Dissociation between Ca2+-triggered synaptic vesicle exocytosis and clathrin-mediated endocytosis at a central synapse

Selective replenishment of two vesicle pools depends on the source of Ca2+ at the Drosophila synapse

After synaptic vesicles (SVs) undergo exocytosis, SV pools are replenished by recycling SVs at nerve terminals. At Drosophila neuromuscular synapses, there are two distinct SV pools (i.e., the exo/endo cycling pool (ECP), which primarily maintains synaptic transmission, and the reserve pool (RP), which participates in synaptic transmission only during tetanic stimulation). Labeling endocytosed vesicular structures with a fluorescent styryl dye, FM1-43, and measuring intracellular Ca2+ concentrations with a Ca2+ indicator, rhod-2, we show here that the ECP is replenished by SVs endocytosed during stimulation, and this process depends on external Ca2+. In contrast, the RP is refilled after cessation of tetanus by a process mediated by Ca2+ released from internal stores.

Snap-shots of clathrin-mediated endocytosis

Clathrin-mediated endocytosis is one of the major entry routes into a eukaryotic cell. It is driven by protein components that aid the selection of cargo and provide the mechanical force needed to both deform the plasma membrane and detach a vesicle. Clathrin-coated vesicles were first observed by electron microscopy in the early 1960s. In subsequent years, many of the characteristic intermediates generated during vesicle formation have been trapped and observed. A variety of electron microscopy techniques, from the analysis of sections through cells to the study of endocytic intermediates formed in vitro, have led to the proposition of a sequence of events and of roles for different proteins during vesicle formation. In this article, these techniques and the insights gained are reviewed, and their role in providing snap-shots of the stages of endocytosis in atomic detail is discussed.

Monitoring of exocytosis and endocytosis of insulin secretory granules in the pancreatic beta-cell line MIN6 using pH-sensitive green fluorescent protein (pHluorin) and confocal laser microscopy

The dynamics of exocytosis/endocytosis of insulin secretory granules in pancreatic beta-cells remains to be clarified. In the present study, we visualized and analysed the motion of insulin secretory granules in MIN6 cells using pH-sensitive green fluorescent protein (pHluorin) fused to either insulin or the vesicle membrane protein, phogrin. In order to monitor insulin exocytosis, pHluorin, which is brightly fluorescent at approximately pH 7.4, but not at approximately pH 5.0, was attached to the C-terminus of insulin. To monitor the motion of insulin secretory granules throughout exocytosis/endocytosis, pHluorin was inserted between the third and fourth amino acids after the identified signal-peptide cleavage site of rat phogrin cDNA. Using this method of cDNA construction, pHluorin was located in the vesicle lumen, which may enable discrimination of the unfused acidic secretory granules from the fused neutralized ones. In MIN6 cells expressing insulin-pHluorin, time-lapse confocal laser scanning microscopy (5 or 10 s intervals) revealed the appearance of fluorescent spots by depolarization after stimulation with 50 mM KCl and 22 mM glucose. The number of these spots in the image at the indicated times was counted and found to be consistent with the results of insulin release measured by RIA during the time course. In MIN6 cells expressing phogrin-pHluorin, data showed that fluorescent spots appeared following high KCl stimulation and remained stationary for a while, moved on the plasma membrane and then disappeared. Thus we demonstrate the visualized motion of insulin granule exocytosis/endocytosis using the pH-sensitive marker, pHluorin.

Decreased synaptic vesicle recycling efficiency and cognitive deficits in amphiphysin 1 knockout mice

The function of the clathrin coat in synaptic vesicle endocytosis is assisted by a variety of accessory factors, among which amphiphysin (amphiphysin 1 and 2) is one of the best characterized. A putative endocytic function of amphiphysin was supported by dominant-negative interference studies. We have now generated amphiphysin 1 knockout mice and found that lack of amphiphysin 1 causes a parallel dramatic reduction of amphiphysin 2 selectively in brain. Cell-free assembly of endocytic protein scaffolds is defective in mutant brain extracts. Knockout mice exhibit defects in synaptic vesicle recycling that are unmasked by stimulation and suggest impairments at multiple stages of the cycle. These defects correlate with increased mortality due to rare irreversible seizures and with major learning deficits, suggesting a critical role of amphiphysin for higher brain functions.

The dephosphins: dephosphorylation by calcineurin triggers synaptic vesicle endocytosis

When nerve terminals in the brain are stimulated, a group of phosphoproteins called the dephosphins are coordinately dephosphorylated by calcineurin, the Ca(2+)-dependent protein phosphatase. Amazingly, the seven presently known dephosphins are not structurally related, yet each has been independently shown to be essential for synaptic vesicle endocytosis (SVE). Nowhere else in biology is there a similar example of the coordinated dephosphorylation of such a large group of proteins each sharing roles in the same biological response. This suggests that dephosphorylation and phosphorylation of the dephosphins is essential for SVE. Recent studies in synaptosomes have confirmed this view, with calcineurin-mediated dephosphorylation of the dephosphins essential for triggering SVE. The phosphorylation cycle of the dephosphins might regulate SVE by targeting the proteins to sites of action and by stimulating the assembly of several large essential endocytic protein complexes.

PIP kinase Igamma is the major PI(4,5)P(2) synthesizing enzyme at the synapse

Disruption of the presynaptically enriched polyphosphoinositide phosphatase synaptojanin 1 leads to an increase of clathrin-coated intermediates and of polymerized actin at endocytic zones of nerve terminals. These changes correlate with elevated levels of PI(4,5)P(2) in neurons. We report that phosphatidylinositol phosphate kinase type Igamma (PIPKIgamma), a major brain PI(4)P 5-kinase, is concentrated at synapses. Synaptojanin 1 and PIPKIgamma antagonize each other in the recruitment of clathrin coats to lipid membranes. Like synaptojanin 1 and other proteins involved in endocytosis, PIPKIgamma undergoes stimulation-dependent dephosphorylation. These results implicate PIPKIgamma in the synthesis of a PI(4,5)P(2) pool that acts as a positive regulator of clathrin coat recruitment and actin function at the synapse.

Role of Rab3 GDP/GTP exchange protein in synaptic vesicle trafficking at the mouse neuromuscular junction

The Rab3 small G protein family consists of four members, Rab3A, -3B, -3C, and -3D. Of these members, Rab3A regulates Ca(2+)-dependent neurotransmitter release. These small G proteins are activated by Rab3 GDP/GTP exchange protein (Rab3 GEP). To determine the function of Rab3 GEP during neurotransmitter release, we have knocked out Rab3 GEP in mice. Rab3 GEP-/- mice developed normally but died immediately after birth. Embryos at E18.5 showed no evoked action potentials of the diaphragm and gastrocnemius muscles in response to electrical stimulation of the phrenic and sciatic nerves, respectively. In contrast, axonal conduction of the spinal cord and the phrenic nerve was not impaired. Total numbers of synaptic vesicles, especially those docked at the presynaptic plasma membrane, were reduced at the neuromuscular junction approximately 10-fold compared with controls, whereas postsynaptic structures and functions appeared normal. Thus, Rab3 GEP is essential for neurotransmitter release and probably for formation and trafficking of the synaptic vesicles.

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.

Visualizing postendocytic traffic of synaptic vesicles at hippocampal synapses

We have investigated mechanisms in postendocytic processing of synaptic vesicles at hippocampal synapses, using synaptobrevin/vesicle-associated membrane protein (VAMP) tagged with variants of the green fluorescent protein. Following exocytosis, VAMP is retrieved at synaptic and adjoining axonal regions. Retrieved VAMP-containing vesicles return to synaptic vesicle clusters at a rate slower than endocytosis. Vesicles containing a different protein, synaptophysin, recluster at a similar rate, suggesting common vesicular intermediates for the two proteins. Activity prolongs the time taken by endocytosed vesicles to return to synapses. Exogenous calcium buffers slow endocytosis but have no additional effect on the time course of reclustering. In contrast, the protein kinase inhibitor staurosporine does not affect endocytosis but slows reclustering. Finally, since VAMP can move freely on surface membranes, sustained synaptic activity leads to mixing of this vesicular component between adjacent synapses.

Implications of lipid microdomains for membrane curvature, budding and fission

Recent studies have highlighted the importance of monolayer and bilayer curvature for the budding and fission of biological membranes. Other lines of research, addressing the structure of planar biological membranes, have revealed the existence of cholesterol-based membrane microdomains. Here, we comment on the significance of microdomains for curved membranes, with special emphasis on budding and fission.

Cycling of synaptic vesicles: how far? How fast!

Synaptic transmission is based on the regulated exocytotic fusion of synaptic vesicles filled with neurotransmitter. In order to sustain neurotransmitter release, these vesicles need to be recycled locally. Recent data suggest that two tracks for the cycling of synaptic vesicles coexist: a slow track in which vesicles fuse completely with the presynaptic plasma membrane, followed by clathrin-mediated recycling of the vesicular components, and a fast track that may correspond to the transient opening and closing of a fusion pore. In this review, we attempt to provide an overview of the components involved in both tracks of vesicle cycling, as well as to identify possible mechanistic links between these two pathways.

Contributions of identifiable neurons and neuron classes to lamprey vertebrate neurobiology

Among the advantages offered by the lamprey brainstem and spinal cord for studies of the structure and function of the nervous system is the unique identifiability of several pairs of reticulospinal neurons in the brainstem. These neurons have been exploited in investigations of the patterns of sensory input to these cells and the patterns of their outputs to spinal neurons, but no doubt these cells could be used much more effectively in exploring their roles in descending control of the spinal cord. The variability of cell positions of neurons in the spinal cord has precluded the recognition of unique spinal neurons. However, classes of nerve cells can be readily defined and characterized within the lamprey spinal cord and this has led to progress in understanding the cellular and synaptic mechanisms of locomotor activity. In addition, both the identifiable reticulospinal cells and the various spinal nerve cell classes and their known synaptic interactions have been used to demonstrate the degree and specificity of regeneration within the lamprey nervous system. The lack of uniquely identifiable cells within the lamprey spinal cord has hampered progress in these areas, especially in gaining a full understanding of the locomotor network and how neuromodulation of the network is accomplished.

Accessory factors in clathrin-dependent synaptic vesicle endocytosis

Clathrin-mediated endocytosis is a special form of vesicle budding important for the internalization of receptors and extracellular ligands, for the recycling of plasma membrane components, and for the retrieval of surface proteins destined for degradation. In nerve terminals, clathrin-mediated endocytosis is crucial for synaptic vesicle recycling. Recent structural studies have provided molecular details of coat assembly. In addition, biochemical and genetic studies have identified numerous accessory proteins that assist the clathrin coat in its function at synapses and in other systems. This review summarizes these advances with a special focus on accessory factors and highlights new aspects of clathrin-mediated endocytosis revealed by the study of these factors.

Dual interaction of synaptotagmin with mu2- and alpha-adaptin facilitates clathrin-coated pit nucleation

The synaptic vesicle protein synaptotagmin was proposed to act as a major docking site for the recruitment of clathrin coats implicated in endocytosis, including the recycling of synaptic vesicles. We show here that the C2B domain of synaptotagmin binds mu2- and alpha-adaptin, two of the four subunits of the endocytic adaptor complex AP-2. mu2 represents the major interacting subunit of AP-2 within this complex. Its binding to synaptotagmin is mediated by a site in subdomain B that is distinct from the binding site for tyrosine-based sorting motifs located in subdomain A. The presence of the C2B domain of synaptotagmin at the surface of liposomes enhances the recruitment of AP-2 and clathrin. Conversely, perturbation of the interaction between synaptotagmin and AP-2 by synprint, the cytoplasmic synaptotagmin-binding domain of N-type calcium channels, inhibits transferrin internalization in living cells. We conclude that a dual interaction of synaptotagmin with the clathrin adaptor AP-2 plays a key physiological role in the nucleation of endocytic clathrin-coated pits.

Drosophila endosomal proteins hook and deep orange regulate synapse size but not synaptic vesicle recycling

To study the function of endosomes at synapses we analyzed the localization and function of two Drosophila endosomal proteins, Hook and Deep orange (Dor), at the larval neuromuscular junction. Hook, a negative regulator of endocytic trafficking, and Dor, a positive regulator of endocytic trafficking, are highly enriched at synapses, especially close to postsynaptic membranes. Mutations in hook (hk) and dor do not affect synaptic vesicle recycling, as assessed by electrophysiological analysis of synaptic transmission and behavioral studies of double mutants with shi(ts) mutations that alter vesicle recycling. However, hk and dor mutations alter the number of presynaptic varicosities (synapse size) in opposing ways. Synapse size is increased in hk(11) mutants and is decreased in dor(4) mutants. Double mutants for dor and hk show a dor-like phenotype. These effects on synapse size parallel known functions of Hook and Dor in endocytosis and strongly indicate a role for endocytic trafficking in the regulation of synapse size in vivo. Our observations suggest a model in which Hook and Dor function in later stages of endocytosis is essential for regulating synaptic plasma membrane composition but not synaptic vesicle recycling.

Two endocytic recycling routes selectively fill two vesicle pools in frog motor nerve terminals

We have identified and characterized two vesicle recycling pathways in frog motor nerve terminals. We exploited the differential staining properties of FM dyes of varying hydrophobicity to label selectively two different vesicle pools, using optical imaging and electron microscopy of photoconverted dyes. During a 1 min tetanus, a rapidly recycling route places vesicles selectively into a small readily releasable pool comprising about 20% of vesicles. After the tetanus, a much slower pathway (from which FM2-10 but not FM1-43 can be rinsed) delivers vesicles via infoldings and cisternae selectively to a reserve pool with a halftime of about 8 min. Mixing between the two pools is slow. During stimulation at 30 Hz, 10-15 s is required to mobilize and release dye from the reserve pool.

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.

Fission and uncoating of synaptic clathrin-coated vesicles are perturbed by disruption of interactions with the SH3 domain of endophilin

Coordination between sequential steps in synaptic vesicle endocytosis, including clathrin coat formation, fission, and uncoating, appears to involve proteinprotein interactions. Here, we show that compounds that disrupt interactions of the SH3 domain of endophilin with dynamin and synaptojanin impair synaptic vesicle endocytosis in a living synapse. Two distinct endocytic intermediates accumulated. Free clathrin-coated vesicles were induced by a peptide-blocking endophilin's SH3 domain and by antibodies to the proline-rich domain (PRD) of synaptojanin. Invaginated clathrin-coated pits were induced by the same peptide and by the SH3 domain of endophilin. We suggest that the SH3 domain of endophilin participates in both fission and uncoating and that it may be a key component of a molecular switch that couples the fission reaction to uncoating.

Sequential steps in clathrin-mediated synaptic vesicle endocytosis

Synaptic vesicles are recycled with remarkable speed and precision in nerve terminals. A major recycling pathway involves clathrin-mediated endocytosis at endocytic zones located around sites of release. Different 'accessory' proteins linked to this pathway have been shown to alter the shape and composition of lipid membranes, to modify membrane-coat protein interactions, and to influence actin polymerization. These include the GTPase dynamin, the lysophosphatidic acid acyl transferase endophilin, and the phosphoinositide phosphatase synaptojanin. Protein perturbation studies in living nerve terminals are now beginning to link the actions of these proteins with morphologically defined steps of endocytosis.

Synaptophysin regulates clathrin-independent endocytosis of synaptic vesicles

The GTPase dynamin I is required for synaptic vesicle (SV) endocytosis. Our observation that dynamin binds to the SV protein synaptophysin in a Ca(2+)-dependent fashion suggested the possibility that a dynamin/synaptophysin complex functions in SV recycling. In this paper we show that disruption of the dynamin/synaptophysin interaction by peptide injection into the squid giant synapse preterminal results in a decrease in transmitter release during high-frequency stimulation, indicating an inhibition of SV recycling. Electron microscopy of these synapses reveals a depletion of SVs, demonstrating a block of vesicle retrieval after fusion. In addition, we observed an increase in clathrin-coated vesicles, indicating that the peptide does not block clathrin-dependent endocytosis. We conclude that the dynamin/synaptophysin complex functions in a clathrin-independent mechanism of SV endocytosis that is required for efficient synaptic transmission.

Activity and calcium-dependent mechanisms maintain reliable interneuron synaptic transmission in a rhythmic neural network

Inputs from glutamatergic excitatory interneurons (EIN) to motor neurons in the lamprey spinal cord locomotor network exhibit activity-dependent depression during spike trains. The mechanism underlying this depression has been examined here, and its relevance to transmitter release during rhythmic activity has been investigated. The depression of EIN inputs was greater after larger initial EPSPs and reduced in low-calcium Ringer's solution, effects that are consistent with depression caused by depletion of releasable transmitter stores. However, the depression was greater at lower stimulation frequencies and could be reversed by increasing the stimulation frequency. In addition, high-calcium Ringer's solution and the slow intracellular calcium chelator EGTA-AM, which both failed to affect the amplitude of low frequency-evoked EPSPs, reduced and increased the depression, respectively. These results are inconsistent with a simple depletion mechanism but suggest that ongoing activity and calcium-dependent mechanisms oppose depletion. The network relevance of this mechanism was examined using physiologically relevant bursts to simulate EIN spiking during rhythmic activity. Although considerably more EPSPs were evoked than during spike trains, burst-evoked EPSPs did not depress. However, single EPSPs evoked at the interburst interval depressed, and burst transmission was disrupted by EGTA-AM, again suggesting the involvement of activity and calcium-dependent mechanisms. By responding to the calcium changes evoked by increased interneuron activity, this mechanism can monitor transmitter requirements caused by EIN spiking, allowing reliable transmission across different patterns of network activity. However, not all types of spinal interneurons exhibit reliable burst transmission, suggesting specificity of this mechanism to a subset of neurons.

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.

Botulinum neurotoxin A blocks synaptic vesicle exocytosis but not endocytosis at the nerve terminal

The supply of synaptic vesicles in the nerve terminal is maintained by a temporally linked balance of exo- and endocytosis. Tetanus and botulinum neurotoxins block neurotransmitter release by the enzymatic cleavage of proteins identified as critical for synaptic vesicle exocytosis. We show here that botulinum neurotoxin A is unique in that the toxin-induced block in exocytosis does not arrest vesicle membrane endocytosis. In the murine spinal cord, cell cultures exposed to botulinum neurotoxin A, neither K(+)-evoked neurotransmitter release nor synaptic currents can be detected, twice the ordinary number of synaptic vesicles are docked at the synaptic active zone, and its protein substrate is cleaved, which is similar to observations with tetanus and other botulinal neurotoxins. In marked contrast, K(+) depolarization, in the presence of Ca(2+), triggers the endocytosis of the vesicle membrane in botulinum neurotoxin A-blocked cultures as evidenced by FM1-43 staining of synaptic terminals and uptake of HRP into synaptic vesicles. These experiments are the first demonstration that botulinum neurotoxin A uncouples vesicle exo- from endocytosis, and provide evidence that Ca(2+) is required for synaptic vesicle membrane retrieval.

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.

Inhibition of neurotransmitter release in the lamprey reticulospinal synapse by antibody-mediated disruption of SNAP-25 function

Exocytosis - syntaxin - synaptobrevin - SNARE synaptic vesicle The lamprey giant reticulospinal synapse can be used to manipulate the molecular machinery of synaptic vesicle exocytosis by presynaptic microinjection. Here we test the effect of disrupting the function of the SNARE protein SNAP-25. Polyclonal SNAP-25 antibodies were shown in an in vitro assay to inhibit the binding between syntaxin and SNAP-25. When microinjected presynaptically, these antibodies produced a potent inhibition of the synaptic response. Ba2+ spikes recorded in the presynaptic axon were not altered, indicating that the effect was not due to a reduced presynaptic Ca2+ entry. Electron microscopic analysis showed that synaptic vesicle clusters had a similar organization in synapses of antibody-injected axons as in control axons, and the number of synaptic vesicles in apparent contact with the presynaptic plasma membrane was also similar. Clathrin-coated pits, which normally occur at the plasma membrane around stimulated synapses, were not detected after injection of SNAP-25 antibodies, consistent with a blockade of vesicle cycling. Thus, SNAP-25 antibodies, which disrupt the interaction with syntaxin, inhibit neurotransmitter release without affecting the number of synaptic vesicles at the plasma membrane. These results provide further support to the view that the formation of SNARE complexes is critical for membrane fusion, but not for the targeting of synaptic vesicles to the presynaptic membrane.

The calcineurin-dynamin 1 complex as a calcium sensor for synaptic vesicle endocytosis

Exocytosis of synaptic vesicles is calcium-dependent, with synaptotagmin serving as the calcium sensor. Endocytosis of synaptic vesicles has also been postulated as a calcium-dependent process; however, an endocytic calcium sensor has not been found. We now report a physical association between the calcium-dependent phosphatase calcineurin and dynamin 1, a component of the synaptic endocytic machinery. The calcineurin-dynamin 1 interaction is calcium-dependent, with an EC(50) for calcium in the range of 0.1-0. 4 microM. Disruption of the calcineurin-dynamin 1 interaction inhibits clathrin-mediated endocytosis. Thus, the calcium-dependent formation of the calcineurin-dynamin 1 complex, delivered to the other endocytic coat proteins, provides a calcium-sensing mechanism that facilitates endocytosis.

Endophilin/SH3p4 is required for the transition from early to late stages in clathrin-mediated synaptic vesicle endocytosis

Endophilin/SH3p4 is a protein highly enriched in nerve terminals that binds the GTPase dynamin and the polyphosphoinositide phosphatase synaptojanin, two proteins implicated in synaptic vesicle endocytosis. We show here that antibody-mediated disruption of endophilin function in a tonically stimulated synapse leads to a block in the invagination of clathrin-coated pits adjacent to the active zone and therefore to a block of synaptic vesicle recycling. We also show that in a cell-free system, endophilin is not associated with clathrin coats and is a functional partner of dynamin. Our findings suggest that endophilin is part of a biochemical machinery that acts in trans to the clathrin coat from early stages to vesicle fission.

Correlation of miniature synaptic activity and evoked release probability in cultures of cortical neurons

Spontaneous miniature synaptic activity is caused by action potential (AP)-independent release of transmitter vesicles and is regulated at the level of single synapses. In cultured cortical neurons we have used this spontaneous vesicle turnover to load the styryl dye FM1-43 into synapses with high rates of miniature synaptic activity. Automated selection procedures restricted analysis to synapses with sufficient levels of miniature activity-mediated FM1-43 uptake. After FM1-43 loading, vesicular FM1-43 release in response to AP stimulation was recorded at single synapses as a measure of release probability. We find that synapses with high rates of miniature activity possess significantly enhanced evoked release rates compared with a control population. Because the difference in release rates between the two populations is [Ca(2+)](o)-dependent, it is most likely caused by a difference in release probability. Within the subpopulation of synapses with high miniature activity, we find that the probabilities for miniature and AP-evoked release are correlated at single synaptic sites. Furthermore, the degree of miniature synaptic activity is correlated with the vesicle pool size. These findings suggest that both evoked and miniature vesicular release are regulated in parallel and that the frequency of miniature synaptic activity can be used as an indicator for evoked release efficacy.

Presynaptic mitochondria and the temporal pattern of neurotransmitter release

Mitochondria are critical for the function of nerve terminals as the cycling of synaptic vesicle membrane requires an efficient supply of ATP. In addition, the presynaptic mitochondria take part in functions such as Ca2+ buffering and neurotransmitter synthesis. To learn more about presynaptic mitochondria, we have examined their organization in two types of synapse in the lamprey, both of which are glutamatergic but are adapted to different temporal patterns of activity. The first is the giant lamprey reticulospinal synapse, which is specialized to transmit phasic signals (i.e. bursts of impulses). The second is the synapse established by sensory dorsal column axons, which is adapted to tonic activity. In both cases, the presynaptic axons were found to contain two distinct types of mitochondria; small 'synaptic' mitochondria, located near release sites, and larger mitochondria located in more central parts of the axon. The size of the synapse-associated mitochondria was similar in both types of synapse. However, their number differed considerably. Whereas the reticulospinal synapses contained only single mitochondria within 1 micron distance from the edge of the active zone (on average 1.2 per active zone, range of 1-3), the tonic dorsal column synapses were surrounded by clusters of mitochondria (4.5 per active zone, range of 3-6), with individual mitochondria sometimes apparently connected by intermitochondrial contacts. In conjunction with studies of crustacean neuromuscular junctions, these observations indicate that the temporal pattern of transmitter release is an important determinant of the organization of presynaptic mitochondria.

The interaction of epsin and Eps15 with the clathrin adaptor AP-2 is inhibited by mitotic phosphorylation and enhanced by stimulation-dependent dephosphorylation in nerve terminals

Clathrin-mediated endocytosis was shown to be arrested in mitosis due to a block in the invagination of clathrin-coated pits. A Xenopus mitotic phosphoprotein, MP90, is very similar to an abundant mammalian nerve terminal protein, epsin, which binds the Eps15 homology (EH) domain of Eps15 and the alpha-adaptin subunit of the clathrin adaptor AP-2. We show here that both rat epsin and Eps15 are mitotic phosphoproteins and that their mitotic phosphorylation inhibits binding to the appendage domain of alpha-adaptin. Both epsin and Eps15, like other cytosolic components of the synaptic vesicle endocytic machinery, undergo constitutive phosphorylation and depolarization-dependent dephosphorylation in nerve terminals. Furthermore, their binding to AP-2 in brain extracts is enhanced by dephosphorylation. Epsin together with Eps15 was proposed to assist the clathrin coat in its dynamic rearrangements during the invagination/fission reactions. Their mitotic phosphorylation may be one of the mechanisms by which the invagination of clathrin-coated pits is blocked in mitosis and their stimulation-dependent dephosphorylation at synapses may contribute to the compensatory burst of endocytosis after a secretory stimulus.