F-actin is concentrated in nonrelease domains at frog neuromuscular junctions

Membrane compression by synaptic vesicle exocytosis triggers ultrafast endocytosis

Compensatory endocytosis keeps the membrane surface area of secretory cells constant following exocytosis. At chemical synapses, clathrin-independent ultrafast endocytosis maintains such homeostasis. This endocytic pathway is temporally and spatially coupled to exocytosis; it initiates within 50 ms at the region immediately next to the active zone where vesicles fuse. However, the coupling mechanism is unknown. Here, we demonstrate that filamentous actin is organized as a ring, surrounding the active zone at mouse hippocampal synapses. Assuming the membrane area conservation is due to this actin ring, our theoretical model suggests that flattening of fused vesicles exerts lateral compression in the plasma membrane, resulting in rapid formation of endocytic pits at the border between the active zone and the surrounding actin-enriched region. Consistent with model predictions, our data show that ultrafast endocytosis requires sufficient compression by exocytosis of multiple vesicles and does not initiate when actin organization is disrupted, either pharmacologically or by ablation of the actin-binding protein Epsin1. Our work suggests that membrane mechanics underlie the rapid coupling of exocytosis to endocytosis at synapses.

Control of Synapse Structure and Function by Actin and Its Regulators

Neurons transmit and receive information at specialized junctions called synapses. Excitatory synapses form at the junction between a presynaptic axon terminal and a postsynaptic dendritic spine. Supporting the shape and function of these junctions is a complex network of actin filaments and its regulators. Advances in microscopic techniques have enabled studies of the organization of actin at synapses and its dynamic regulation. In addition to highlighting recent advances in the field, we will provide a brief historical perspective of the understanding of synaptic actin at the synapse. We will also highlight key neuronal functions regulated by actin, including organization of proteins in the pre- and post- synaptic compartments and endocytosis of ion channels. We review the evidence that synapses contain distinct actin pools that differ in their localization and dynamic behaviors and discuss key functions for these actin pools. Finally, whole exome sequencing of humans with neurodevelopmental and psychiatric disorders has identified synaptic actin regulators as key disease risk genes. We briefly summarize how genetic variants in these genes impact neurotransmission via their impact on synaptic actin.

Hypothesis Relating the Structure, Biochemistry and Function of Active Zone Material Macromolecules at a Neuromuscular Junction

This report integrates knowledge of in situ macromolecular structures and synaptic protein biochemistry to propose a unified hypothesis for the regulation of certain vesicle trafficking events (i.e., docking, priming, Ca²⁺-triggering, and membrane fusion) that lead to neurotransmitter secretion from specialized “active zones” of presynaptic axon terminals. Advancements in electron tomography, to image tissue sections in 3D at nanometer scale resolution, have led to structural characterizations of a network of different classes of macromolecules at the active zone, called “Active Zone Material’. At frog neuromuscular junctions, the classes of Active Zone Material macromolecules “top-masts”, “booms”, “spars”, “ribs” and “pins” direct synaptic vesicle docking while “pins”, “ribs” and “pegs” regulate priming to influence Ca²⁺-triggering and membrane fusion. Other classes, “beams”, “steps”, “masts”, and “synaptic vesicle luminal filaments’ likely help organize and maintain the structural integrity of active zones. Extensive studies on the biochemistry that regulates secretion have led to comprehensive characterizations of the many conserved proteins universally involved in these trafficking events. Here, a hypothesis including a partial proteomic atlas of Active Zone Material is presented which considers the common roles, binding partners, physical features/structure, and relative positioning in the axon terminal of both the proteins and classes of macromolecules involved in the vesicle trafficking events. The hypothesis designates voltage-gated Ca²⁺ channels and Ca²⁺-gated K⁺ channels to ribs and pegs that are connected to macromolecules that span the presynaptic membrane at the active zone. SNARE proteins (Syntaxin, SNAP25, and Synaptobrevin), SNARE-interacting proteins Synaptotagmin, Munc13, Munc18, Complexin, and NSF are designated to ribs and/or pins. Rab3A and Rabphillin-3A are designated to top-masts and/or booms and/or spars. RIM, Bassoon, and Piccolo are designated to beams, steps, masts, ribs, spars, booms, and top-masts. Spectrin is designated to beams. Lastly, the luminal portions of SV2 are thought to form the bulk of the observed synaptic vesicle luminal filaments. The goal here is to help direct future studies that aim to bridge Active Zone Material structure, biochemistry, and function to ultimately determine how it regulates the trafficking events in vivo that lead to neurotransmitter secretion.

A Na,K-ATPase-Fodrin-Actin Membrane Cytoskeleton Complex is Required for Endothelial Fenestra Biogenesis

Fenestrae are transcellular plasma membrane pores that mediate blood–tissue exchange in specialised vascular endothelia. The composition and biogenesis of the fenestra remain enigmatic. We isolated and characterised the protein composition of large patches of fenestrated plasma membrane, termed sieve plates. Loss-of-function experiments demonstrated that two components of the sieve plate, moesin and annexin II, were positive and negative regulators of fenestra formation, respectively. Biochemical analyses showed that moesin is involved in the formation of an actin–fodrin submembrane cytoskeleton that was essential for fenestra formation. The link between the fodrin cytoskeleton and the plasma membrane involved the fenestral pore protein PV-1 and Na,K-ATPase, which is a key regulator of signalling during fenestra formation both in vitro and in vivo. These findings provide a conceptual framework for fenestra biogenesis, linking the dynamic changes in plasma membrane remodelling to the formation of a submembrane cytoskeletal signalling complex.

Pilocarpine-induced epilepsy is associated with actin cytoskeleton reorganization in the mossy fiber-CA3 synapses

Dramatic structural changes have been demonstrated in the mossy fiber-CA3 synapses in the post status epilepticus (SE) animals, suggesting a potential reorganization of filamentous actin (F-actin) network occurring in the hippocampus. However, until now the long-term effects of SE on the synaptic F-actin have still not been reported. In this study, phalloidin labeling combined with confocal microscopy and protein analyses were adopted to investigate the effects of pilocarpine treatment on the F-actin in the C57BL/6 mice. As compared to the controls, there was ∼ 43% reduction in F-actin density in the post SE mice. Quantitative analysis showed that the labeling density and the puncta number were significantly decreased after pilocarpine treatment (p<0.01, n=5 mice per group, Student's t-test). The puncta of F-actin in the post SE group tended to be highly clustered, while those in the controls were generally distributed evenly. The mean puncta size of F-actin puncta was 0.73±0.19μm(2) (n=1102 puncta from 5 SE mice) in the experimental group, significantly larger than that in the controls (0.51±0.10μm(2), n=1983 puncta from 5 aged-matched control mice, p<0.01, Student's t-test). These observations were well consistent with the alterations of postsynaptic densities in the same region, revealed by immunostaining of PSD95, suggesting the reorganization of F-actin occurred mainly postsynaptically. Our results are indicative of important cytoskeletal changes in the mossy fiber-CA3 synapses after pilocarpine treatment, which may contribute to the excessive excitatory output in the hippocampal trisynaptic circuit.

The rearrangement of filamentous actin in mossy fiber synapses in pentylenetetrazol-kindled C57BL/6 mice

Chemical kindling, as an experimental model of epileptogenesis, is induced by repetitive administration of subconvulsive amount of excitatory drugs. Kindled mice do not typically display spontaneous recurrent seizures, but are instead characterized by enhanced seizure susceptibility to convulsive stimulations. In order to provide insights into the aberrant synaptic plasticity during kindling, this study investigated the effect of pentylenetetrazol (PTZ) kindling on filamentous actin (F-actin) in mossy fiber synapses in C57BL/6 mice. Phalloidin labeling of F-actin showed that F-actin puncta were increased in number in the stratum lucidum of CA3 region in the hippocampus after kindling. The rearrangement of F-actin seemed to occur presynaptically, since synapsin I, a specific marker for mossy fiber terminals, was also up-regulated. Such subtle structural modifications occurring in the synapses are thought to contribute to the long-lasting increased sensitivity in the PTZ-kindled C57BL/6 mice.

Embryonic and larval neural connectivity: progressive changes in synapse form and function at the neuromuscular junction mediated by cytoskeletal regulation

During development, precise formation of millions of synaptic connections is critical for the formation of a functional nervous system. Synaptogenesis is a complex multistep process in which axons follow gradients of secreted and cell surface guidance cues to reach their target area, at which point they must accurately distinguish their specific target. Upon target recognition, the axonal growth cone undergoes rapid growth and morphological changes, ultimately forming a functional synapse that continues to remodel during activity-dependent plasticity. Significant evidence suggests that the underlying actin and microtubule (MT) cytoskeletons are key effectors throughout synaptogenesis downstream of numerous receptors and signaling pathways. An increasing number of cytoskeletal-associated proteins have been shown to influence actin and MT stability and dynamics and many of these regulators have been implicated during synaptic morphogenesis using both mammalian and invertebrate model systems. In this review, we present an overview of the role cytoskeletal regulators play during the formation of the Drosophila neuromuscular junction.

Actin- and dynamin-dependent maturation of bulk endocytosis restores neurotransmission following synaptic depletion

Bulk endocytosis contributes to the maintenance of neurotransmission at the amphibian neuromuscular junction by regenerating synaptic vesicles. How nerve terminals internalize adequate portions of the presynaptic membrane when bulk endocytosis is initiated before the end of a sustained stimulation is unknown. A maturation process, occurring at the end of the stimulation, is hypothesised to precisely restore the pools of synaptic vesicles. Using confocal time-lapse microscopy of FM1-43-labeled nerve terminals at the amphibian neuromuscular junction, we confirm that bulk endocytosis is initiated during a sustained tetanic stimulation and reveal that shortly after the end of the stimulation, nerve terminals undergo a maturation process. This includes a transient bulging of the plasma membrane, followed by the development of large intraterminal FM1-43-positive donut-like structures comprising large bulk membrane cisternae surrounded by recycling vesicles. The degree of bulging increased with stimulation frequency and the plasmalemma surface retrieved following the transient bulging correlated with the surface membrane internalized in bulk cisternae and recycling vesicles. Dyngo-4a, a potent dynamin inhibitor, did not block the initiation, but prevented the maturation of bulk endocytosis. In contrast, cytochalasin D, an inhibitor of actin polymerization, hindered both the initiation and maturation processes. Both inhibitors hampered the functional recovery of neurotransmission after synaptic depletion. Our data confirm that initiation of bulk endocytosis occurs during stimulation and demonstrates that a delayed maturation process controlled by actin and dynamin underpins the coupling between exocytosis and bulk endocytosis.

Membrane trafficking events underlying axon repair, growth, and regeneration

Two central challenges for the field of neurobiology are to understand how axons grow and make proper synaptic connections under normal conditions and how they repair their membranes and mount regenerative responses after injury. At the most reductionist level, the first step toward addressing these challenges is to delineate the cellular and molecular processes by which an axon extends from its cell body. Underlying axon extension are questions of appropriate timing and mechanisms that establish or maintain the axon's polarity, initiate growth cone formation, and promote axon outgrowth and synapse formation. After injury, the problem is even more complicated because the neuron must also repair its damaged membrane, redistribute or manufacture what it needs in order to survive, and grow and form new synapses within a more mature, complex environment. While other reviews have focused extensively on the signaling events and cytoskeletal rearrangements that support axon outgrowth and regeneration, we focus this review instead on the underlying membrane trafficking events underlying these processes. Though the mechanisms are still under active investigation, the key roles played by membrane trafficking events during axon repair, growth, and regeneration have been elucidated through elegant comparative studies in both invertebrate and vertebrate organisms. Taken together, a model emerges indicating that the critical requirements for ensuring proper membrane sealing and axon extension include iterative bouts of SNARE mediated exocytosis, endocytosis, and functional links between vesicles and the actin cytoskeleton, similar to the mechanisms utilized during synaptic transmission. This article is part of a Special Issue entitled 'Neuronal Function'.

Changes in the distribution of calcium calmodulin-dependent protein kinase II at the presynaptic bouton after depolarization

Phosphorylation of synapsin I by CaMKII has been reported to mobilize synaptic vesicles from the reserve pool. In the present study, the distributions of alpha-CaMKII and of synapsin I were compared in synaptic boutons of unstimulated and stimulated hippocampal neurons in culture by immunogold electron microscopy. CaMKII and synapsin I are located in separate domains in presynaptic terminals of unstimulated neurons. Label for alpha -CaMKII typically surrounds synaptic vesicle clusters and is absent from the inside of the cluster in control synapses. In contrast, intense labeling for synapsin I is found within the vesicle clusters. Following 2 minutes of depolarization in high K(+), synaptic vesicles decluster and CaMKII label disperses and mingles with vesicles and synapsin I. These results indicate that, under resting conditions, CaMKII has limited access to the synapsin I in synaptic vesicle clusters. The peripheral distribution of CaMKII around vesicle clusters suggests that CaMKII-mediated declustering progresses from the periphery towards the center, with the depth of penetration into the synaptic vesicle cluster depending on the duration of CaMKII activation. Depolarization also promotes a significant increase in CaMKII immunolabel near the presynaptic active zone. Activity-induced redistribution of CaMKII leaves it in a position to facilitate phosphorylation of additional presynaptic proteins regulating neurotransmitter release.

Synaptic vesicle mobility and presynaptic F-actin are disrupted in a N-ethylmaleimide-sensitive factor allele of Drosophila

N-ethylmaleimide sensitive factor (NSF) can dissociate the soluble NSF attachment receptor (SNARE) complex, but NSF also participates in other intracellular trafficking functions by virtue of SNARE-independent activity. Drosophila that express a neural transgene encoding a dominant-negative form of NSF2 show an 80% reduction in the size of releasable synaptic vesicle pool, but no change in the number of vesicles in nerve terminal boutons. Here we tested the hypothesis that vesicles in the NSF2 mutant terminal are less mobile. Using a combination of genetics, pharmacology, and imaging we find a substantial reduction in vesicle mobility within the nerve terminal boutons of Drosophila NSF2 mutant larvae. Subsequent analysis revealed a decrease of filamentous actin in both NSF2 dominant-negative and loss-of-function mutants. Lastly, actin-filament disrupting drugs also decrease vesicle movement. We conclude that a factor contributing to the NSF mutant phenotype is a reduction in vesicle mobility, which is associated with decreased presynaptic F-actin. Our data are consistent with a model in which actin filaments promote vesicle mobility and suggest that NSF participates in establishing or maintaining this population of actin.

Deletion of synapsins I and II genes alters the size of vesicular pools and rabphilin phosphorylation

Previous studies established that genetic deletion of synapsins, synaptic vesicle-associated phosphoproteins that regulate neurotransmitter release, decreases the number of synaptic vesicles in nerve terminals. To investigate whether these changes affect the release properties of the remaining synaptic vesicles, we used a radioactive labeling technique to measure release independently of the total number of synaptic vesicles. 3H-glutamate and 14C-gamma-amino-butyric-acid (GABA) release from isolated nerve terminals prepared from the neocortex of synapsins I and II double knock-out mice (DKO) was assayed and compared to wild-type preparations. Hyperosmotic shock-evoked 3H-glutamate was reduced by 20+/-3% from DKO nerve terminals and potassium depolarization-evoked glutamate release was also decreased by 28+/-2%. Surprisingly, sucrose or potassium depolarization-evoked release of 14C-GABA was increased by 32+/-4% and 29+/-5%, respectively. The basal efflux of both 3H-glutamate and 14C-GABA increased by 17+/-2% and 12+/-2% from DKO nerve terminals. As lack of synapsins I and II, major phosphoproteins of synaptic vesicles, may lead to deregulation of phosphorylation events, we compared phosphorylation state of another synaptic vesicle protein, rabphilin. In DKO nerve terminals, membrane-associated rabphilin level was reduced by approximately 0.28-fold, its phosphorylation at 234serine was increased by approximately 1.61-fold whereas cytosolic rabphilin levels showed both more dramatic reduction in abundance, approximately 16.5-fold, and increase in phosphorylation, approximately 4.8-fold. Collectively, these data suggest that deletion of major synapsin isoforms leads to (1) deregulation of basal neurotransmission causing "leaky" basal release, (2) changes in either the size or mobilization of releasable or reserve pools, and (3) a decrease in rabphilin abundance accompanied by an increase in basal phosphorylation of the remaining rabphilin.

Giant reticulospinal synapse in lamprey: molecular links between active and periactive zones

Deciphering the function of synaptic release sites is central to understanding neuronal communication. Here, we review studies of the lamprey giant reticulospinal synapse, a model that can be used to dissect synaptic vesicle trafficking at single release sites. The presynaptic axon is large and contains active zones that are spatially separated from each other. During activity, synaptic vesicle membrane is shuttled between the active zone and the periactive zone at which endocytosis occurs. Recent studies have shown that the periactive zone contains an actin-rich cytomatrix that expands during synaptic activity. This cytomatrix has been implicated in multiple functions that include (1) activity-dependent trafficking of proteins between the synaptic vesicle cluster and the periactive zone, (2) synaptic vesicle endocytosis, and (3) the movement of newly formed synaptic vesicles to the vesicle cluster. The actin cytomatrix thus provides a link between the active zone and the periactive zone; this link appears to be critical for sustained cycling of synaptic vesicles.

Actin polymerization regulates clathrin coat maturation during early stages of synaptic vesicle recycling at lamprey synapses

Although it is established that presynaptic actin participates in synaptic vesicle recycling at several synapses, the earliest stages at which actin polymerization is employed during this process are still unclear. To address this, we prevented actin polymerization at lamprey synapses by applying latrunculin B or swinholide A. Latrunculin and swinholide depolymerize actin by sequestering actin monomers and, in addition, swinholide can sever existing actin filaments. When injected into individual presynaptic axons of the intact spinal cord, fluorescently labeled monomeric actin rapidly incorporated in a calcium-dependent manner into a stable, filamentous actin network concentrated at endocytic zones. This pool of actin was disrupted completely by latrunculin. At stimulated synapses, specific disruption of actin polymerization with latrunculin and swinholide induced a selective increase in unconstricted clathrin-coated pits and, in the case of swinholide, an additional increase in the size of plasma membrane evaginations. These results indicate that actin polymerization participates initially in the maturation of clathrin-coated pits during early stages of synaptic vesicle recycling.

The actin cytoskeleton: integrating form and function at the synapse

Synapses are highly specialized intercellular junctions that mediate the transmission of information between axons and target cells. A fundamental property of synapses is their ability to modify the efficacy of synaptic communication through various forms of synaptic plasticity. Recent developments in imaging techniques have revealed that synapses exhibit a high degree of morphological plasticity under basal conditions and also in response to neuronal activity that induces alterations in synaptic strength. The underlying molecular basis for this morphological plasticity has attracted much attention, yet its functional significance to the mechanisms of synaptic transmission and synaptic plasticity remains elusive. These morphological changes ultimately require the dynamic actin cytoskeleton, which is the major structural component of synapses. Delineating the physiological roles of the actin cytoskeleton in supporting synaptic transmission and synaptic plasticity, therefore, paves the way for gaining molecular insights into when and how synaptic machineries couple synapse form and function.

Systematic analysis of genes required for synapse structure and function

Chemical synapses are complex structures that mediate rapid intercellular signalling in the nervous system. Proteomic studies suggest that several hundred proteins will be found at synaptic specializations. Here we describe a systematic screen to identify genes required for the function or development of Caenorhabditis elegans neuromuscular junctions. A total of 185 genes were identified in an RNA interference screen for decreased acetylcholine secretion; 132 of these genes had not previously been implicated in synaptic transmission. Functional profiles for these genes were determined by comparing secretion defects observed after RNA interference under a variety of conditions. Hierarchical clustering identified groups of functionally related genes, including those involved in the synaptic vesicle cycle, neuropeptide signalling and responsiveness to phorbol esters. Twenty-four genes encoded proteins that were localized to presynaptic specializations. Loss-of-function mutations in 12 genes caused defects in presynaptic structure.

Synaptic vesicle pools

Communication between cells reaches its highest degree of specialization at chemical synapses. Some synapses talk in a 'whisper'; others 'shout'. The 'louder' the synapse, the more synaptic vesicles are needed to maintain effective transmission, ranging from a few hundred (whisperers) to nearly a million (shouters). These vesicles reside in different 'pools', which have been given a bewildering array of names. In this review, we focus on five tissue preparations in which synaptic vesicle pools have been identified and thoroughly characterized. We argue that, in each preparation, each vesicle can be assigned to one of three distinct pools.

Lipid regulation of the synaptic vesicle cycle

Membrane vesicle cycling is orchestrated through the combined actions of proteins and lipids. At neuronal synapses, this orchestration must meet the stringent demands of speed, fidelity and sustainability of the synaptic vesicle cycle that mediates neurotransmission. Historically, the lion's share of the attention has been focused on the proteins that are involved in this cycle; but, in recent years, it has become clear that the previously unheralded plasma membrane and vesicle lipids are also key regulators of this cycle. This article reviews recent insights into the roles of lipid-modifying enzymes and lipids in the acute modulation of neurotransmission.

Regulation of synaptic vesicle recycling by calcineurin in different vesicle pools

The synaptic vesicles keep recycling by the processes of endocytosis and exocytosis to maintain the normal synaptic transmission. The synaptic vesicles are classified as the readily releasable pool (RRP) and the reserve pool (RP). In the endocytosis process, calcineurin (CaN), a Ca2+/calmodulin-dependent protein phosphatase, has been shown to play important roles. However, it is unclear about its roles in different vesicle pools. Here, we investigated the role of CaN in the regulation of vesicle recycling in the RRP and RP. Vesicle recycling was monitored by using fluorescent dyes FM1-43 and FM4-64 in the primary cultures of hippocampal neurons. Inhibition of CaN by FK506 and cyclosporin A suppressed the endocytosis in the RP, but not in the RRP. Inhibition of CaN also restrained the exocytic process triggered by 10 Hz stimulation, but had no effect on 3-5 Hz stimulation-induced exocytosis. FK506 also reduced the total vesicle pool size in the synaptic terminals. A synthesized CaN inhibitory peptide showed the similar effects as FK506 and cyclosporin A. These results revealed a novel mechanism that CaN plays critical roles in the distinct vesicle recycling processes.

The synaptic vesicle cycle

Neurotransmitter release is mediated by exocytosis of synaptic vesicles at the presynaptic active zone of nerve terminals. To support rapid and repeated rounds of release, synaptic vesicles undergo a trafficking cycle. The focal point of the vesicle cycle is Ca2+-triggered exocytosis that is followed by different routes of endocytosis and recycling. Recycling then leads to the docking and priming of the vesicles for another round of exo- and endocytosis. Recent studies have led to a better definition than previously available of how Ca2+ triggers exocytosis and how vesicles recycle. In particular, insight into how Munc18-1 collaborates with SNARE proteins in fusion, how the vesicular Ca2+ sensor synaptotagmin 1 triggers fast release, and how the vesicular Rab3 protein regulates release by binding to the active zone proteins RIM1 alpha and RIM2 alpha has advanced our understanding of neurotransmitter release. The present review attempts to relate these molecular data with physiological results in an emerging view of nerve terminals as macromolecular machines.

Protein-lipid interactions and phosphoinositide metabolism in membrane traffic: insights from vesicle recycling in nerve terminals

Great progress has been made in the elucidation of the function of proteins in membrane traffic. Less is known about the regulatory role of lipids in membrane dynamics. Studies of nerve terminals, compartments highly specialized for the recycling of synaptic vesicles, have converged with studies from other systems to reveal mechanisms in protein-lipid interactions that affect membrane shape as well as the fusion and fission of vesicles. Phosphoinositides have emerged as major regulators of the binding of cytosolic proteins to the bilayer. Phosphorylation on different positions of the inositol ring generates different isomers that are heterogeneously distributed on cell membranes and that together with membrane proteins generate a "dual keys" code for the recruitment of cytosolic proteins. This code helps controlling vectoriality of membrane transport. Powerful methods for the detection of lipids are rapidly advancing this field, thus complementing the broad range of information about biological systems that can be obtained from genomic and proteomic approaches.

The structural organization of the readily releasable pool of synaptic vesicles

The defining morphological feature of chemical synapses is the vesicle cluster in the presynaptic nerve terminal. It has generally been assumed that vesicles closest to release sites are recruited first during nerve activity. We tested this by selectively labeling the "readily releasable" pool, those vesicles released first during physiological stimulation. The readily releasable vesicles were not clustered close to the presynaptic membrane but instead were dispersed almost randomly throughout the vesicle cluster. Thus, vesicles are not recruited according to proximity to release sites but are mobilized differently, perhaps by being peeled from the surface of the cluster.

Effects of wortmannin and latrunculin A on slow endocytosis at the frog neuromuscular junction

Phosphoinositides are key regulators of synaptic vesicle cycling and endocytic traffic; the actin cytoskeleton also seems to be involved in modulating these processes. We investigated the effects of perturbing phosphoinositide signalling and actin dynamics on vesicle cycling in frog motor nerve terminals, using fluorescence and electron microscopy, and electrophysiology. Antibody staining for beta-actin revealed that actin surrounds but does not overlap with synaptic vesicle clusters. Latrunculin A, which disrupts actin filaments by binding actin monomers, and wortmannin, an inhibitor of phosphatidyl inositol-3-kinase (PI3-kinase), each disrupted the pattern of presynaptic actin staining, but not vesicle clusters in resting terminals. Latrunculin A, but not wortmannin, also reduced vesicle mobilization and exocytosis. Both drugs inhibited the stimulation-induced uptake of the styryl dye FM1-43 and blocked vesicle reformation from internalized membrane objects after tetanic stimulation. These results are consistent with a role of PI3-kinase and the actin cytoskeleton in the slow pathway of vesicle endocytosis, used primarily by reserve pool vesicles.

Actin assembly and endocytosis: from yeast to mammals

Internalization of receptors, lipids, pathogens, and other cargo at the plasma membrane involves several different pathways and requires coordinated interactions between a variety of protein and lipid molecules. The actin cytoskeleton is an integral part of the cell cortex, and there is growing evidence that F-actin plays a direct role in these endocytic events. Genetic studies in yeast have firmly established a functional connection between actin and endocytosis. Identification of several proteins that may function at the interface between actin and the endocytic machinery has provided further evidence for this association in both yeast and mammalian cells. Several of these proteins are directly involved in regulating actin assembly and could thus harness forces produced during actin polymerization to facilitate specific steps in the endocytic process. Recent microscopy studies in mammalian cells provide powerful evidence that localized recruitment and polymerization of actin occurs at endocytic sites. In this review, we focus on progress made in elucidating the functions of the actin cytoskeleton in endocytosis.

The amyloid precursor protein and its regulatory protein, FE65, in growth cones and synapses in vitro and in vivo

Although the Alzheimer amyloid protein precursor (APP) has been studied intensely for more than a decade, its function in neurons is unresolved. Much less is known about its binding partner FE65. We have shown recently that APP and FE65 synergistically regulate the movement of transfected cells. It remained to be shown whether endogenous APP and FE65 could play a similar role in vivo. Here, we show that FE65, like APP, is expressed at high levels in neurons. Using a combination of immunofluorescence, live imaging, and subcellular fractionation, we find that FE65 and APP localize in vitro and in vivo to the most motile regions of neurons, the growth cones. Within growth cones, APP and FE65 concentrate in actin-rich lamellipodia. Finally, APP and FE65 interact in nerve terminals, where they associate with Rab5-containing synaptic organelles but not with synaptic vesicles. Our data are consistent with a role for the APP/FE65 complex in regulation of actin-based membrane motility in neurons, which could be important for highly dynamic processes such as neurite growth and synapse modification.

Intersectin 1L guanine nucleotide exchange activity is regulated by adjacent src homology 3 domains that are also involved in endocytosis

Intersectin 1L is a scaffolding protein involved in endocytosis that also has guanine nucleotide exchange activity for Cdc42. In the context of the full-length protein, the catalytic exchange activity of the DH domain is repressed. Here we use biochemical methods to dissect the mechanism for this inhibition. We demonstrate that the intersectin 1L SH3 domains, which bind endocytic proteins, directly inhibit the activity of the DH domain in assays for both binding and exchange of Cdc42. This inhibitory mechanism seems to act through steric hindrance of Cdc42 binding by an intramolecular interaction between the intersectin 1L SH3 domain region and the adjacent DH domain. Surprisingly, the mode of SH3 domain binding is other than through the proline peptide binding pocket. The dual role of the SH3 domains in endocytosis and repression of exchange activity suggests that the intersectin 1L exchange activity is regulated by endocytosis. We show that the endocytic protein, dynamin, competes for binding to the SH3 domains with the neural Wiskott-Aldrich Syndrome protein, an actin filament nucleation protein that is a substrate for activated Cdc42. Swapping of SH3 domain binding partners might act as a switch controlling the actin nucleation activity of intersectin 1L.

Endocytosis and the cytoskeleton

In this review we describe the potential roles of the actin cytoskeleton in receptor-mediated endocytosis in mammalian cells and summarize the efforts of recent years in establishing a relationship between these two cellular functions. With molecules such as dynamin, syndapin, HIP1R, Abp1, synaptojanin, N-WASP, intersectin, and cortactin a set of molecular links is now available and it is likely that their further characterization will reveal the basic principles of a functional interconnection between the membrane cytoskeleton and the vesicle-budding machinery. We will therefore discuss proteins involved in endocytic clathrin coat formation and accessory factors to control and regulate coated vesicle formation but we will also focus on actin cytoskeletal components such as the Arp2/3 complex, spectrin, profilin, and motor proteins involved in actin dynamics and organization. Additionally, we will discuss how phosphoinositides, such as PI(4,5)P2, small GTPases thought to control the actin cytoskeleton, such as Rho, Rac, and Cdc42, or membrane trafficking, such as Rab GTPases and ARF proteins, and different kinases may participate in the functional connection of actin and endocytosis. We will compare the concepts and different molecular mechanisms involved in mammalian cells with yeast as well as with specialized cells, such as epithelial cells and neurons, because different model organisms often offer complementary advantages for further studies in this thriving field of current cell biological research.

CELLBIOLOGY OF THEPRESYNAPTICTERMINAL

The chemical synapse is a specialized intercellular junction that operates nearly autonomously to allow rapid, specific, and local communication between neurons. Focusing our attention on the presynaptic terminal, we review the current understanding of how synaptic morphology is maintained and then the mechanisms in synaptic vesicle exocytosis and recycling.

Bulk membrane retrieval in the synaptic terminal of retinal bipolar cells

The mechanism of bulk membrane uptake at the synapse remains poorly defined, although exocytosis of synaptic vesicles is followed by compensatory membrane retrieval into both small vesicles and large cisternas or vacuoles. We investigated bulk retrieval in the presynaptic terminal of retinal bipolar cells. Fluorescence imaging of the membrane dye FM1-43 indicated that Ca2+-triggered exocytosis was followed by endocytosis into small vesicles and larger vacuoles that could be selectively labeled using large fluorescent dextrans. Disruption of actin filaments with cytochalasin D or latrunculin B inhibited the formation and transport of vacuoles, but exocytosis and endocytosis continued at normal rates. Bulk retrieval was linked to remodeling of the actin network, and both processes were inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). The regulation of F-actin dynamics by Ca2+ and PI 3-kinase therefore played an important role in compensatory endocytosis at this synapse, but this role was confined to bulk membrane uptake. Capacitance measurements demonstrated that fast endocytosis and refilling of the rapidly releasable pool of vesicles were not dependent on F-actin or PI 3-kinase activity. The basic properties of bulk membrane retrieval at this synapse were very similar to macropinocytosis described in non-neural cells. Bulk retrieval did not play an essential role in maintaining the vesicle cycle during maintained stimulation, but we suggest that it may play a role in the structural plasticity of this synaptic terminal.

Temporal and spatial coordination of exocytosis and endocytosis

In secretory cells, exocytosis and compensatory endocytosis are tightly coupled membrane trafficking processes that control the surface area and composition of the plasma membrane. While exocytic and endocytic processes have been studied independently in great detail, at present there is much interest in understanding the mode of their coupling. This review discusses emerging insights into the coupling of these processes, both in the chemical synapses of neurons and in non-neuronal cells.

Delayed reentry of recycling vesicles into the fusion-competent synaptic vesicle pool in synaptojanin 1 knockout mice

Synaptojanin 1 is a polyphosphoinositide phosphatase implicated in synaptic vesicle recycling. We used FM1-43 imaging and electron microscopy in cultured cortical neurons from control and synaptojanin 1 knockout mice to study how the absence of this protein affects specific steps of the synaptic vesicle cycle. Exoendocytosis after a moderate stimulus was unchanged. However, during prolonged stimulation, the regeneration of fusion-competent synaptic vesicles was severely impaired. In stimulated nerve terminals, there was a persistent accumulation of clathrin-coated vesicles and a backup of newly reformed vesicles in the cytomatrix-rich area around the synaptic vesicle cluster. These findings demonstrate that synaptojanin 1 function is needed for the progression of recycling vesicles to the functional synaptic vesicle pool.

Impaired recycling of synaptic vesicles after acute perturbation of the presynaptic actin cytoskeleton

Actin is an abundant component of nerve terminals that has been implicated at multiple steps of the synaptic vesicle cycle, including reversible anchoring, exocytosis, and recycling of synaptic vesicles. In the present study we used the lamprey reticulospinal synapse to examine the role of actin at the site of synaptic vesicle recycling, the endocytic zone. Compounds interfering with actin function, including phalloidin, the catalytic subunit of Clostridium botulinum C2 toxin, and N-ethylmaleimide-treated myosin S1 fragments were microinjected into the axon. In unstimulated, phalloidin-injected axons actin filaments formed a thin cytomatrix adjacent to the plasma membrane around the synaptic vesicle cluster. The filaments proliferated after stimulation and extended toward the vesicle cluster. Synaptic vesicles were tethered along the filaments. Injection of N-ethylmaleimide-treated myosin S1 fragments caused accumulation of aggregates of synaptic vesicles between the endocytic zone and the vesicle cluster, suggesting that vesicle transport was inhibited. Phalloidin, as well as C2 toxin, also caused changes in the structure of clathrin-coated pits in stimulated synapses. Our data provide evidence for a critical role of actin in recycling of synaptic vesicles, which seems to involve functions both in endocytosis and in the transport of recycled vesicles to the synaptic vesicle cluster.

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

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

Dynamin at actin tails

Dynamin, the product of the shibire gene of Drosophila, is a GTPase critically required for endocytosis. Some studies have suggested a functional link between dynamin and the actin cytoskeleton. This link is of special interest, because there is evidence implicating actin dynamics in endocytosis. Here we show that endogenous dynamin 2, as well as green fluorescence protein fusion proteins of both dynamin 1 and 2, is present in actin comets generated by Listeria or by type I PIP kinase (PIPK) overexpression. In PIPK-induced tails, dynamin is further enriched at the interface between the tails and the moving organelles. Dynamin mutants harboring mutations in the GTPase domain inhibited nucleation of actin tails induced by PIPK and moderately reduced their speed. Although dynamin localization to the tails required its proline-rich domain, expression of a dynamin mutant lacking this domain also diminished tail formation. In addition, this mutant disrupted a membrane-associated actin scaffold (podosome rosette) previously shown to include dynamin. These findings suggest that dynamin is part of a protein network that controls nucleation of actin from membranes. At endocytic sites, dynamin may couple the fission reaction to the polymerization of an actin pool that functions in the separation of the endocytic vesicles from the plasma membrane.

Filamentous actin is concentrated in specific subpopulations of neuronal and glial structures in rat central nervous system

This paper is the second in a series of studies on the light and electron microscopic distribution of filamentous actin (F-actin) in the rat central nervous system (CNS) using phalloidin tagged with the fluorophore eosin followed by fluorescence photooxidation. A previous report described the selective localization of high concentrations of F-actin in subpopulations of dendritic spines in hippocampus, cerebellum and neostriatum. Dendritic spines were the most intensely stained structures in the CNS, but several other structures were notable for their consistent staining for F-actin. Although the majority of cell bodies, axons and large dendrites were unlabeled, mossy fibers and Schaffer collaterals in the hippocampal formation, basket cell axons in the cerebellar pinceau, and granule cell dendrites in the glomeruli of the cerebellar cortex routinely showed strong F-actin labeling. Staining was observed in all three glial cell types. Labeling was consistently observed in the astrocytic processes surrounding the Purkinje cell soma and primary dendrite. Intense but sporadic staining was observed in the perinodal glia of the Node of Ranvier. A few examples of labeled oligodendrocyte processes were also seen in the neostriatum. Labeling was observed in microglia in every brain region examined, although the labeling was present in the lumen of the endoplasmic reticulum and the nuclear membrane, leading to questions about its specificity. Perycites apposed to the blood vessels also showed very consistent labeling. Our results suggest that selected structures in the adult CNS in addition to dendritic spines are enriched in F-actin.

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

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

Endocytotic mechanisms in synapses

Nerve terminals are highly enriched in proteins needed for endocytosis. Although constitutive and ligand-stimulated endocytosis take place in nerve terminals, the primary type is compensatory endocytosis--the process by which a cell retrieves the additional membrane added to cell surface by a regulated secretory event. This process has been extensively characterized using electrophysiological techniques. Except for an unusual form of coupled exo- and endocytosis called kiss-and-run release, compensatory endocytosis appears to use basically the same clathrin-mediated mechanisms as the constitutive and ligand stimulated type. The remarkable speed and selectivity of compensatory endocytosis may be achieved by concentrating the machinery at specialized sites in the nerve terminal adjacent to exocytosis sites and by the use of neuronal isoforms of the proteins that mediate endocytosis.

Stages of synapse development defined by dependence on F-actin

It has been widely speculated that actin plays a central role in CNS synapse assembly, but such a requirement for actin filaments (F-actin) has not yet been demonstrated experimentally. We used hippocampal neurons grown in culture and the actin depolymerizing agent, latrunculin A, to examine directly the relationship between F-actin and synapse formation and maturation. During the first week in culture, actin depolymerization results in a near complete loss of synapses defined by synaptophysin-labeled vesicle clusters, synaptic vesicle recycling, and ultrastructure. Over the second week in culture, F-actin becomes increasingly stable, but actin depolymerization no longer disrupts basic synaptic structure. There is, however, a reduction in the number and size of synaptophysin-labeled clusters and in the size of vesicle clusters undergoing FM4-64 recycling, suggesting that synaptic vesicle anchoring remains partially dependent on F-actin. By 18 d in culture, synaptophysin clusters and synaptic vesicle recycling are largely resistant to F-actin depolymerization. The decrease in synapse dependence on F-actin correlates well with the acquisition and retention of presynaptic scaffolding proteins such as Bassoon and postsynaptic scaffolding proteins such as those of the postsynaptic density-95 family. Increased activity stabilizes F-actin and its associated proteins at synaptic sites, suggesting a correlation between active synapses, actin stability, and synapse stability. Our findings demonstrate that F-actin is essential for the development and maintenance of young synapses. Because F-actin is also highly regulatable, we propose that F-actin may be a principal target for stabilizing or destabilizing signals that ultimately result in synapse maintenance or elimination.

Drosophila alpha- and beta-spectrin mutations disrupt presynaptic neurotransmitter release

Spectrins are plasma membrane-associated cytoskeletal proteins implicated in several aspects of synaptic development and function, including presynaptic vesicle tethering and postsynaptic receptor aggregation. To test these hypotheses, we characterized Drosophila mutants lacking either alpha- or beta-spectrin. The Drosophila genome contains only one alpha-spectrin and one conventional beta-spectrin gene, making it an ideal system to genetically manipulate spectrin levels and examine the resulting synaptic alterations. Both spectrin proteins are strongly expressed in the Drosophila neuromusculature and highly enriched at the glutamatergic neuromuscular junction. Protein null alpha- and beta-spectrin mutants are embryonic lethal and display severely disrupted neurotransmission without altered morphological synaptogenesis. Contrary to current models, the absence of spectrins does not alter postsynaptic glutamate receptor field function or the ultrastructural localization of presynaptic vesicles. However, the subcellular localization of numerous synaptic proteins is disrupted, suggesting that the defects in presynaptic neurotransmitter release may be attributable to inappropriate assembly, transport, or localization of proteins required for synaptic function.

Phosphoinositides in membrane traffic at the synapse

Inositol phospholipids represent a minor fraction of membrane phospholipids; yet they play important regulatory functions in signaling pathways and membrane traffic. The phosphorylated inositol ring can act either as a precursor for soluble intracellular messengers or as a binding site for cytosolic or membrane proteins. Hence, phosphorylation-dephosphorylation of phosphoinositides represents a mechanism for regulation of recruitment to the membrane of coat proteins, cytoskeletal scaffolds or signaling complexes and for the regulation of membrane proteins. Recent work suggests that phosphoinositide metabolism has an important role in membrane traffic at the synapse. PtdIns(4,5)P(2) generation is implicated in the secretion of at least a subset of neurotransmitters. Furthermore, PtdIns(4,5)P(2) plays a role in the nucleation of clathrin coats and of an actin-based cytoskeletal scaffold at endocytic zones of synapses, and PtdIns(4,5)P(2) dephosphorylation accompanies the release of newly formed vesicles from these interactions. Thus, the reversible phosphorylation of inositol phospholipids may be one of the mechanisms governing the timing and vectorial progression of synaptic vesicle membranes during their exocytic-endocytic cycle.

Accessory factors in clathrin-dependent synaptic vesicle endocytosis

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