The stoned proteins regulate synaptic vesicle recycling in the presynaptic terminal

Dominant and recessive congenital myasthenic syndromes caused by SYT2 mutations

INTRODUCTION/AIMS

We studied a patient with a congenital myasthenic syndrome (CMS) caused by a dominant mutation in the synaptotagmin 2 gene (SYT2) and compared the clinical features of this patient with those of a previously described patient with a recessive mutation in the same gene.

METHODS

We performed electrodiagnostic (EDX) studies, genetic studies, muscle biopsy, microelectrode recordings and electron microscopy (EM).

RESULTS

Both patients presented with muscle weakness and bulbar deficits, which were worse in the recessive form. EDX studies showed presynaptic failure, which was more prominent in the recessive form. Microelectrode studies in the dominant form showed a marked reduction of the quantal content, which increased linearly with higher frequencies of nerve stimulation. The MEPP frequencies were normal at rest but increased markedly with higher frequencies of nerve stimulation. The EM demonstrated overdeveloped postsynaptic folding, and abundant endosomes, multivesicular bodies and degenerative lamellar bodies inside small nerve terminals.

DISCUSSION

The recessive form of CMS caused by a SYT2 mutation showed far more severe clinical manifestations than the dominant form. The pathogenesis of the dominant form likely involves a dominant-negative effect due to disruption of the dual function of synaptotagmin as a Ca²⁺ -sensor and modulator of synaptic vesicle exocytosis.

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

The synaptic cleft manifests enriched glycosylation, with structured glycans coordinating signaling between presynaptic and postsynaptic cells. Glycosylated signaling ligands orchestrating communication are tightly regulated by secreted glycan-binding lectins. Using the Drosophila neuromuscular junction (NMJ) as a model glutamatergic synapse, we identify a new Ca2+-binding (C-type) lectin, Lectin-galC1 (LGC1), which modulates presynaptic function and neurotransmission strength. We find that LGC1 is enriched in motoneuron presynaptic boutons and secreted into the NMJ extracellular synaptomatrix. We show that LGC1 limits locomotor peristalsis and coordinated movement speed, with a specific requirement for synaptic function, but not NMJ architecture. LGC1 controls neurotransmission strength by limiting presynaptic active zone (AZ) and postsynaptic glutamate receptor (GluR) aligned synapse number, reducing both spontaneous and stimulation-evoked synaptic vesicle (SV) release, and capping SV cycling rate. During high-frequency stimulation (HFS), mutants have faster synaptic depression and impaired recovery while replenishing depleted SV pools. Although LGC1 removal increases the number of glutamatergic synapses, we find that LGC1-null mutants exhibit decreased SV density within presynaptic boutons, particularly SV pools at presynaptic active zones. Thus, LGC1 regulates NMJ neurotransmission to modulate coordinated movement.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

APache Is an AP2-Interacting Protein Involved in Synaptic Vesicle Trafficking and Neuronal Development

Synaptic transmission is critically dependent on synaptic vesicle (SV) recycling. Although the precise mechanisms of SV retrieval are still debated, it is widely accepted that a fundamental role is played by clathrin-mediated endocytosis, a form of endocytosis that capitalizes on the clathrin/adaptor protein complex 2 (AP2) coat and several accessory factors. Here, we show that the previously uncharacterized protein KIAA1107, predicted by bioinformatics analysis to be involved in the SV cycle, is an AP2-interacting clathrin-endocytosis protein (APache). We found that APache is highly enriched in the CNS and is associated with clathrin-coated vesicles via interaction with AP2. APache-silenced neurons exhibit a severe impairment of maturation at early developmental stages, reduced SV density, enlarged endosome-like structures, and defects in synaptic transmission, consistent with an impaired clathrin/AP2-mediated SV recycling. Our data implicate APache as an actor in the complex regulation of SV trafficking, neuronal development, and synaptic plasticity.

Synaptic Vesicle Endocytosis in Different Model Systems

Neurotransmission in complex animals depends on a choir of functionally distinct synapses releasing neurotransmitters in a highly coordinated manner. During synaptic signaling, vesicles fuse with the plasma membrane to release their contents. The rate of vesicle fusion is high and can exceed the rate at which synaptic vesicles can be re-supplied by distant sources. Thus, local compensatory endocytosis is needed to replenish the synaptic vesicle pools. Over the last four decades, various experimental methods and model systems have been used to study the cellular and molecular mechanisms underlying synaptic vesicle cycle. Clathrin-mediated endocytosis is thought to be the predominant mechanism for synaptic vesicle recycling. However, recent studies suggest significant contribution from other modes of endocytosis, including fast compensatory endocytosis, activity-dependent bulk endocytosis, ultrafast endocytosis, as well as kiss-and-run. Currently, it is not clear whether a universal model of vesicle recycling exist for all types of synapses. It is possible that each synapse type employs a particular mode of endocytosis. Alternatively, multiple modes of endocytosis operate at the same synapse, and the synapse toggles between different modes depending on its activity level. Here we compile review and research articles based on well-characterized model systems: frog neuromuscular junctions, C. elegans neuromuscular junctions, Drosophila neuromuscular junctions, lamprey reticulospinal giant axons, goldfish retinal ribbon synapses, the calyx of Held, and rodent hippocampal synapses. We will compare these systems in terms of their known modes and kinetics of synaptic vesicle endocytosis, as well as the underlying molecular machineries. We will also provide the future development of this field.

Safeguards of Neurotransmission: Endocytic Adaptors as Regulators of Synaptic Vesicle Composition and Function

Communication between neurons relies on neurotransmitters which are released from synaptic vesicles (SVs) upon Ca²⁺ stimuli. To efficiently load neurotransmitters, sense the rise in intracellular Ca²⁺ and fuse with the presynaptic membrane, SVs need to be equipped with a stringently controlled set of transmembrane proteins. In fact, changes in SV protein composition quickly compromise neurotransmission and most prominently give rise to epileptic seizures. During exocytosis SVs fully collapse into the presynaptic membrane and consequently have to be replenished to sustain neurotransmission. Therefore, surface-stranded SV proteins have to be efficiently retrieved post-fusion to be used for the generation of a new set of fully functional SVs, a process in which dedicated endocytic sorting adaptors play a crucial role. The question of how the precise reformation of SVs is achieved is intimately linked to how SV membranes are retrieved. For a long time both processes were believed to be two sides of the same coin since Clathrin-mediated endocytosis (CME), the proposed predominant SV recycling mode, will jointly retrieve SV membranes and proteins. However, with the recent proposal of Clathrin-independent SV recycling pathways SV membrane retrieval and SV reformation turn into separable events. This review highlights the progress made in unraveling the molecular mechanisms mediating the high-fidelity retrieval of SV proteins and discusses how the gathered knowledge about SV protein recycling fits in with the new notions of SV membrane endocytosis.

MAN1 Restricts BMP Signaling During Synaptic Growth in Drosophila

Bone morphogenic protein (BMP) signaling is crucial for coordinated synaptic growth and plasticity. Here, we show that the nuclear LEM-domain protein MAN1 is a negative regulator of synaptic growth at Drosophila larval and adult neuromuscular junctions (NMJs). Loss of MAN1 is associated with synaptic structural defects, including floating T-bars, membrane attachment defects, and accumulation of vesicles between perisynaptic membranes and membranes of the subsynaptic reticulum. In addition, MAN1 mutants accumulate more heterogeneously sized vesicles and multivesicular bodies in larval and adult synapses, the latter indicating that MAN1 may function in synaptic vesicle recycling and endosome-to-lysosome trafficking. Synaptic overgrowth in MAN1 is sensitive to BMP signaling levels, and loss of key BMP components attenuate BMP-induced synaptic overgrowth. Based on these observations, we propose that MAN1 negatively regulates accumulation and distribution of BMP signaling components to ensure proper synaptic growth and integrity at larval and adult NMJs.

Genetic Dissection of Nutrition-Induced Plasticity in Insulin/Insulin-Like Growth Factor Signaling and Median Life Span in a Drosophila Multiparent Population

The nutritional environments that organisms experience are inherently variable, requiring tight coordination of how resources are allocated to different functions relative to the total amount of resources available. A growing body of evidence supports the hypothesis that key endocrine pathways play a fundamental role in this coordination. In particular, the insulin/insulin-like growth factor signaling (IIS) and target of rapamycin (TOR) pathways have been implicated in nutrition-dependent changes in metabolism and nutrient allocation. However, little is known about the genetic basis of standing variation in IIS/TOR or how diet-dependent changes in expression in this pathway influence phenotypes related to resource allocation. To characterize natural genetic variation in the IIS/TOR pathway, we used >250 recombinant inbred lines (RILs) derived from a multiparental mapping population, the Drosophila Synthetic Population Resource, to map transcript-level QTL of genes encoding 52 core IIS/TOR components in three different nutritional environments [dietary restriction (DR), control (C), and high sugar (HS)]. Nearly all genes, 87%, were significantly differentially expressed between diets, though not always in ways predicted by loss-of-function mutants. We identified cis (i.e., local) expression QTL (eQTL) for six genes, all of which are significant in multiple nutrient environments. Further, we identified trans (i.e., distant) eQTL for two genes, specific to a single nutrient environment. Our results are consistent with many small changes in the IIS/TOR pathways. A discriminant function analysis for the C and DR treatments identified a pattern of gene expression associated with the diet treatment. Mapping the composite discriminant function scores revealed a significant global eQTL within the DR diet. A correlation between the discriminant function scores and the median life span (r = 0.46) provides evidence that gene expression changes in response to diet are associated with longevity in these RILs.

Puzzling Out Synaptic Vesicle 2 Family Members Functions

Synaptic vesicle proteins 2 (SV2) were discovered in the early 80s, but the clear demonstration that SV2A is the target of efficacious anti-epileptic drugs from the racetam family stimulated efforts to improve understanding of its role in the brain. Many functions have been suggested for SV2 proteins including ions or neurotransmitters transport or priming of SVs. Moreover, several recent studies highlighted the link between SV2 and different neuronal disorders such as epilepsy, Schizophrenia (SCZ), Alzheimer's or Parkinson's disease. In this review article, we will summarize our present knowledge on SV2A function(s) and its potential role(s) in the pathophysiology of various brain disorders.

Transmission, Development, and Plasticity of Synapses

Chemical synapses are sites of contact and information transfer between a neuron and its partner cell. Each synapse is a specialized junction, where the presynaptic cell assembles machinery for the release of neurotransmitter, and the postsynaptic cell assembles components to receive and integrate this signal. Synapses also exhibit plasticity, during which synaptic function and/or structure are modified in response to activity. With a robust panel of genetic, imaging, and electrophysiology approaches, and strong evolutionary conservation of molecular components, Drosophila has emerged as an essential model system for investigating the mechanisms underlying synaptic assembly, function, and plasticity. We will discuss techniques for studying synapses in Drosophila, with a focus on the larval neuromuscular junction (NMJ), a well-established model glutamatergic synapse. Vesicle fusion, which underlies synaptic release of neurotransmitters, has been well characterized at this synapse. In addition, studies of synaptic assembly and organization of active zones and postsynaptic densities have revealed pathways that coordinate those events across the synaptic cleft. We will also review modes of synaptic growth and plasticity at the fly NMJ, and discuss how pre- and postsynaptic cells communicate to regulate plasticity in response to activity.

Overlapping functions of stonin 2 and SV2 in sorting of the calcium sensor synaptotagmin 1 to synaptic vesicles

Brain function depends on neurotransmission, and alterations in this process are linked to neurological disorders. Neurotransmitter release requires the rapid recycling of synaptic vesicles (SVs) by endocytosis. How synapses maintain the molecular composition of SVs during recycling is poorly understood. We demonstrate that overlapping functions of two completely distinct proteins, the vesicle protein SV2A/B and the adaptor stonin 2, mediate endocytic sorting of the vesicular calcium sensor synaptotagmin 1. As SV2A is the target of the commonly used antiepileptic drug levetiracetam and is linked to late onset Alzheimer’s disease, our findings bear implications for the treatment of neurological and neurodegenerative disorders.

Neurotransmission involves the calcium-regulated exocytic fusion of synaptic vesicles (SVs) and the subsequent retrieval of SV membranes followed by reformation of properly sized and shaped SVs. An unresolved question is whether each SV protein is sorted by its own dedicated adaptor or whether sorting is facilitated by association between different SV proteins. We demonstrate that endocytic sorting of the calcium sensor synaptotagmin 1 (Syt1) is mediated by the overlapping activities of the Syt1-associated SV glycoprotein SV2A/B and the endocytic Syt1-adaptor stonin 2 (Stn2). Deletion or knockdown of either SV2A/B or Stn2 results in partial Syt1 loss and missorting of Syt1 to the neuronal surface, whereas deletion of both SV2A/B and Stn2 dramatically exacerbates this phenotype. Selective missorting and degradation of Syt1 in the absence of SV2A/B and Stn2 impairs the efficacy of neurotransmission at hippocampal synapses. These results indicate that endocytic sorting of Syt1 to SVs is mediated by the overlapping activities of SV2A/B and Stn2 and favor a model according to which SV protein sorting is guarded by both cargo-specific mechanisms as well as association between SV proteins.

The AP2 clathrin adaptor protein complex regulates the abundance of GLR-1 glutamate receptors in the ventral nerve cord of Caenorhabditis elegans

Regulation of glutamate receptor trafficking controls synaptic strength and plasticity. This study takes advantage of viable, null mutations in subunits of the clathrin adaptor protein 2 (AP2) complex in Caenorhabditis elegans to reveal a novel and unexpected AP2-dependent trafficking step for glutamate receptors early in the secretory pathway.

Regulation of glutamate receptor (GluR) abundance at synapses by clathrin-mediated endocytosis can control synaptic strength and plasticity. We take advantage of viable, null mutations in subunits of the clathrin adaptor protein 2 (AP2) complex in Caenorhabditis elegans to characterize the in vivo role of AP2 in GluR trafficking. In contrast to our predictions for an endocytic adaptor, we found that levels of the GluR GLR-1 are decreased at synapses in the ventral nerve cord (VNC) of animals with mutations in the AP2 subunits APM-2/μ2, APA-2/α, or APS-2/σ2. Rescue experiments indicate that APM-2/μ2 functions in glr-1–expressing interneurons and the mature nervous system to promote GLR-1 levels in the VNC. Genetic analyses suggest that APM-2/μ2 acts upstream of GLR-1 endocytosis in the VNC. Consistent with this, GLR-1 accumulates in cell bodies of apm-2 mutants. However, GLR-1 does not appear to accumulate at the plasma membrane of the cell body as expected, but instead accumulates in intracellular compartments including Syntaxin-13– and RAB-14–labeled endosomes. This study reveals a novel role for the AP2 clathrin adaptor in promoting the abundance of GluRs at synapses in vivo, and implicates AP2 in the regulation of GluR trafficking at an early step in the secretory pathway.

AP180 couples protein retrieval to clathrin-mediated endocytosis of synaptic vesicles

How clathrin-mediated endocytosis (CME) retrieves vesicle proteins into newly formed synaptic vesicles (SVs) remains a major puzzle. Besides its roles in stimulating clathrin-coated vesicle formation and regulating SV size, the clathrin assembly protein AP180 has been identified as a key player in retrieving SV proteins. The mechanisms by which AP180 recruits SV proteins are not fully understood. Here, we show that following acute inactivation of AP180 in Drosophila, SV recycling is severely impaired at the larval neuromuscular synapse based on analyses of FM 1-43 uptake and synaptic ultrastructure. More dramatically, AP180 activity is important to maintain the integrity of SV protein complexes at the plasma membrane during endocytosis. These observations suggest that AP180 normally clusters SV proteins together during recycling. Consistent with this notion, SV protein composition and distribution are altered in AP180 mutant flies. Finally, AP180 co-immunoprecipitates with SV proteins, including the vesicular glutamate transporter and neuronal synaptobrevin. These results reveal a new mode by which AP180 couples protein retrieval to CME of SVs. AP180 is also genetically linked to Alzheimer's disease. Hence, the findings of this study may provide new mechanistic insight into the role of AP180 dysfunction in Alzheimer's disease.

Cortical surface area correlates with STON2 gene Ser307Pro polymorphism in first-episode treatment-naïve patients with schizophrenia

BACKGROUND

Evidence shows that STON2 gene is associated with synaptic function and schizophrenia. This study aims to explore the relationship between two functional polymorphisms (Ser307Pro and Ala851Ser) of STON2 gene and the cortical surface area in first-episode treatment-naïve patients with schizophrenia and healthy controls.

METHODOLOGY/PRINCIPAL FINDINGS

Magnetic resonance imaging of the whole cortical surface area, which was computed by an automated surface-based technique (FreeSurfer), was obtained from 74 first-episode treatment-naïve patients with schizophrenia and 55 healthy controls. Multiple regression analysis was performed to investigate the effect of genotype subgroups on the cortical surface area. A significant genotype-by-diagnosis effect on the cortical surface area was observed. Pro-allele carriers of Ser307Pro polymorphism had larger right inferior temporal surface area than Ser/Ser carriers in the patients with schizophrenia; however, no significant difference was found in the same area in the healthy controls. The Ala851Ser polymorphism of STON2 gene was not significantly associated with the cortical surface area in patients with schizophrenia and healthy controls.

CONCLUSIONS/SIGNIFICANCE

The present study demonstrated that the functional variant of the STON2 gene could alter cortical surface area on the right inferior temporal and contribute to the pathogenesis of schizophrenia.

AP2 hemicomplexes contribute independently to synaptic vesicle endocytosis

The clathrin adaptor complex AP2 is thought to be an obligate heterotetramer. We identify null mutations in the α subunit of AP2 in the nematode Caenorhabditis elegans. α-adaptin mutants are viable and the remaining μ2/β hemicomplex retains some function. Conversely, in μ2 mutants, the alpha/sigma2 hemicomplex is localized and is partially functional. α-μ2 double mutants disrupt both halves of the complex and are lethal. The lethality can be rescued by expression of AP2 components in the skin, which allowed us to evaluate the requirement for AP2 subunits at synapses. Mutations in either α or μ2 subunits alone reduce the number of synaptic vesicles by about 30%; however, simultaneous loss of both α and μ2 subunits leads to a 70% reduction in synaptic vesicles and the presence of large vacuoles. These data suggest that AP2 may function as two partially independent hemicomplexes. DOI:http://dx.doi.org/10.7554/eLife.00190.001.

Compromised fidelity of endocytic synaptic vesicle protein sorting in the absence of stonin 2

Neurotransmission depends on the exocytic fusion of synaptic vesicles (SVs) and their subsequent reformation either by clathrin-mediated endocytosis or budding from bulk endosomes. How synapses are able to rapidly recycle SVs to maintain SV pool size, yet preserve their compositional identity, is poorly understood. We demonstrate that deletion of the endocytic adaptor stonin 2 (Stn2) in mice compromises the fidelity of SV protein sorting, whereas the apparent speed of SV retrieval is increased. Loss of Stn2 leads to selective missorting of synaptotagmin 1 to the neuronal surface, an elevated SV pool size, and accelerated SV protein endocytosis. The latter phenotype is mimicked by overexpression of endocytosis-defective variants of synaptotagmin 1. Increased speed of SV protein retrieval in the absence of Stn2 correlates with an up-regulation of SV reformation from bulk endosomes. Our results are consistent with a model whereby Stn2 is required to preserve SV protein composition but is dispensable for maintaining the speed of SV recycling.

LRRK2 controls an EndoA phosphorylation cycle in synaptic endocytosis

LRRK2 is a kinase mutated in Parkinson's disease, but how the protein affects synaptic function remains enigmatic. We identified LRRK2 as a critical regulator of EndophilinA. Using genetic and biochemical studies involving Lrrk loss-of-function mutants and Parkinson-related LRRK2(G2019S) gain-of-kinase function, we show that LRRK2 affects synaptic endocytosis by phosphorylating EndoA at S75, a residue in the BAR domain. We show that LRRK2-mediated EndoA phosphorylation has profound effects on EndoA-dependent membrane tubulation and membrane association in vitro and in vivo and on synaptic vesicle endocytosis at Drosophila neuromuscular junctions in vivo. Our work uncovers a regulatory mechanism that indicates that reduced LRRK2 kinase activity facilitates EndoA membrane association, while increased kinase activity inhibits membrane association. Consequently, both too much and too little LRRK2-dependent EndoA phosphorylation impedes synaptic endocytosis, and we propose a model in which LRRK2 kinase activity is part of an EndoA phosphorylation cycle that facilitates efficient vesicle formation at synapses.

UNC-41/stonin functions with AP2 to recycle synaptic vesicles in Caenorhabditis elegans

The recycling of synaptic vesicles requires the recovery of vesicle proteins and membrane. Members of the stonin protein family (Drosophila Stoned B, mammalian stonin 2) have been shown to link the synaptic vesicle protein synaptotagmin to the endocytic machinery. Here we characterize the unc-41 gene, which encodes the stonin ortholog in the nematode Caenorhabditis elegans. Transgenic expression of Drosophila stonedB rescues unc-41 mutant phenotypes, demonstrating that UNC-41 is a bona fide member of the stonin family. In unc-41 mutants, synaptotagmin is present in axons, but is mislocalized and diffuse. In contrast, UNC-41 is localized normally in synaptotagmin mutants, demonstrating a unidirectional relationship for localization. The phenotype of snt-1 unc-41 double mutants is stronger than snt-1 mutants, suggesting that UNC-41 may have additional, synaptotagmin-independent functions. We also show that unc-41 mutants have defects in synaptic vesicle membrane endocytosis, including a ∼50% reduction of vesicles in both acetylcholine and GABA motor neurons. These endocytic defects are similar to those observed in apm-2 mutants, which lack the µ2 subunit of the AP2 adaptor complex. However, no further reduction in synaptic vesicles was observed in unc-41 apm-2 double mutants, suggesting that UNC-41 acts in the same endocytic pathway as µ2 adaptin.

Ca2+ regulates the Drosophila Stoned-A and Stoned-B proteins interaction with the C2B domain of Synaptotagmin-1

The dicistronic Drosophila stoned gene is involved in exocytosis and/or endocytosis of synaptic vesicles. Mutations in either stonedA or stonedB cause a severe disruption of neurotransmission in fruit flies. Previous studies have shown that the coiled-coil domain of the Stoned-A and the µ-homology domain of the Stoned-B protein can interact with the C2B domain of Synaptotagmin-1. However, very little is known about the mechanism of interaction between the Stoned proteins and the C2B domain of Synaptotagmin-1. Here we report that these interactions are increased in the presence of Ca(2+). The Ca(2+)-dependent interaction between the µ-homology domain of Stoned-B and C2B domain of Synaptotagmin-1 is affected by phospholipids. The C-terminal region of the C2B domain, including the tryptophan-containing motif, and the Ca(2+) binding loop region that modulate the Ca(2+)-dependent oligomerization, regulates the binding of the Stoned-A and Stoned-B proteins to the C2B domain. Stoned-B, but not Stoned-A, interacts with the Ca(2+)-binding loop region of C2B domain. The results indicate that Ca(2+)-induced self-association of the C2B domain regulates the binding of both Stoned-A and Stoned-B proteins to Synaptotagmin-1. The Stoned proteins may regulate sustainable neurotransmission in vivo by binding to Ca(2+)-bound Synaptotagmin-1 associated synaptic vesicles.

The early steps of endocytosis: from cargo selection to membrane deformation

Clathrin-mediated endocytosis mediates the internalization of signaling and nutrient receptors, ion channels and regulates the endocytic recycling of pre- and postsynaptic membrane proteins. During early stages endocytic adaptors recognize sorting signals within this diverse array of cargo proteins destined for internalization. Cargo sequestration is mechanistically coupled to membrane deformation, a process involving BAR domain proteins, resulting in the generation of endocytic intermediates that finally undergo dynamin-mediated fission. Here we summarize recent insights gathered from a combination of structural, biochemical, and cell biological studies that have revealed a remarkable complexity of the machinery for endocytic sorting and membrane deformation.

An Arf-like small G protein, ARL-8, promotes the axonal transport of presynaptic cargoes by suppressing vesicle aggregation

Presynaptic assembly requires the packaging of requisite proteins into vesicular cargoes in the cell soma, their long-distance microtubule-dependent transport down the axon, and, finally, their reconstitution into functional complexes at prespecified sites. Despite the identification of several molecules that contribute to these events, the regulatory mechanisms defining such discrete states remain elusive. We report the characterization of an Arf-like small G protein, ARL-8, required during this process. arl-8 mutants prematurely accumulate presynaptic cargoes within the proximal axon of several neuronal classes, with a corresponding failure to assemble presynapses distally. This proximal accumulation requires the activity of several molecules known to catalyze presynaptic assembly. Dynamic imaging studies reveal that arl-8 mutant vesicles exhibit an increased tendency to form immotile aggregates during transport. Together, these results suggest that arl-8 promotes a trafficking identity for presynaptic cargoes, facilitating their efficient transport by repressing premature self-association.

Stoned

The stoned proteins, stoned A (STNA) and stoned B (STNB), are essential for normal vesicle trafficking in Drosophila melanogaster neurons, and deletion of the stoned locus is lethal. Although there is a growing body of research aimed at defining the roles of these proteins, particularly for STNB where homologues have now been identified in all multicellular species, their functions and mechanisms of action are not yet established. The two proteins are structurally unrelated, consistent with two distinct cellular functions. The evidence suggests a critical requirement for stoned proteins in recycling/regulation or specification of a competent synaptic vesicle pool. As stoned proteins may be specific to a particular pathway of endocytosis, studies of their function are likely to be valuable in distinguishing between the different mechanisms of membrane retrieval and their respective contributions to synaptic vesicle recycling, a subject of considerable scientific debate. In this review, we examine the published literature on stoned and comment on the available data, conclusions from these analyses and how they may relate to alternative models of vesicle cycling.

Tweek, an evolutionarily conserved protein, is required for synaptic vesicle recycling

Synaptic vesicle endocytosis is critical for maintaining synaptic communication during intense stimulation. Here we describe Tweek, a conserved protein that is required for synaptic vesicle recycling. tweek mutants show reduced FM1-43 uptake, cannot maintain release during intense stimulation, and harbor larger than normal synaptic vesicles, implicating it in vesicle recycling at the synapse. Interestingly, the levels of a fluorescent PI(4,5)P(2) reporter are reduced at tweek mutant synapses, and the probe is aberrantly localized during stimulation. In addition, various endocytic adaptors known to bind PI(4,5)P(2) are mislocalized and the defects in FM1-43 dye uptake and adaptor localization are partially suppressed by removing one copy of the phosphoinositide phosphatase synaptojanin, suggesting a role for Tweek in maintaining proper phosphoinositide levels at synapses. Our data implicate Tweek in regulating synaptic vesicle recycling via an action mediated at least in part by the regulation of PI(4,5)P(2) levels or availability at the synapse.

A synaptic vesicle-associated Ca2+ channel promotes endocytosis and couples exocytosis to endocytosis

Synaptic vesicle (SV) exo- and endocytosis are tightly coupled to sustain neurotransmission in presynaptic terminals, and both are regulated by Ca(2+). Ca(2+) influx triggered by voltage-gated Ca(2+) channels is necessary for SV fusion. However, extracellular Ca(2+) has also been shown to be required for endocytosis. The intracellular Ca(2+) levels (<1 microM) that trigger endocytosis are typically much lower than those (>10 microM) needed to induce exocytosis, and endocytosis is inhibited when the Ca(2+) level exceeds 1 microM. Here, we identify and characterize a transmembrane protein associated with SVs that, upon SV fusion, localizes at periactive zones. Loss of Flower results in impaired intracellular resting Ca(2+) levels and impaired endocytosis. Flower multimerizes and is able to form a channel to control Ca(2+) influx. We propose that Flower functions as a Ca(2+) channel to regulate synaptic endocytosis and hence couples exo- with endocytosis.

Molecular and genetic characterization of the interactions between the Drosophila stoned-B protein and DAP-160 (intersectin)

The stoned locus of Drosophila produces a dicistronic transcript and encodes two proteins, stoned-A (STNA) and stoned-B (STNB). Both proteins are located at synaptic terminals. The STNB protein contains a domain that has homology with the mu-subunit of the AP (adaptor protein) complex, as well as a number of NPF (Asp-Pro-Phe) motifs known to bind EH (Eps15 homology) domains. Mutations at the stoned locus interact synergistically with mutations at the shibire (dynamin) locus and alter synaptic vesicle endocytosis. The STNB protein has also been shown to interact with synaptic vesicles via synaptogamin-I. We initiated an investigation of the possible interaction of DAP-160 (dynamin-associated protein of 160 kDa), a Drosophila member of the intersectin family, with the STNB protein. We show here that both of the viable stoned alleles interacted with a genetic construct that reduces DAP-160 levels to 25% of normal. One of these stoned alleles contains a substitution resulting in a stop codon in the open reading frame encoding STNB. This allele also shows markedly reduced levels of both DAP-160 and dynamin. As anticipated, the NPF motifs in STNB are found to be high-affinity binding motifs for the EH domains of DAP-160. One of the SH3 (Src homology 3) domains of DAP-160 also interacts with STNB. Finally, we show that immunoprecipitation of STNB from fly head extracts co-precipitates with DAP-160, and we conclude that the interaction of the STNB protein with both synaptotagmin I and DAP-160 may regulate synaptic vesicle recycling by recruiting dynamin to a pre-fission complex.

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.

Polyunsaturated fatty acids influence synaptojanin localization to regulate synaptic vesicle recycling

The lipid polyunsaturated fatty acids are highly enriched in synaptic membranes, including synaptic vesicles, but their precise function there is unknown. Caenorhabditis elegans fat-3 mutants lack long-chain polyunsaturated fatty acids (LC-PUFAs); they release abnormally low levels of serotonin and acetylcholine and are depleted of synaptic vesicles, but the mechanistic basis of these defects is unclear. Here we demonstrate that synaptic vesicle endocytosis is impaired in the mutants: the synaptic vesicle protein synaptobrevin is not efficiently retrieved after synaptic vesicles fuse with the presynaptic membrane, and the presynaptic terminals contain abnormally large endosomal-like compartments and synaptic vesicles. Moreover, the mutants have abnormally low levels of the phosphoinositide phosphatase synaptojanin at release sites and accumulate the main synaptojanin substrate phosphatidylinositol 4,5-bisphosphate at these sites. Both synaptobrevin and synaptojanin mislocalization can be rescued by providing exogenous arachidonic acid, an LC-PUFA, suggesting that the endocytosis defect is caused by LC-PUFA depletion. By showing that the genes fat-3 and synaptojanin act in the same endocytic pathway at synapses, our findings suggest that LC-PUFAs are required for efficient synaptic vesicle recycling, probably by modulating synaptojanin localization at synapses.

RAB-10 regulates glutamate receptor recycling in a cholesterol-dependent endocytosis pathway

Regulated endocytosis of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) is critical for synaptic plasticity. However, the specific combination of clathrin-dependent and -independent mechanisms that mediate AMPAR trafficking in vivo have not been fully characterized. Here, we examine the trafficking of the AMPAR subunit GLR-1 in Caenorhabditis elegans. GLR-1 is localized on synaptic membranes, where it regulates reversals of locomotion in a simple behavioral circuit. Animals lacking RAB-10, a small GTPase required for endocytic recycling of intestinal cargo, are similar in phenotype to animals lacking LIN-10, a postsynaptic density 95/disc-large/zona occludens-domain containing protein: GLR-1 accumulates in large accretions and animals display a decreased frequency of reversals. Mutations in unc-11 (AP180) or itsn-1 (Intersectin 1), which reduce clathrin-dependent endocytosis, suppress the lin-10 but not rab-10 mutant phenotype, suggesting that LIN-10 functions after clathrin-mediated endocytosis. By contrast, cholesterol depletion, which impairs lipid raft formation and clathrin-independent endocytosis, suppresses the rab-10 but not the lin-10 phenotype, suggesting that RAB-10 functions after clathrin-independent endocytosis. Animals lacking both genes display additive GLR-1 trafficking defects. We propose that RAB-10 and LIN-10 recycle AMPARs from intracellular endosomal compartments to synapses along distinct pathways, each with distinct sensitivities to cholesterol and the clathrin-mediated endocytosis machinery.

Genetic modifiers of Drosophila palmitoyl-protein thioesterase 1-induced degeneration

Infantile neuronal ceroid lipofuscinosis (INCL) is a pediatric neurodegenerative disease caused by mutations in the human CLN1 gene. CLN1 encodes palmitoyl-protein thioesterase 1 (PPT1), suggesting an important role for the regulation of palmitoylation in normal neuronal function. To further elucidate Ppt1 function, we performed a gain-of-function modifier screen in Drosophila using a collection of enhancer-promoter transgenic lines to suppress or enhance the degeneration produced by overexpression of Ppt1 in the adult visual system. Modifier genes identified in our screen connect Ppt1 function to synaptic vesicle cycling, endo-lysosomal trafficking, synaptic development, and activity-dependent remodeling of the synapse. Furthermore, several homologs of the modifying genes are known to be regulated by palmitoylation in other systems and may be in vivo substrates for Ppt1. Our results complement recent work on mouse Ppt1(-/-) cells that shows a reduction in synaptic vesicle pools in primary neuronal cultures and defects in endosomal trafficking in human fibroblasts. The pathways and processes implicated by our modifier loci shed light on the normal cellular function of Ppt1. A greater understanding of Ppt1 function in these cellular processes will provide valuable insight into the molecular etiology of the neuronal dysfunction underlying the disease.

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.

Clathrin interaction and subcellular localization of Ce-DAB-1, an adaptor for protein secretion in Caenorhabditis elegans

Growth factors must be secreted appropriately to co-ordinate cell proliferation, specification and movement during development and to control cell numbers and migrations in adult animals. Previous results showed that the secretion of the Caenorhabditis elegans fibroblast growth factor homologue, EGL-17, from vulval precursor cells in vivo involves the cytoplasmic adaptor protein Ce-DAB-1 and two lipoprotein receptors that bind Ce-DAB-1 and EGL-17. Here, we confirm the Ce-DAB-1 requirement for EGL-17 secretion using mutant animals. In vitro, Ce-DAB-1 binds to clathrin and APT-4, the C. elegans homologue of the alpha-adaptin subunit of adaptor protein 2 (AP2), and weakly to the gamma-appendage domains of APT-1 (AP1gamma-adaptin) and APT-9 (GGA protein). In tissue-culture cells, Ce-DAB-1 localizes to various compartments, including AP2-containing vesicles near the cell surface and perinuclear vesicles that contain AP1. The latter also contain Rab8, but not Rab5 or Rab11, as well as proteins en route from the trans Golgi network (TGN) to the surface. In vivo, EGL-17 secretion was inhibited by depletion of apt-1, apt-9 or ce-rab-8 and partially inhibited by RNAi of ce-rab-5, consistent with an important role for these proteins in the secretion of EGL-17 in vivo. These results suggest that Ce-DAB-1 might co-ordinate the assembly of endocytic or secretory vesicles in vivo and may mediate EGL-17 secretion directly, by recruiting clathrin to lipoprotein receptors at the TGN, or indirectly, by affecting lipoprotein receptor endocytosis and recycling.

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.

Stonin 2 Is an AP-2-Dependent Endocytic Sorting Adaptor for Synaptotagmin Internalization and Recycling

Clathrin-mediated endocytosis is involved in the internalization, recycling, and degradation of cycling membrane receptors as well as in the biogenesis of synaptic vesicle proteins. While many constitutively internalized cargo proteins are recognized directly by the clathrin adaptor complex AP-2, stimulation-dependent endocytosis of membrane proteins is often facilitated by specialized sorting adaptors. Although clathrin-mediated endocytosis appears to be a major pathway for presynaptic vesicle cycling, no sorting adaptor dedicated to synaptic vesicle membrane protein endocytosis has been indentified in mammals. Here, we show that stonin 2, a mammalian ortholog of Drosophila stoned B, facilitates clathrin/AP-2-dependent internalization of synaptotagmin and targets it to a recycling vesicle pool in living neurons. The ability of stonin 2 to facilitate endocytosis of synaptotagmin is dependent on its association with AP-2, an intact mu-homology domain, and functional AP-2 heterotetramers. Our data identify stonin 2 as an AP-2-dependent endocytic sorting adaptor for synaptotagmin internalization and recycling.

Effective Translation of the Second Cistron in Two Drosophila Dicistronic Transcripts Is Determined by the Absence of In-frame AUG Codons in the First Cistron

The novel dicistronic transcript encoded by the Drosophila melanogaster stoned gene was recognized as being unusual in that the protein encoded by the first open reading frame, stoned-A (STNA), contains no internal methionine residues in a protein of 93 kDa. The dicistronic nature of the stoned locus and the lack of methionine residues in STNA is conserved across dipteran species. A second methionine-free cistron, encoding Snapin, was identified in Drosophila and also found to be dicistronic, the second open reading frame (ORF) encoding a methyltransferase. We have replaced the methyltransferase cistron with green fluorescent protein (GFP) and used this dicistronic construct to show that the GFP cistron is translated in Drosophila S2 cells. The insertion of in-frame AUG codons into the snapin ORF attenuates the translation of GFP, and the level of attenuation correlates with the number of inserted AUGs. Increasing the efficiency of translation-initiation of the Snapin cistron also attenuates the translation of GFP. This indicates that failure to initiate translation at the first AUG allows ribosomes to scan through the Snapin ORF and to initiate translation of the second cistron, unless new AUG codons are inserted. These data are used to interpret the expression of the stoned locus and in particular, to explain the altered stoned protein levels in the stoned-temperature-sensitive mutant allele, which replaces a lysine with a methionine codon early in the first, stonedA, cistron.

Mouse chromaffin cells have two populations of dense core vesicles

The quantal hypothesis states that neurotransmitter is released in discrete packages, quanta, thought to represent the neurotransmitter content of individual vesicles. If true, then vesicle size should influence quantal size. Although chromaffin cells are generally thought to have a single population of secretory vesicles, our electron microscopy analysis suggested two populations as the size distribution was best described as the sum of two Gaussians. The average volume difference was fivefold. To test whether this difference in volume affected quantal size, neurotransmitter release from permeabilized cells exposed to 100 microM Ca2+ was measured with amperometry. Quantal content was bimodally distributed with both large and small events; the distribution of vesicle sizes predicted by amperometry was extremely similar to those measured with electron microscopy. In addition, each population of events exhibited distinct release kinetics. These results suggest that chromaffin cells have two populations of dense core vesicles (DCV) with unique secretory properties and which may represent two distinct synthetic pathways for DCV biogenesis or alternatively they may represent different stages of biosynthesis.

AP180 maintains the distribution of synaptic and vesicle proteins in the nerve terminal and indirectly regulates the efficacy of Ca2+-triggered exocytosis

AP180 plays an important role in clathrin-mediated endocytosis of synaptic vesicles (SVs) and has also been implicated in retrieving SV proteins. In Drosophila, deletion of its homologue, Like-AP180 (LAP), has been shown to increase the size of SVs but decrease the number of SVs and transmitter release. However, it remains elusive whether a reduction in the total vesicle pool directly affects transmitter release. Further, it is unknown whether the lap mutation also affects vesicle protein retrieval and synaptic protein localization and, if so, how it might affect exocytosis. Using a combination of electrophysiology, optical imaging, electron microscopy, and immunocytochemistry, we have further characterized the lap mutant and hereby show that LAP plays additional roles in maintaining both normal synaptic transmission and protein distribution at synapses. While increasing the rate of spontaneous vesicle fusion, the lap mutation dramatically reduces impulse-evoked transmitter release at steps downstream of calcium entry and vesicle docking. Notably, lap mutations disrupt calcium coupling to exocytosis and reduce calcium cooperativity. These results suggest a primary defect in calcium sensors on the vesicles or on the release machinery. Consistent with this hypothesis, three vesicle proteins critical for calcium-mediated exocytosis, synaptotagmin I, cysteine-string protein, and neuronal synaptobrevin, are all mislocalized to the extrasynaptic axonal regions along with Dap160, an active zone marker (nc82), and glutamate receptors in the mutant. These results suggest that AP180 is required for either recycling vesicle proteins and/or maintaining the distribution of both vesicle and synaptic proteins in the nerve terminal.

Synapse scaffolding: intersection of endocytosis and growth

The Drosophila dynamin-associated protein Dap160, homolog of the vertebrate Intersectins, is thought likely to act as a molecular scaffold in the synaptic periactive zone. New mutant analyses have revealed separable roles for Dap160 in the regulation of vesicular endocytosis and synaptic growth.

SV2B regulates synaptotagmin 1 by direct interaction

SV2 proteins are abundant synaptic vesicle proteins expressed in two major (SV2A and SV2B) and one minor (SV2C) isoform. SV2A and SV2B have been shown to be involved in the regulation of synaptic vesicle exocytosis. Previous studies found that SV2A, but not SV2B, can interact with the cytoplasmic domain of synaptotagmin 1, a Ca2+ sensor for synaptic vesicle exocytosis. To determine whether SV2B can interact with full-length synaptotagmin 1, we performed immunoprecipitations from brain protein extracts and found that SV2B interacts strongly with synaptotagmin 1 in a detergent-resistant, Ca2+ -independent manner. In contrast, an interaction between native SV2A and synaptotagmin 1 was not detectable under these conditions. The SV2B-synaptotagmin 1 complex also contained the synaptic t-SNARE proteins, syntaxin 1 and SNAP-25, suggesting that SV2B may participate in exocytosis by modulating the interaction of synaptotagmin 1 with t-SNARE proteins. Analysis of retinae in SV2B knock-out mice revealed a strong reduction in the level of synaptotagmin 1 in rod photoreceptor synapses, which are unique in that they express only the SV2B isoform. In contrast, other synaptic vesicle proteins were not affected by SV2B knock out, indicating a specific role for SV2B in the regulation of synaptotagmin 1 levels at certain synapses. These experiments suggest that the SV2B-synaptotagmin 1 complex is involved in the regulation of synaptotagmin 1 stability and/or trafficking. This study has demonstrated a new role of SV2B as a regulator of synaptotagmin 1 that is likely mediated by direct interaction of these two synaptic proteins.

Dap160/Intersectin Acts as a Stabilizing Scaffold Required for Synaptic Development and Vesicle Endocytosis

We describe the isolation of mutations in dynamin-associated protein 160 kDa (dap160), the Drosophila homolog of intersectin, a putative adaptor for proteins involved in endocytosis, cytoskeletal regulation, and signaling. We show that partial loss-of-function mutants display temperature-sensitive (ts) paralysis, whereas null mutants show ts defects in endocytosis. Loss-of-function mutants exhibit bouton overgrowth at larval neuromuscular junctions (NMJs), but evoked neurotransmission is normal. Mutant NMJs show a mild endocytic defect at 22 degrees C, which is strongly enhanced at 34 degrees C. The levels of dynamin, synaptojanin and endophilin are severely reduced in dap160 mutant NMJs, suggesting that Dap160 serves to stabilize an endocytic macromolecular complex. Electron microscopy reveals fewer vesicles, aberrant large vesicles, and an accumulation of endocytic intermediates at active and periactive zones in mutant terminals. Our data suggest that Dap160, like dynamin, is involved in synaptic vesicle retrieval at active and periactive zones.

Functional dissection of the interactions of stonin 2 with the adaptor complex AP-2 and synaptotagmin

Synaptic vesicle recycling is in part mediated by clathrin-mediated endocytosis. This process involves the coordinated assembly of clathrin and adaptor proteins and the concomitant selection of cargo proteins. Here, we demonstrate that the endocytotic protein stonin 2 localizes to axonal vesicle clusters through its micro-homology domain. Interaction of this domain with synaptotagmin I is sufficient to recruit stonin 2 to the plasmalemma. The N-terminal domain of stonin 2 harbors multiple AP-2-interaction motifs that bind to the clathrin adaptor complex AP-2. These motifs with the consensus sequence WVxF are capable of binding to the alpha-adaptin ear domain and to micro2. Mutation of the tyrosine motif-binding pocket of micro2 abolishes recognition of the WVxF peptide, suggesting that association with stonin 2 renders AP-2 incompetent to sort tyrosine motif-containing cargo proteins. We hypothesize that stonin 2 may function as an AP-2-dependent sorting adaptor for synaptic vesicle recycling.

Cellular bases of activity-dependent paralysis inDrosophila stress-sensitive mutants

Stress-sensitive mutants in Drosophila have been shown to exhibit activity-dependent defects in neurotransmission. Using the neuromuscular junction (NMJ), this study investigates synaptic function more specifically in two stress-sensitive mutants: stress-sensitive B (sesB), which encodes a mitochondrial ADP/ATP translocase (ANT); and Atpalpha(2206), a conditional mutant of the Na+/K+ ATPase alpha-subunit. Mechanical shock induces a period of brief paralysis in both homozygous and double heterozygous mutants, but further analysis revealed distinct activity-dependent neurotransmission lesions in each mutant. Basal neurotransmission appeared similar to wild-type controls in both mutants under low frequency stimulation. High frequency stimulation, however, caused pronounced synaptic fatigue as well as slow and incomplete synaptic recovery in sesB mutants while Atpalpha(2206) mutants displayed an increase (25-fold) in synaptic failures. Perhaps to compensate for these activity dependent defects, the neuromuscular synapse was found to be overgrown in both mutants. Passive electrotonic stimulation, which initiates synaptic transmission independent of action potentials, ameliorated synaptic failures and resulted in increased neurotransmission amplitude in Atpalpha(2206) mutants. In addition, spontaneous synaptic vesicle fusion rates were increased in Atpalpha(2206) mutants, suggesting that, in the absence of action potential requirements, these synaptic terminals are healthy, if not hyperactive. Dye labeling studies revealed aberrant synaptic vesicle cycling in sesB mutants indicating a reduction of functional synaptic vesicles. We therefore postulate that both stress-sensitive mutants harbor unique neurotransmission defects: Atpalpha(2206) mutants are unable to maintain ionic gradients required during repetitive action potential propagation, and sesB mutants cannot maintain synaptic vesicle cycling during periods of high demand.

An essential role of Rab5 in uniformity of synaptic vesicle size

Rab5 small GTPase is a famous regulator of endocytic vesicular transport from plasma membrane to early endosomes. In neurons, Rab5 is found not only on endocytic vesicles in cell bodies but also on synaptic vesicles in nerve terminals. However, the function of Rab5 on synaptic vesicles remains unclear. Here, we elucidate the function of Rab5 on synaptic vesicles with in vivo and in vitro experiments using Drosophila photoreceptor cells. Functional inhibition of Rab5 with Rab5N142I, a dominant negative version of Drosophila Rab5, induced enlargement of synaptic vesicles. This enlargement was, however, suppressed by enhancing synaptic vesicle recycling under light illumination. In addition, synaptic vesicles prepared from Rab5N142I-expressing flies exhibited homotypic fusion in vitro. These results indicate that Rab5 functions to keep the size of synaptic vesicles uniform by preventing their homotypic fusion. By contrast, Rab5 was not involved in the endocytic reformation of synaptic vesicles, contrary to expectation from its conventional function. Furthermore, we electrophysiologically and behaviourally showed that the function of Rab5 is essential for efficient signal transmission across synapses.

Role of Drosophila Rab5 during endosomal trafficking at the synapse and evoked neurotransmitter release

During constitutive endocytosis, internalized membrane traffics through endosomal compartments. At synapses, endocytosis of vesicular membrane is temporally coupled to action potential-induced exocytosis of synaptic vesicles. Endocytosed membrane may immediately be reused for a new round of neurotransmitter release without trafficking through an endosomal compartment. Using GFP-tagged endosomal markers, we monitored an endosomal compartment in Drosophila neuromuscular synapses. We showed that in conditions in which the synaptic vesicles pool is depleted, the endosome is also drastically reduced and only recovers from membrane derived by dynamin-mediated endocytosis. This suggests that membrane exchange takes place between the vesicle pool and the synaptic endosome. We demonstrate that the small GTPase Rab5 is required for endosome integrity in the presynaptic terminal. Impaired Rab5 function affects endo- and exocytosis rates and decreases the evoked neurotransmitter release probability. Conversely, Rab5 overexpression increases the release efficacy. Therefore, the Rab5-dependent trafficking pathway plays an important role for synaptic performance.

Hsc70 is required for endocytosis and clathrin function in Drosophila

By screening for Drosophila mutants exhibiting aberrant bride of sevenless (Boss) staining patterns on eye imaginal disc epithelia, we have recovered a point mutation in Hsc70-4, the closest homologue to bovine clathrin uncoating ATPase. Although the mutant allele was lethal, analysis of mutant clones generated by FLP/FRT recombination demonstrated that the Sevenless-mediated internalization of Boss was blocked in mutant Hsc70-4 eye disc epithelial cells. Endocytosis of other probes was also greatly inhibited in larval Garland cells. Immunostaining and EM analysis of the mutant cells revealed disruptions in the organization of endosomal/lysosomal compartments, including a substantial reduction in the number of clathrin-coated structures in Garland cells. The Hsc70-4 mutation also interacted genetically with a dominant-negative mutant of dynamin, a gene required for the budding of clathrin-coated vesicles (CCVs). Consistent with these phenotypes, recombinant mutant Hsc70 proteins exhibited diminished clathrin uncoating activity in vitro. Together, these data provide genetic support for the long-suspected role of Hsc70 in clathrin-mediated endocytosis, at least in part by inhibiting the uncoating of CCVs.

The synaptic vesicle cycle: exocytosis and endocytosis in Drosophila and C. elegans

Advances in the study of Drosophila melanogaster and Caenorhabditis elegans have provided key insights into the processes of neurotransmission and neuromodulation. Work in the past year has revealed that Unc-13 and Rab3a-interacting molecule regulate the conformational state of syntaxin to prime synaptic vesicle fusion. Analyses of synaptotagmin support its role as a putative calcium sensor triggering vesicular fusion and highlight the possible role of SNARE complex oligomerization in the fusion mechanism. Characterization of endophilin mutants demonstrates that kiss-and-run endocytosis is a major component of synaptic vesicle recycling. In neuromodulation, dcaps mutants provide the first genetic insight into possible roles of the CAPS protein in mediating dense core vesicle fusion and modulating synaptic vesicle fusion.