at high but not at low frequencies. Both clathrin-mediated recycling and mobilization of vesicles from the reserve pool are thus important in the replenishment of synaptic vesicles. The efficacy of each mechanism appears to differ between synapses which operate with different patterns of activity.

Anti-synapsin monoclonal antibodies: epitope mapping and inhibitory effects on phosphorylation and Grb2 binding

The synapsins are a family of major neuron-specific synaptic vesicle-associated phosphoproteins which play important roles in synaptic function. In an effort to identify molecular tools which can be used to perturb the activity of the synapsins in in vitro as well as in vivo experiments, we have localized the epitopes of a panel of monoclonal antibodies (mAbs) raised against synapsins I and II and have characterized their ability to interfere with the interactions of the synapsins with protein kinases, actin and Src homology-3 (SH3) domains. The epitopes of the six mAbs were found to be concentrated in the N-terminal region within domains A and B for the synapsin II-reactive mAbs 19.4, 19.11, 19.51 and 19.21, and in two C-terminal clusters in the proline-rich domains D for synapsin I (mAbs 10.22, 19.51, 19.11 and 19.8) and G for synapsin II (mAb 19.8). The synapsin II-specific mAbs 19.4 and 19.21, whose overlapping epitopes are adjacent to phosphorylation site 1, specifically inhibited synapsin II phosphorylation by endogenous or exogenous cAMP-dependent protein kinase. While all the anti-synapsin I mAbs were unable to affect the interactions of synapsin I both with Ca2+/calmodulin-dependent protein kinase II and with actin monomers and filaments, mAbs 19.8 and 19.51 were found to inhibit the binding of Grb2 SH3 domains to the proline-rich C-terminal region of synapsin I.

Phosphorylation-dependent effects of synapsin IIa on actin polymerization and network formation

The synapsins are a family of synaptic vesicle phosphoproteins which play a key role in the regulation of neurotransmitter release and synapse formation. In the case of synapsin I, these biological properties have been attributed to its ability to interact with both synaptic vesicles and the actin-based cytoskeleton. Although synapsin II shares some of the biological properties of synapsin I, much less is known of its molecular properties. We have investigated the interactions of recombinant rat synapsin Ila with monomeric and filamentous actin and the sensitivity of those interactions to phosphorylation, and found that: i) dephosphorylated synapsin II stimulates actin polymerization by binding to actin monomers and forming actively elongating nuclei and by facilitating the spontaneous nucleation/elongation processes; ii) dephosphorylated synapsin II induces the formation of thick and ordered bundles of actin filaments with greater potency than synapsin I; iii) phosphorylation by protein kinase A markedly inhibits the ability of synapsin II to interact with both actin monomers and filaments. The results indicate that the interactions of synapsin II with actin are similar but not identical to those of synapsin I and suggest that synapsin II may play a major structural role in mature and developing nerve terminals, which is only partially overlapping with the role played by synapsin I.

Kinetic analysis of the phosphorylation-dependent interactions of synapsin I with rat brain synaptic vesicles

1. Synapsin I, a major synaptic vesicle (SV)-associated phosphoprotein, is involved in the regulation of neurotransmitter release and synapse formation. By binding to both phospholipid and protein components of SV with high affinity and in a phosphorylation-dependent fashion, synapsin I is believed to cluster SV and to attach them to the actin-based cytoskeleton of the nerve terminal. 2. In the present study we have investigated the kinetic aspects of synapsin I-SV interactions and the mechanisms of their modulation by ionic strength and site-specific phosphorylation, using fluorescence resonance energy transfer between suitable fluorophores linked to synapsin I and to the membrane bilayer. 3. The binding of synapsin I to the phospholipid and protein components of SV has fast kinetics: mean time constants ranged between 1 and 4 s for association and 9 and 11's for ionic strength-induced dissociation at 20 degrees C. The interaction with the phospholipid component consists predominantly of a hydrophobic binding with the core of the membrane which may account for the membrane stabilizing effect of synapsin I. 4. Phosphorylation of synapsin I by either SV-associated or purified exogenous Ca2+/calmodulin-dependent protein kinase II (CaMPKII) inhibited the association rate and the binding to SV at steady state by acting on the ionic strength-sensitive component of the binding. When dephosphorylated synapsin I was previously bound to SV, exposure of SV to Ca2+/calmodulin in the presence of ATP triggered a prompt dissociation of synapsin I with a time constant similar to that of ionic strength-induced dissociation. 5. In conclusion, the reversible interactions between synapsin I and SV are highly regulated by site-specific phosphorylation and have kinetics of the same order of magnitude as the kinetics of SV recycling determined in mammalian neurons under comparable temperature conditions. These findings are consistent with the hypothesis that synapsin I associates with, and dissociates from, SV during the exo-endocytotic cycle. The on-vesicle phosphorylation of synapsin I by the SV-associated CaMPKII, and the subsequent dissociation of the protein from the vesicle membrane, though not involved in mediating exocytosis of primed vesicles evoked by a single stimulus, may represent a prompt and efficient mechanism for the modulation of neurotransmitter release and presynaptic plasticity.

Synapsin I interacts with c-Src and stimulates its tyrosine kinase activity

Synapsin I is a synaptic vesicle-associated phosphoprotein that has been implicated in the formation of presynaptic specializations and in the regulation of neurotransmitter release. The nonreceptor tyrosine kinase c-Src is enriched on synaptic vesicles, where it accounts for most of the vesicle-associated tyrosine kinase activity. Using overlay, affinity chromatography, and coprecipitation assays, we have now shown that synapsin I is the major binding protein for the Src homology 3 (SH3) domain of c-Src in highly purified synaptic vesicle preparations. The interaction was mediated by the proline-rich domain D of synapsin I and was not significantly affected by stoichiometric phosphorylation of synapsin I at any of the known regulatory sites. The interaction of purified c-Src and synapsin I resulted in a severalfold stimulation of tyrosine kinase activity and was antagonized by the purified c-Src-SH3 domain. Depletion of synapsin I from purified synaptic vesicles resulted in a decrease of endogenous tyrosine kinase activity. Portions of the total cellular pools of synapsin I and Src were coprecipitated from detergent extracts of rat brain synaptosomal fractions using antibodies to either protein species. The interaction between synapsin I and c-Src, as well as the synapsin I-induced stimulation of tyrosine kinase activity, may be physiologically important in signal transduction and in the modulation of the function of axon terminals, both during synaptogenesis and at mature synapses.