Kinetic efficiency of endocytosis at mammalian CNS synapses requires synaptotagmin I

Expression, localization, and functional role for synaptotagmins in pancreatic acinar cells

Secretagogue-induced changes in intracellular Ca(2+) play a pivotal role in secretion in pancreatic acini yet the molecules that respond to Ca(2+) are uncertain. Zymogen granule (ZG) exocytosis is regulated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. In nerve and endocrine cells, Ca(2+)-stimulated exocytosis is regulated by the SNARE-associated family of proteins termed synaptotagmins. This study examined a potential role for synaptotagmins in acinar secretion. RT-PCR revealed that synaptotagmin isoforms 1, 3, 6, and 7 are present in isolated acini. Immunoblotting and immunofluorescence using three different antibodies demonstrated synaptotagmin 1 immunoreactivity in apical cytoplasm and ZG fractions of acini, where it colocalized with vesicle-associated membrane protein 2. Synaptotagmin 3 immunoreactivity was detected in membrane fractions and colocalized with an endolysosomal marker. A potential functional role for synaptotagmin 1 in secretion was indicated by results that introduction of synaptotagmin 1 C2AB domain into permeabilized acini inhibited Ca(2+)-dependent exocytosis by 35%. In contrast, constructs of synaptotagmin 3 had no effect. Confirmation of these findings was achieved by incubating intact acini with an antibody specific to the intraluminal domain of synaptotagmin 1, which is externalized following exocytosis. Externalized synaptotagmin 1 was detected exclusively along the apical membrane. Treatment with CCK-8 (100 pM, 5 min) enhanced immunoreactivity by fourfold, demonstrating that synaptotagmin is inserted into the apical membrane during ZG fusion. Collectively, these data indicate that acini express synaptotagmin 1 and support that it plays a functional role in secretion whereas synaptotagmin 3 has an alternative role in endolysosomal membrane trafficking.

Probing the functional equivalence of otoferlin and synaptotagmin 1 in exocytosis

Cochlear inner hair cells (IHCs) use Ca(2+)-dependent exocytosis of glutamate to signal sound information. Otoferlin (Otof), a C(2) domain protein essential for IHC exocytosis and hearing, may serve as a Ca(2+) sensor in vesicle fusion in IHCs that seem to lack the classical neuronal Ca(2+) sensors synaptotagmin 1 (Syt1) and Syt2. Support for the Ca(2+) sensor of fusion hypothesis for otoferlin function comes from biochemical experiments, but additional roles in late exocytosis upstream of fusion have been indicated by physiological studies. Here, we tested the functional equivalence of otoferlin and Syt1 in three neurosecretory model systems: auditory IHCs, adrenal chromaffin cells, and hippocampal neurons. Long-term and short-term ectopic expression of Syt1 in IHCs of Otof (-/-) mice by viral gene transfer in the embryonic inner ear and organotypic culture failed to rescue their Ca(2+) influx-triggered exocytosis. Conversely, virally mediated overexpression of otoferlin did not restore phasic exocytosis in Syt1-deficient chromaffin cells or neurons but enhanced asynchronous release in the latter. We further tested exocytosis in Otof (-/-) hippocampal neurons and in Syt1(-/-) IHCs but found no deficits in vesicle fusion. Expression analysis of different synaptotagmin isoforms indicated that Syt1 and Syt2 are absent from mature IHCs. Our data argue against a simple functional equivalence of the two C(2) domain proteins in exocytosis of IHC ribbon synapses, chromaffin cells, and hippocampal synapses.

Erk1/2 inhibit synaptic vesicle exocytosis through L-type calcium channels

L-type calcium channels play only a minor role in basal neurotransmitter release in brain neurons but contribute significantly after induction of plasticity. Very little is known about mechanisms that enable L-type calcium channel participation in neurotransmitter release. Here, using mouse primary cortical neurons, we found that inhibition of Erk1/2 (extracellular signal-regulated kinases 1 and 2) enhanced synaptic vesicle exocytosis by increasing calcium influx through L-type calcium channels. Furthermore, inhibition of Erk1/2 increased the surface fraction of these channels. These findings indicate a novel inhibitory effect of Erk1/2 on synaptic transmission through L-type calcium channels.

Botulinum neurotoxin D uses synaptic vesicle protein SV2 and gangliosides as receptors

Botulinum neurotoxins (BoNTs) include seven bacterial toxins (BoNT/A-G) that target presynaptic terminals and act as proteases cleaving proteins required for synaptic vesicle exocytosis. Here we identified synaptic vesicle protein SV2 as the protein receptor for BoNT/D. BoNT/D enters cultured hippocampal neurons via synaptic vesicle recycling and can bind SV2 in brain detergent extracts. BoNT/D failed to bind and enter neurons lacking SV2, which can be rescued by expressing one of the three SV2 isoforms (SV2A/B/C). Localization of SV2 on plasma membranes mediated BoNT/D binding in both neurons and HEK293 cells. Furthermore, chimeric receptors containing the binding sites for BoNT/A and E, two other BoNTs that use SV2 as receptors, failed to mediate the entry of BoNT/D suggesting that BoNT/D binds SV2 via a mechanism distinct from BoNT/A and E. Finally, we demonstrated that gangliosides are essential for the binding and entry of BoNT/D into neurons and for its toxicity in vivo, supporting a double-receptor model for this toxin.

BoNTs are a family of seven bacterial toxins (BoNT/A-G). Among the seven BoNTs, whether BoNT/D uses the same entry pathways and similar receptor-binding strategies as other BoNTs is not known. Previous studies have suggested that BoNT/D does not need a protein receptor nor ganglioside co-receptor, in contrast to all other BoNTs. Here we demonstrate that BoNT/D uses synaptic vesicle protein SV2 as its protein receptor and gangliosides as co-receptor, thus supporting the “double-receptor” model as a central theme for this class of toxins. Furthermore, we found that BoNT/D utilizes a SV2 binding mechanism distinct from BoNT/A and BoNT/E, two other BoNTs that use SV2 as receptors. This indicates that different BoNTs can develop their distinct mechanisms to target a common receptor protein.

Clathrin-mediated endocytosis at the synaptic terminal: bridging the gap between physiology and molecules

It has long been known that the maintenance of fast communication between neurons requires that presynaptic terminals recycle the small vesicles from which neurotransmitter is released. But the mechanisms that retrieve vesicles from the cell surface are still not understood. Although we have a wealth of information about the molecular details of endocytosis in non-neuronal cells, it is clear that endocytosis at the synapse is faster and regulated in distinct ways. A satisfying understanding of these processes will require molecular events to be manipulated while observing endocytosis in living synapses. Here, we review recent work that seeks to bridge the gap between physiology and molecules to unravel the endocytic machinery operating at the synaptic terminal.

Control of exocytosis by synaptotagmins and otoferlin in auditory hair cells

In pre-hearing mice, vesicle exocytosis at cochlear inner hair cell (IHC) ribbon synapses is triggered by spontaneous Ca(2+) spikes. At the onset of hearing, IHC exocytosis is then exclusively driven by graded potentials, and is characterized by higher Ca(2+) efficiency and improved synchronization of vesicular release. The molecular players involved in this transition are still unknown. Here we addressed the involvement of synaptotagmins and otoferlin as putative Ca(2+) sensors in IHC exocytosis during postnatal maturation of the cochlea. Using cell capacitance measurements, we showed that Ca(2+)-evoked exocytosis in mouse IHCs switches from an otoferlin-independent to an otoferlin-dependent mechanism at postnatal day 4. During this early exocytotic period, several synaptotagmins (Syts), including Syt1, Syt2 and Syt7, were detected in IHCs. The exocytotic response as well as the release of the readily releasable vesicle pool (RRP) was, however, unchanged in newborn mutant mice lacking Syt1, Syt2 or Syt7 (Syt1(-/-), Syt2(-/-) and Syt7(-/-) mice). We only found a defect in RRP recovery in Syt1(-/-) mice which was apparent as a strongly reduced response to repetitive stimulations. In post-hearing Syt2(-/-) and Syt7(-/-) mutant mice, IHC synaptic exocytosis was unaffected. The transient expression of Syt1 and Syt2, which were no longer detected in IHCs after the onset of hearing, indicates that these two most common Ca(2+)-sensors in CNS synapses are not involved in mature IHCs. We suggest that otoferlin underlies highly efficient Ca(2+)-dependent membrane-membrane fusion, a process likely essential to increase the probability and synchrony of vesicle fusion events at the mature IHC ribbon synapse.

Cotrafficking of SV2 and synaptotagmin at the synapse

Synaptic vesicles are specialized cycling endosomes that contain a unique constellation of membrane proteins. Proteins are sorted to vesicles by short amino acid sequences that serve as binding sites for clathrin adaptor proteins. Here we show that a tyrosine-based endocytosis motif in the vesicle protein SV2 is required for trafficking to synaptic vesicles of both SV2 and the calcium sensor protein synaptotagmin. Aberrant neurotransmission in cultured hippocampal neurons lacking SV2 was rescued by expression of wild-type SV2A, but not by SV2A-Y46A, a mutant containing a disrupted endocytosis motif in SV2A's cytoplasmic N terminus. Neurons expressing SV2A-Y46A had significantly more SV2 on the plasma membrane, indicating reduced internalization. A screen for proteins that preferentially bound wild-type SV2A identified multiple endocytosis-related proteins, and in vitro binding studies confirmed binding to the clathrin adaptors AP2, EPS15, and amphiphysin 2/Bin1. Neurons lacking SV2 contained less synaptotagmin and had a higher proportion of synaptotagmin on the plasma membrane. Expression of either wild-type SV2A or SV2A-Y46A restored synaptotagmin expression levels; however, only wild-type SV2A restored a normal proportion of synaptotagmin on the plasma membrane. These findings indicate that SV2 influences the expression and trafficking of synaptotagmin via separate mechanisms. Synaptic vesicles immunoisolated from SV2A/B double knock-out mice had significantly less synaptotagmin than vesicles isolated from wild-type mice. Our results indicate that SV2 plays a major role in regulating the amount of synaptotagmin in synaptic vesicles and provide an explanation for the observation that synapses lacking SV2 have fewer vesicles competent for calcium-induced fusion.

Involvement of Ca2+ channel synprint site in synaptic vesicle endocytosis

The synaptic protein interaction (synprint) site of the voltage-gated Ca(2+) channel (VGCC) alpha1 subunit can interact with proteins involved in exocytosis, and it is therefore thought to be essential for exocytosis of synaptic vesicles. Here we report that the synprint site can also directly bind the mu subunit of AP-2, an adaptor protein for clathrin-mediated endocytosis, in competition with the synaptotagmin 1 (Syt 1) C2B domain. In brain lysates, the AP-2-synprint interaction occurred over a wide range of Ca(2+) concentrations but was inhibited at high Ca(2+) concentrations, in which Syt 1 interacted with synprint site. At the calyx of Held synapse in rat brainstem slices, direct presynaptic loading of the synprint fragment peptide blocked endocytic, but not exocytic, membrane capacitance changes. We propose that the VGCC synprint site is involved in synaptic vesicle endocytosis, rather than exocytosis, in the nerve terminal, via Ca(2+)-dependent interactions with AP-2 and Syt.

Calcium dependence of exo- and endocytotic coupling at a glutamatergic synapse

The mechanism coupling exocytosis and endocytosis remains to be elucidated at central synapses. Here, we show that the mechanism linking these two processes is dependent on microdomain-[Ca2+](i) similar to that which triggers exocytosis, as well as the exocytotic protein synaptobrevin/VAMP. Furthermore, block of endocytosis has a limited, retrograde action on exocytosis, delaying recruitment of release-ready vesicles and enhancing short-term depression. This effect sets in so rapidly that it cannot be explained by the nonavailability of recycled vesicles. Rather, we postulate that perturbation of a step linking exocytosis and endocytosis temporarily prevents new vesicles from docking at specialized sites for exocytosis.

Ca(2+) and calmodulin initiate all forms of endocytosis during depolarization at a nerve terminal

Although endocytosis maintains synaptic transmission, how endocytosis is initiated is unclear. We found that calcium influx initiated all forms of endocytosis at a single nerve terminal in rodents, including clathrin-dependent slow endocytosis, bulk endocytosis, rapid endocytosis and endocytosis overshoot (excess endocytosis), with each being evoked with a correspondingly higher calcium threshold. As calcium influx increased, endocytosis gradually switched from very slow endocytosis to slow endocytosis to bulk endocytosis to rapid endocytosis and to endocytosis overshoot. The calcium-induced endocytosis rate increase was a result of the speeding up of membrane invagination and fission. Pharmacological experiments suggested that the calcium sensor mediating these forms of endocytosis is calmodulin. In addition to its role in recycling vesicles, calcium/calmodulin-initiated endocytosis facilitated vesicle mobilization to the readily releasable pool, probably by clearing fused vesicle membrane at release sites. Our findings provide a unifying mechanism for the initiation of various forms of endocytosis that are critical in maintaining exocytosis.

A neurodegenerative disease affecting synaptic connections in Drosophila mutant for the tumor suppressor morphogen Patched

The tumor suppressor morphogen, Patched (Ptc), has an extensive homology to the Niemann-Pick-C 1 (NPC1) protein. The NPC disease is a paediatric, progressive and fatal neurodegenerative disorder thought to be due to an abnormal accumulation of cholesterol in neurons. Here, we report that patched mutant adults develop a progressive neurodegenerative disease and their brain contains membranous and lamellar inclusions. There is also a significant reduction in the number of synaptic terminals in the brain of the mutant adults. Interestingly, feeding cholesterol to wild type flies generates inclusions in the brain, but does not cause the disease. However, feeding cholesterol to mutant flies increases synaptic connections and suppresses the disease. Our results suggest that sequestration of cholesterol in the mutant brain in the form of membranous material and inclusions affects available pool of cholesterol for cellular functions. This, in turn, negatively affects the synaptic number and contributes to the disease-state. Consistent with this, in ptc mutants there is a reduction in the pool of cholesterol esters, and cholesterol-mediated suppression of the disease accompanies an increase in cholesterol esters. We further show that Ptc does not function directly in this process since gain of function for Hedgehog also induces the same disease with a reduction in the level of cholesterol esters. We believe that loss of function for ptc causes neurodegeneration via two distinct ways: de-repression of genes that interfere with lipid trafficking, and de-repression of genes outside of the lipid trafficking; the functions of both classes of genes ultimately converge on synaptic connections.

Autapses and networks of hippocampal neurons exhibit distinct synaptic transmission phenotypes in the absence of synaptotagmin I

Synaptotagmin-I (syt-I) is required for rapid neurotransmitter release in mouse hippocampal neurons. However, contradictory results have been reported regarding evoked and spontaneous secretion from syt-I knock-out (KO) neurons. Here, we compared synaptic transmission in two different hippocampal neuron preparations: autaptic cultures in which a single isolated cell innervates itself, and dissociated mass cultures in which individual cells are innervated by neighboring cells. In autaptic cultures, the total extent of evoked release, size of readily releasable pool of synaptic vesicles, and release probability were unchanged in syt-I KO neurons. In contrast, in cultures containing multiple interconnected neurons, total evoked release, the number of docked vesicles, and release probability, were significantly reduced in syt-I KO neurons. Using a micronetwork system in which we varied the number of cells on an island, we found that the frequency of spontaneous synaptic vesicle fusion events (minis) was unchanged in syt-I KO neurons when two or fewer cells were present on an island. However, in micronetworks composed of three or more neurons, mini frequency was increased threefold to fivefold in syt-I KO neurons compared with wild type. Moreover, interneuronal synapses exhibited higher rates of spontaneous release than autaptic synapses. This higher rate was attributable to an increase in release probability because excitatory hippocampal neurons in micronetworks formed a set number of synapses per cell regardless of the number of connected neurons. Thus, aspects of synaptic transmission differ between autaptic and dissociated cultures, and the synaptic transmission phenotype, resulting from loss of syt-I, is dictated by the connectivity of neurons.

Live imaging of synaptic vesicle release and retrieval in dopaminergic neurons

Dopaminergic (DA) neurons represent <0.01% of neurons in the human brain, but are essential for normal neurological and psychiatric function. The majority of these neurons reside in the ventral midbrain, but they exert their profound influences on brain function through projections to both the cortex and the basal ganglia. These projections secrete dopamine from small clear synaptic vesicles (SVs) in axonal varicosities. DA signaling has unique spatial and temporal characteristics as compared to the fast, focal synaptic transmission of excitatory and inhibitory neurons. However, as with fast-acting neurotransmitters, DA SVs must be locally recycled for use following exocytosis. Little is known about these DA SV recycling properties and how they might impact efficacy of DA neurotransmission. Here we used the pH-sensitive fluorescent probe synaptopHluorin to investigate SV recycling in DA neurons and compared their properties to prototypical fast neurotransmitter synapses of the hippocampus. These measurements showed that DA SVs, like hippocampal SVs, have a resting pH of ∼5.6. However, compared to hippocampal neurons, DA neurons show limited depletion of the recycling pool of vesicles as the stimulus frequency is increased from 5 to 30 Hz. Additional measurements show that exocytosis rates at this frequency are comparable between hippocampal and DA neurons. Thus, limited vesicle depletion likely arises from a stimulus frequency-dependent acceleration of DA SV endocytosis or re-acidification. Our observations imply differential regulation of endocytic–exocytic balance in DA neurons. Finally, our assay can also be used to investigate the effects of genetic and chemical modulation of the SV cycle.

The Rho-linked mental retardation protein OPHN1 controls synaptic vesicle endocytosis via endophilin A1

Neurons transmit information at chemical synapses by releasing neurotransmitters that are stored in synaptic vesicles (SVs) at the presynaptic site. After release, these vesicles need to be efficiently retrieved in order to maintain synaptic transmission. In concurrence, malfunctions in SV recycling have been associated with cognitive disorders. Oligophrenin-1 (OPHN1) encodes a Rho-GTPase-activating protein (Rho-GAP) whose loss of function causes X-linked mental retardation. OPHN1 is highly expressed in the brain and present both pre- and postsynaptically in neurons. Previous studies report that postsynaptic OPHN1 is important for dendritic spine morphogenesis, but its function at the presynaptic site remains largely unexplored. Here, we present evidence that reduced or defective OPHN1 signaling impairs SV cycling at hippocampal synapses. In particular, we show that OPHN1 knockdown affects the kinetic efficiency of endocytosis. We further demonstrate that OPHN1 forms a complex with endophilin A1, a protein implicated in membrane curvature generation during SV endocytosis and, importantly, that OPHN1's interaction with endophilin A1 and its Rho-GAP activity are important for its function in SV endocytosis. Our findings suggest that defects in efficient SV retrieval may contribute to the pathogenesis of OPHN1-linked cognitive impairment.

Synaptotagmin IV: a multifunctional regulator of peptidergic nerve terminals

Many members of the synaptotagmin (Syt) protein family bind Ca(2+) and trigger exocytosis, but some Syt proteins appear to have no Ca(2+)-dependent actions and their biological functions remain obscure. Syt IV is an activity-induced brain protein with no known Ca(2+)-dependent interactions and its subcellular localization and biological functions have sparked considerable controversy. We found Syt IV on both micro- and dense-core vesicles in posterior pituitary nerve terminals in mice. In terminals from Syt IV knockout mice compared with those from wild types, low Ca(2+) entry triggered more exocytosis, high Ca(2+) entry triggered less exocytosis and endocytosis was accelerated. In Syt IV knockouts, dense-core and microvesicle fusion was enhanced in cell-attached patches and dense-core vesicle fusion pores had conductances that were half as large as those in wild types. Given the neuroendocrine functions of the posterior pituitary, changes in Syt IV levels could be involved in endocrine transitions involving alterations in the release of the neuropeptides oxytocin and vasopressin.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

Antisense in vivo knockdown of synaptotagmin I by HVJ-liposome mediated gene transfer attenuates ischemic brain damage in neonatal rats

Synaptic release of the excitatory amino acid glutamate is considered as an important mechanism in the pathogenesis of ischemic brain damage in neonates. Synaptotagmin I is one of exocytosis-related proteins at nerve terminals and considered to accelerate the exocytosis of synaptic vesicles by promoting fusion between the vesicles and plasma membrane. To test the possibility that antisense in vivo knockdown of synaptotagmin I modulates the exocytotic release of glutamate, thus suppressing the excitotoxic intracellular processes leading to neuronal death following ischemia in the neonatal brain, we injected antisense oligodeoxynucleotides (ODNs) targeting synaptotagmin I (0.3 (AS), 0.15 (0.5 AS), or 0.03 microg (0.1 AS), or vehicle) into the lateral ventricles of 7-day-old rats by using a hemagglutinating virus of Japan (HVJ)-liposome mediated gene transfer technique. At 10 days of age, these rats were subjected to an electrical coagulation of the right external and internal carotid arteries, then the insertion of a solid nylon thread into the right common carotid artery toward the ascending aorta up to 10-12 mm from the upper edge of the sternocleidomastoid muscle. Cerebral ischemia was induced by clamping the left external and internal carotid arteries with a clip, and ended by removing the clip 2h later. Twenty-four hours after the end of ischemia, the extent of ischemic brain damage was neuropathologically and quantitatively evaluated in the neocortex and striatum. While the relative volume of damage in the cerebral cortex and striatum of the vehicle group was extended to 40% and 13.7%, respectively, that in the AS group was significantly reduced to 4.8% and 0.6%. In the 0.5 AS group, the relative volume of ischemic damage in the cerebral cortex and striatum was reduced to 20.5% and 15.4%, respectively, and the difference between the 0.5 AS group and vehicle group was statistically significant in the neocortex, but not in the striatum. These results indicated that antisense in vivo knockdown of synaptotagmin I successfully attenuated ischemic brain damage in neonatal rats and that the effect was dose-dependent. It was also suggested that this treatment was more effective in the neocortex than in the striatum in neonatal rats.

Stoned B mediates sorting of integral synaptic vesicle proteins

A continuous supply of fusion-competent synaptic vesicles is essential for sustainable neurotransmission. Drosophila mutations of the dicistronic stoned locus disrupt normal vesicle cycling and cause functional deficits in synaptic transmission. Although both Stoned A and B proteins putatively participate in reconstituting synaptic vesicles, their precise function is still unclear. Here we investigate the effects of progressive depletion of Stoned B protein (STNB) on the release properties of neuromuscular synapses using a novel set of synthetic stnB hypomorphic alleles. Decreasing neuronal STNB expression to < or =35% of wild-type level causes a strong reduction in excitatory junctional current amplitude at low stimulation frequencies and a marked slowing in synaptic depression during high-frequency stimulation, suggesting vesicle depletion is attenuated by decreased release probability. Recovery from synaptic depression after prolonged stimulation is also decelerated in mutants, indicating a delayed recovery of fusion-ready vesicles. These phenotypes appear not to be due to a diminished vesicle population, since the docked vesicle pool is ultrastructurally unaffected, and the total number of vesicles is only slightly reduced in these hypomorphs, unlike lethal stoned mutants. Therefore, we conclude that STNB not only functions as an essential component of the endocytic complex for vesicle reconstitution, as previously proposed, but also regulates the competence of recycled vesicles to undergo fusion. In support of such role of STNB, synaptic levels of the vesicular glutamate transporter (vGLUT) and synaptotagmin-1 are strongly reduced with diminishing STNB function, while other synaptic proteins are largely unaffected. We conclude that STNB organizes the endocytic sorting of a subset of integral synaptic vesicle proteins thereby regulating the fusion-competence of the recycled vesicle.

Molecular basis of synaptic vesicle cargo recognition by the endocytic sorting adaptor stonin 2

Synaptic transmission depends on clathrin-mediated recycling of synaptic vesicles (SVs). How select SV proteins are targeted for internalization has remained elusive. Stonins are evolutionarily conserved adaptors dedicated to endocytic sorting of the SV protein synaptotagmin. Our data identify the molecular determinants for recognition of synaptotagmin by stonin 2 or its Caenorhabditis elegans orthologue UNC-41B. The interaction involves the direct association of clusters of basic residues on the surface of the cytoplasmic domain of synaptotagmin 1 and a β strand within the μ–homology domain of stonin 2. Mutation of K783, Y784, and E785 to alanine within this stonin 2 β strand results in failure of the mutant stonin protein to associate with synaptotagmin, to accumulate at synapses, and to facilitate synaptotagmin internalization. Synaptotagmin-binding–defective UNC-41B is unable to rescue paralysis in C. elegans stonin mutant animals, suggesting that the mechanism of stonin-mediated SV cargo recognition is conserved from worms to mammals.

Pharmacology of neurotransmitter release: measuring exocytosis

Neurotransmission in the nervous system is initiated at presynaptic terminals by fusion of synaptic vesicles with the plasma membrane and subsequent exocytic release of chemical transmitters. Currently, there are multiple methods to detect neurotransmitter release from nerve terminals, each with their own particular advantages and disadvantages. For instance, most commonly employed methods monitor actions of released chemical substances on postsynaptic receptors or artificial substrates such as carbon fibers. These methods are closest to the physiological setting because they have a rapid time resolution and they measure the action of the endogenous neurotransmitters rather than the signals emitted by exogenous probes. However, postsynaptic receptors only indirectly report neurotransmitter release in a form modified by the properties of receptors themselves, which are often nonlinear detectors of released substances. Alternatively, released chemical substances can be detected biochemically, albeit on a time scale slower than electrophysiological methods. In addition, in certain preparations, where presynaptic terminals are accessible to whole cell recording electrodes, fusion of vesicles with the plasma membrane can be monitored using capacitance measurements. In the last decade, in addition to electrophysiological and biochemical methods, several fluorescence imaging modalities have been introduced which report synaptic vesicle fusion, endocytosis, and recycling. These methods either take advantage of styryl dyes that can be loaded into recycling vesicles or exogenous expression of synaptic vesicle proteins tagged with a pH-sensitive GFP variant at regions facing the vesicle lumen. In this chapter, we will provide an overview of these methods with particular emphasis on their relative strengths and weaknesses and discuss the types of information one can obtain from them.

Mechanism of botulinum neurotoxin B and G entry into hippocampal neurons

Botulinum neurotoxins (BoNTs) target presynaptic nerve terminals by recognizing specific neuronal surface receptors. Two homologous synaptic vesicle membrane proteins, synaptotagmins (Syts) I and II, bind toxins BoNT/B and G. However, a direct demonstration that Syts I/II mediate toxin binding and entry into neurons is lacking. We report that BoNT/B and G fail to bind and enter hippocampal neurons cultured from Syt I knockout mice. Wild-type Syts I and II (but not Syt I loss-of-function toxin-binding domain mutants) restored binding and entry of BoNT/B and G in Syt I–null neurons, thus demonstrating that Syts I/II are protein receptors for BoNT/B and G. Furthermore, mice lacking complex gangliosides exhibit reduced sensitivity to BoNT/G, and binding and entry of BoNT/A, B, and G into hippocampal neurons lacking gangliosides is diminished. These data suggest that gangliosides are the shared coreceptor for BoNT/A, B, and G, supporting a double-receptor model for these three BoNTs for which the protein receptors are known.

Synaptotagmin VII splice variants alpha, beta, and delta are expressed in pancreatic beta-cells and regulate insulin exocytosis

Synaptotagmins (SYT) are calcium-binding proteins that participate in regulated exocytosis. Although SYTI to IX isoforms are expressed in insulin-producing cell lines, hitherto only SYTIX has been associated with native beta-cell insulin granules and implicated in exocytosis. SYTVII was also proposed to regulate insulin exocytosis, but its subcellular location and number of alternative splice variants produced remain controversial. Only transcripts of SYTVII alpha, beta, and a novel splice variant delta are expressed in beta-cells and INS-1E cells. Western blotting revealed that INS-1E cells predominantly produced SYTVII alpha and low levels of SYTVII beta, whereas SYTVII delta was undetectable. The protein colocalized with insulin granules but not with synaptic-like microvesicles. Overexpression of SYTVII alpha resulted in decreased insulin granule content with a concomitant translocation of the variant to the plasma membrane, while SYTVII beta retained largely a granular pattern. Overexpressed SYTVII delta exhibited a distribution different to that of insulin granules and inhibited exocytosis when assessed by whole cell patch clamp capacitance recording. Silencing of SYTVII alpha by targeted RNA interference suppressed secretion, while repression of beta slightly increased release. Our results demonstrate that SYTVII is expressed on insulin granules and that only SYTVII alpha is implicated in exocytosis under physiological conditions.

Presynaptic G-protein-coupled receptors regulate synaptic cleft glutamate via transient vesicle fusion

When synaptic vesicles fuse with the plasma membrane, they may completely collapse or fuse transiently. Transiently fusing vesicles remain structurally intact and therefore have been proposed to represent a form of rapid vesicle recycling. However, the impact of a transient synaptic vesicle fusion event on neurotransmitter release, and therefore on synaptic transmission, has yet to be determined. Recently, the molecular mechanism by which a serotonergic presynaptic G-protein-coupled receptor (GPCR) regulates synaptic vesicle fusion and inhibits synaptic transmission was identified. By making paired electrophysiological recordings in the presence and absence of low-affinity antagonists, we now demonstrate that activation of this presynaptic GPCR lowers the peak synaptic cleft glutamate concentration independently of the probability of vesicle fusion. Furthermore, this change in cleft glutamate concentration differentially inhibits synaptic NMDA and AMPA receptor-mediated currents. We conclude that a presynaptic GPCR regulates the profile of glutamate in the synaptic cleft through altering the mechanism of vesicle fusion leading to qualitative as well as quantitative changes in neural signaling.

Clathrin-mediated endocytosis at synapses

Neurons are communication specialists that convert electrical into chemical signals at specialized cell-cell junctions termed synapses. Arrival of an action potential triggers calcium-regulated exocytosis of neurotransmitter (NT) from small synaptic vesicles (SVs), which then diffuses across the synaptic cleft and binds to postsynaptic receptors to elicit specific changes within the postsynaptic cell. Endocytosis of pre- and postsynaptic membrane proteins including SV components and postsynaptic NT receptors is essential for the proper functioning of the synapse. During the past several years, we have witnessed enormous progress in our understanding of the mechanics of clathrin-mediated endocytosis (CME) and its role in regulating exo-endocytic vesicle cycling at synapses. Here we summarize the molecular machinery used for recognition of synaptic membrane protein cargo and its clathrin-dependent internalization, and describe the inventory of tools that can be used to monitor vesicle cycling at synapses or to inhibit CME in a stage-specific manner.

Synaptotagmin I binds intestinal epithelial NHE3 and mediates cAMP- and Ca2+-induced endocytosis by recruitment of AP2 and clathrin

Apical membrane sodium hydrogen exchanger 3 (NHE3), a major pathway for non-nutrient-dependent intestinal Na(+) absorption, is tightly regulated by second messenger systems that affect its functional activity and membrane trafficking. However, the events and components involved in NHE3 regulation are only partially understood. We report that the adaptor protein synaptotagmin I (Syt I) plays a pivotal role in cAMP- and Ca(2+)-induced cargo recognition of NHE3 and initiation of its endocytosis. Both mouse small intestine (jejunum) and Caco-2BBe Syt I coimmunoprecipitated with NHE3, particularly following increases in cellular cAMP or Ca(2+). Following short interfering RNA (siRNA) suppression of Syt I expression, cAMP- and Ca(2+)-induced inhibition of NHE3 activity were still observed but NHE3 endocytosis was blocked, as assessed by (22)Na influx and apical membrane biotin labeling, respectively. Similar effects on NHE3 inhibition and endocytosis were observed by siRNA suppression of either the mu-subunit of the adaptor protein 2 (AP2) complex or the heavy chain of clathrin. Coimmunoprecipitation analyses of NHE3 with these adaptor proteins revealed that cAMP- and Ca(2+)-induced NHE3-Syt I interaction preceded and was required for recruitment of AP2 and the clathrin complex. Confocal microscopy confirmed both the time sequence and protein associations of these events. We conclude that Syt I plays a pivotal role in mediating cAMP- and Ca(2+)-induced endocytosis of NHE3 (but not in inhibition of activity) through cargo recognition of NHE3 and subsequent recruitment of AP2-clathrin assembly required for membrane endocytosis.

Distinct roles of the C2A and the C2B domain of the vesicular Ca2+ sensor synaptotagmin 9 in endocrine beta-cells

Synaptotagmins form a family of calcium-sensor proteins implicated in exocytosis, and these vesicular transmembrane proteins are endowed with two cytosolic calcium-binding C2 domains, C2A and C2B. Whereas the isoforms syt1 and syt2 have been studied in detail, less is known about syt9, the calcium sensor involved in endocrine secretion such as insulin release from large dense core vesicles in pancreatic beta-cells. Using cell-based assays to closely mimic physiological conditions, we observed SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor)-independent translocation of syt9C2AB to the plasma membrane at calcium levels corresponding to endocrine exocytosis, followed by internalization to endosomes. The use of point mutants and truncations revealed that initial translocation required only the C2A domain, whereas the C2B domain ensured partial pre-binding of syt9C2AB to the membrane and post-stimulatory localization to endosomes. In contrast with the known properties of neuronal and neuroendocrine syt1 or syt2, the C2B domain of syt9 did not undergo calcium-dependent membrane binding despite a high degree of structural homology as observed through molecular modelling. The present study demonstrates distinct intracellular properties of syt9 with different roles for each C2 domain in endocrine cells.

Synaptotagmins I and IX function redundantly in regulated exocytosis but not endocytosis in PC12 cells

Synaptotagmin I is considered to be a Ca2+ sensor for fast vesicle exocytosis. Because Ca2+-dependent vesicle exocytosis persists in synaptotagmin I mutants, there must be additional Ca2+ sensors. Multiple synaptotagmin isoforms co-reside on vesicles, which suggests that other isoforms complement synaptotagmin I function. We found that full downregulation of synaptotagmins I and IX, which co-reside on vesicles in PC12 cells, completely abolished Ca2+-dependent vesicle exocytosis. By contrast, Ca2+-dependent exocytosis persisted in cells expressing only synaptotagmin I or only synaptotagmin IX, which indicated a redundancy in function for these isoforms. Although either isoform was sufficient to confer Ca2+ regulation on vesicle exocytosis, synaptotagmins I and IX conferred faster and slower release rates, respectively, indicating that individual isoforms impart distinct kinetic properties to vesicle exocytosis. The downregulation of synaptotagmin I but not synaptotagmin IX impaired compensatory vesicle endocytosis, which revealed a lack of isoform redundancy and functional specialization of synaptotagmin I for endocytic retrieval.

'Fractional recovery' analysis of a presynaptic synaptotagmin 1-anchored endocytic protein complex

Background

The integral synaptic vesicle protein and putative calcium sensor, synaptotagmin 1 (STG), has also been implicated in synaptic vesicle (SV) recovery. However, proteins with which STG interacts during SV endocytosis remain poorly understood. We have isolated an STG-associated endocytic complex (SAE) from presynaptic nerve terminals and have used a novel fractional recovery (FR) assay based on electrostatic dissociation to identify SAE components and map the complex structure. The location of SAE in the presynaptic terminal was determined by high-resolution quantitative immunocytochemistry at the chick ciliary ganglion giant calyx-type synapse.

Methodology/Principle Findings

The first step in FR analysis was to immunoprecipitate (IP) the complex with an antibody against one protein component (the IP-protein). The immobilized complex was then exposed to a high salt (1150 mM) stress-test that caused shedding of co-immunoprecipitated proteins (co-IP-proteins). A Fractional Recovery ratio (FR: recovery after high salt/recovery with control salt as assayed by Western blot) was calculated for each co-IP-protein. These FR values reflect complex structure since an easily dissociated protein, with a low FR value, cannot be intermediary between the IP-protein and a salt-resistant protein. The structure of the complex was mapped and a blueprint generated with a pair of FR analyses generated using two different IP-proteins. The blueprint of SAE contains an AP180/X/STG/stonin 2/intersectin/epsin core (X is unknown and epsin is hypothesized), and an AP2 adaptor, H-/L-clathrin coat and dynamin scission protein perimeter. Quantitative immunocytochemistry (ICA/ICQ method) at an isolated calyx-type presynaptic terminal indicates that this complex is associated with STG at the presynaptic transmitter release face but not with STG on intracellular synaptic vesicles.

Conclusions/Significance

We hypothesize that the SAE serves as a recognition site and also as a seed complex for clathrin-mediated synaptic vesicle recovery. The combination of FR analysis with quantitative immunocytochemistry provides a novel and effective strategy for the identification and characterization of biologically-relevant multi-molecular complexes.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

C2B polylysine motif of synaptotagmin facilitates a Ca2+-independent stage of synaptic vesicle priming in vivo

Synaptotagmin I, a synaptic vesicle protein required for efficient synaptic transmission, contains a highly conserved polylysine motif necessary for function. Using Drosophila, we examined in which step of the synaptic vesicle cycle this motif functions. Polylysine motif mutants exhibited an apparent decreased Ca2+ affinity of release, and, at low Ca2+, an increased failure rate, increased facilitation, and increased augmentation, indicative of a decreased release probability. Disruption of Ca2+ binding, however, cannot account for all of the deficits in the mutants; rather, the decreased release probability is probably due to a disruption in the coupling of synaptotagmin to the release machinery. Mutants exhibited a major slowing of recovery from synaptic depression, which suggests that membrane trafficking before fusion is disrupted. The disrupted process is not endocytosis because the rate of FM 1-43 uptake was unchanged in the mutants, and the polylysine motif mutant synaptotagmin was able to rescue the synaptic vesicle depletion normally found in syt(null) mutants. Thus, the polylysine motif functions after endocytosis and before fusion. Finally, mutation of the polylysine motif inhibits the Ca2+-independent ability of synaptotagmin to accelerate SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-mediated fusion. Together, our results demonstrate that the polylysine motif is required for efficient Ca2+-independent docking and/or priming of synaptic vesicles in vivo.

Distinct endocytic pathways control the rate and extent of synaptic vesicle protein recycling

Synaptic vesicles have been proposed to form through two mechanisms: one directly from the plasma membrane involving clathrin-dependent endocytosis and the adaptor protein AP2, and the other from an endosomal intermediate mediated by the adaptor AP3. However, the relative role of these two mechanisms in synaptic vesicle recycling has remained unclear. We now find that vesicular glutamate transporter VGLUT1 interacts directly with endophilin, a component of the clathrin-dependent endocytic machinery. In the absence of its interaction with endophilin, VGLUT1 recycles more slowly during prolonged, high-frequency stimulation. Inhibition of the AP3 pathway with brefeldin A rescues the rate of recycling, suggesting a competition between AP2 and -3 pathways, with endophilin recruiting VGLUT1 toward the faster AP2 pathway. After stimulation, however, inhibition of the AP3 pathway prevents the full recovery of VGLUT1 by endocytosis, implicating the AP3 pathway specifically in compensatory endocytosis.

Silence of synaptotagmin I in INS-1 cells inhibits fast exocytosis and fast endocytosis

Synaptotagmin I (Syt I) is a Ca(2+) sensor for triggering fast synchronized release of neurotransmitters. However, controversy remains whether Syt I is also obligatory for the exocytosis and endocytosis of larger dense core vesicles (LDCVs) in endocrine cells. In this study, we used a short hairpin RNA (shRNA) to silence the expression of Syt I and investigated the roles of Syt I on exocytosis and endocytosis in INS-1 cells. Our results demonstrated that expression of Syt I is remarkably reduced by the Syt I gene targeting shRNA. Using high-time resolution capacitance measurement, we found that the silence of Syt I decreased the calcium sensitivity of fusion of insulin granules and therefore reduced the exocytotic burst triggered by step-like [Ca(2+)](i) elevation. In addition, the occurrence frequency and amplitude of fast endocytosis were remarkably reduced in the silenced cells. We conclude that Syt I not only participates in the Ca(2+)-sensing of LDCV fusion with plasmalemma, but also plays a crucial role in fast endocytosis in INS-1 cells.

Altered synaptic development and active zone spacing in endocytosis mutants

Many types of synapses have highly characteristic shapes and tightly regulated distributions of active zones, parameters that are important to the function of neuronal circuits. The development of terminal arborizations must therefore include mechanisms to regulate the spacing of terminals, the frequency of branching, and the distribution and density of release sites. At present, however, the mechanisms that control these features remain obscure. Here, we report the development of supernumerary or "satellite" boutons in a variety of endocytic mutants at the Drosophila neuromuscular junction. Mutants in endophilin, synaptojanin, dynamin, AP180, and synaptotagmin all show increases in supernumerary bouton structures. These satellite boutons contain releasable vesicles and normal complements of synaptic proteins that are correctly localized within terminals. Interestingly, however, synaptojanin terminals have more active zones per unit of surface area and more dense bodies (T-bars) within these active zones, which may in part compensate for reduced transmission per active zone. The altered structural development of the synapse is selectively encountered in endocytosis mutants and is not observed when synaptic transmission is reduced by mutations in glutamate receptors or when synaptic transmission is blocked by tetanus toxin. We propose that endocytosis plays a critical role in sculpting the structure of synapses, perhaps through the endocytosis of unknown regulatory signals that organize morphogenesis at synaptic terminals.

A pre-synaptic to-do list for coupling exocytosis to endocytosis

Synaptic vesicles are made locally in the nerve terminal during recycling of membrane. Synaptic vesicle proteins must be sorted and concentrated on the plasma membrane, packaged into a budding vesicle of precise size and cut away from the synaptic surface. Adaptors, scaffolds, BAR-domain and ENTH-domain proteins all must be coordinated to carry out this sequence of events prior to the action of dynamin. Details of how this is orchestrated at nerve terminals are just beginning to emerge.

Discrete residues in the c(2)b domain of synaptotagmin I independently specify endocytic rate and synaptic vesicle size

It has been demonstrated that synapses lacking functional synaptotagmin I (Syt I) have a decreased rate of synaptic vesicle endocytosis. Beyond this, the function of Syt I during endocytosis remains undefined. Here, we demonstrate that a decreased rate of endocytosis in syt(null) mutants correlates with a stimulus-dependent perturbation of membrane internalization, assayed ultrastructurally. We then separate the mechanisms that control endocytic rate and vesicle size by mapping these processes to discrete residues in the Syt I C(2)B domain. Mutation of a poly-lysine motif alters vesicle size but not endocytic rate, whereas the mutation of calcium-coordinating aspartate residues (syt-D3,4N) alters endocytic rate but not vesicle size. Finally, slowed endocytic rate in the syt-D3,4N animals, but not syt(null) animals, can be rescued by elevating extracellular calcium concentration, supporting the conclusion that calcium coordination within the C(2)B domain contributes to the control of endocytic rate.

Kissing and pinching: synaptotagmin and calcium do more between bilayers

Building on recent findings that synaptotagmin (Syt) participates in synaptic vesicle endocytosis, Poskanzer et al., in this issue of Neuron, show distinct mechanisms by which Syt functions in this process. Most significantly, they show (1) that calcium binding to Syt determines the rate but not fidelity of vesicle recycling and (2) that mutations in a different Syt domain affect the shape but not rate of formation of recycled synaptic vesicles.

Nigral injection of antisense oligonucleotides to synaptotagmin I using HVJ-liposome vectors causes disruption of dopamine release in the striatum and impaired skill learning

To produce an animal model of a dopa-responsive motor disorder with depletion of dopamine (DA) release in the striatum by dysfunction of the transmitter release machinery of the nigrostriatal DA system, we performed an intra-nigral injection of an HVJ-liposome gene transfer vector containing antisense oligodeoxynucleotides (ODNs) against synaptotagmin I (SytI), a key regulator of Ca(2+)-dependent exocytosis and endocytosis in adult rats. A unilateral intra-nigral injection of HVJ-liposome vectors containing antisense ODNs against SytI (syt-AS) caused a moderate disruption of methamphetamine-induced release of DA in the treated side of the striatum, while the syt-AS treatment did not affect physiological release of DA in the treated striatum. A bilateral intra-nigral injection of HVJ-liposome vectors containing syt-AS induced an impairment of the striatal DA-mediated acquisition of skilled behavior in a rotarod task without any deficits in general motor functions, such as spontaneous locomotor activity, motor adjusting steps, equilibrium function, or muscle strength. These findings suggest that an intra-nigral treatment with HVJ-liposome vectors containing syt-AS may cause a long-lasting nigral knockdown of SytI which, in turn, leads to a moderate dysfunction of the DA release machinery in the terminals of the nigrostriatal DA system and a subsequent mild depletion of DA release in the striatum.

The synaptic vesicle: cycle of exocytosis and endocytosis

Synaptic vesicles are clustered at the presynaptic terminal where they fuse and recycle in response to stimulation. Vesicles appear to be sorted into pools, but we do not yet understand how physiologically defined pools relate to morphological pools. The advent of dynamic imaging approaches has led to an appreciation of the regulation of vesicle mobility. Newly endocytosed vesicles are highly mobile but appear to become transiently trapped as they re-enter the recycling pool. Recent experiments indicate that endocytosis might have a constant rate, but limited capacity. How endocytosis is linked to exocytosis remains unclear, although calcium emerges as an important player.

Drosophila synaptotagmin Inull mutants show severe alterations in vesicle populations but calcium-binding motif mutants do not

Synaptotagmin I is a synaptic vesicle protein postulated to mediate vesicle docking, vesicle recycling, and the Ca(2+) sensing required to trigger vesicle fusion. Analysis of synaptotagmin I knockouts (sytI(NULL) mutants) in both Drosophila and mice led to these hypotheses. Although much research on the mechanisms of synaptic transmission in Drosophila is performed at the third instar neuromuscular junction, the ultrastructure of this synapse has never been analyzed in sytI(NULL) mutants. Here we report severe synaptic vesicle depletion, an accumulation of large vesicles, and decreased vesicle docking at sytI(NULL) third instar neuromuscular junctions. Mutations in synaptotagmin I's C(2)B Ca(2+)-binding motif nearly abolish synaptic transmission and decrease the apparent Ca(2+) affinity of neurotransmitter release. Although this result is consistent with disruption of the Ca(2+) sensor, synaptic vesicle depletion and/or redistribution away from the site of Ca(2+) influx could produce a similar phenotype. To address this question, we examined vesicle distributions at neuromuscular junctions from third instar C(2)B Ca(2+)-binding motif mutants and transgenic wild-type controls. The number of docked vesicles and the overall number of synaptic vesicles in the vicinity of active zones was unchanged in the mutants. We conclude that the near elimination of synaptic transmission and the decrease in the Ca(2+) affinity of release observed in C(2)B Ca(2+)-binding motif mutants is not due to altered synaptic vesicle distribution but rather is a direct result of disrupting synaptotagmin I's ability to bind Ca(2+). Thus, Ca(2+) binding by the C(2)B domain mediates a post-docking step in fusion.

Synaptotagmin IV is necessary for the maturation of secretory granules in PC12 cells

In neuroendocrine PC12 cells, immature secretory granules (ISGs) mature through homotypic fusion and membrane remodeling. We present evidence that the ISG-localized synaptotagmin IV (Syt IV) is involved in ISG maturation. Using an in vitro homotypic fusion assay, we show that the cytoplasmic domain (CD) of Syt IV, but not of Syt I, VII, or IX, inhibits ISG homotypic fusion. Moreover, Syt IV CD binds specifically to ISGs and not to mature secretory granules (MSGs), and Syt IV binds to syntaxin 6, a SNARE protein that is involved in ISG maturation. ISG homotypic fusion was inhibited in vivo by small interfering RNA–mediated depletion of Syt IV. Furthermore, the Syt IV CD, as well as Syt IV depletion, reduces secretogranin II (SgII) processing by prohormone convertase 2 (PC2). PC2 is found mostly in the proform, suggesting that activation of PC2 is also inhibited. Granule formation, and the sorting of SgII and PC2 from the trans-Golgi network into ISGs and MSGs, however, is not affected. We conclude that Syt IV is an essential component for secretory granule maturation.

Synaptic vesicle reuse and its implications

Presynaptic nerve terminals are exquisite vesicle trafficking machines. Neurotransmission is sustained by constant recycling of a handful of vesicles. Therefore, the rate and the pathway of vesicle trafficking can critically determine synaptic efficacy during activity. However, it is yet unclear whether synaptic vesicle recycling becomes rate limiting on a rapid time scale during physiologically relevant forms of activity in the brain. Several forms of synaptic plasticity arise from persistent alterations in the dynamics of vesicle trafficking in presynaptic terminals. What makes presynaptic forms of plasticity particularly interesting is that they not only increase or decrease the amplitude of synaptic responses but also cause frequency-dependent changes in neurotransmission. In this manner, plasticity can alter the information coding in neural circuits beyond simple scaling of synaptic responses. However, studying the synaptic vesicle cycle beyond exocytosis and endocytosis has been difficult. In the past decade, several methods have been developed to infer vesicles' trajectory during their cycle in the synapse. Nevertheless, several questions remain. A better understanding of the role of synaptic vesicle trafficking in neurotransmission will require novel approaches that either combine existing methods or the development of new methods to trace vesicles during their cycle. Recent evidence suggests that various presynaptic proteins involved in the synaptic function and homeostasis are either mutated or altered in their expression in several neurological and psychiatric disorders. Therefore, elucidation of the mechanisms that underlie the synaptic vesicle cycle may reveal novel therapeutic targets for brain disorders.