Real-time imaging of Rab3a and Rab5a reveals differential roles in presynaptic function

Maximal Fusion Capacity and Efficient Replenishment of the Dense Core Vesicle Pool in Hippocampal Neurons

Neuropeptides and neurotrophins, stored in dense core vesicles (DCVs), are together the largest currently known group of chemical signals in the brain. Exocytosis of DCVs requires high-frequency or patterned stimulation, but the determinants to reach maximal fusion capacity and for efficient replenishment of released DCVs are unknown. Here, we systematically studied fusion of DCV with single vesicle resolution on different stimulation patterns in mammalian CNS neurons. We show that tetanic stimulation trains of 50-Hz action potential (AP) bursts maximized DCV fusion, with significantly fewer fusion event during later bursts of the train. This difference was omitted by introduction of interburst intervals but did not increase total DCV fusion. Interburst intervals as short as 5 s were sufficient to restore the fusion capacity. Theta burst stimulation (TBS) triggered less DCV fusion than tetanic stimulation, but a similar fusion efficiency per AP. Prepulse stimulation did not alter this. However, low-frequency stimulation (4 Hz) intermitted with fast ripple stimulation (200 APs at 200 Hz) produced substantial DCV fusion, albeit not as much as tetanic stimulation. Finally, individual fusion events had longer durations with more intense stimulation. These data indicate that trains of 50-Hz AP stimulation patterns triggered DCV exocytosis most efficiently and more intense stimulation promotes longer DCV fusion pore openings.SIGNIFICANCE STATEMENT Neuropeptides and neurotrophins modulate multiple regulatory functions of human body like reproduction, food intake or mood. They are packed into dense core vesicles (DCVs) that undergo calcium and action potential (AP) fusion with the plasma membrane. In order to study the fusion of DCVs in vitro, techniques like perfusion with buffer containing high concentration of potassium or electric field stimulation are needed to trigger the exocytosis of DCVs. Here, we studied the relationship between DCVs fusion properties and different electric field stimulation patterns. We used six different stimulation patterns and showed that trains of 50-Hz action potential bursts triggered DCV exocytosis most efficiently and more intense stimulation promotes longer DCV fusion pore openings.

Endosomal recycling protein Rab11 in Parkin and Pink1 signaling in Drosophila model of Parkinson's disease

Neurodegenerative diseases are progressive disorders of the nervous system primarily affecting the loss of neuronal cells present in the brain. Although most neurodegenerative cases are sporadic, some familial genes are found to be involved in the neurodegenerative diseases. The extensively studied parkin and pink1 gene products are known to be involved in the removal of damaged mitochondria via autophagy (mitophagy), a quality control process. If the function of any of these genes is somehow disrupted, accumulation of damaged mitochondria occurs in the forms of protein aggregates in the cytoplasm, leading to formation of the Lewy-bodies. Autophagy is an important catabolic process where the endosomal Rab proteins are seen to be involved. Rab11, an endosomal recycling protein, serves as an ATG9A carrier that helps in autophagosome formation and maturation. Earlier studies have reported that loss of Rab11 prevents the fusion of autophagosomes with the late endosomes hampering the autophagy pathway resulting in apoptosis of cells. In this study, we have emphasized on the importance and functional role of Rab11 in the molecular pathway of Parkin/Pink1 in Parkinson's disease.

Synaptic Vesicle Recycling and the Endolysosomal System: A Reappraisal of Form and Function

The endolysosomal system is present in all cell types. Within these cells, it performs a series of essential roles, such as trafficking and sorting of membrane cargo, intracellular signaling, control of metabolism and degradation. A specific compartment within central neurons, called the presynapse, mediates inter-neuronal communication via the fusion of neurotransmitter-containing synaptic vesicles (SVs). The localized recycling of SVs and their organization into functional pools is widely assumed to be a discrete mechanism, that only intersects with the endolysosomal system at specific points. However, evidence is emerging that molecules essential for endolysosomal function also have key roles within the SV life cycle, suggesting that they form a continuum rather than being isolated processes. In this review, we summarize the evidence for key endolysosomal molecules in SV recycling and propose an alternative model for membrane trafficking at the presynapse. This includes the hypotheses that endolysosomal intermediates represent specific functional SV pools, that sorting of cargo to SVs is mediated via the endolysosomal system and that manipulation of this process can result in both plastic changes to neurotransmitter release and pathophysiology via neurodegeneration.

Rab3a, a small GTP-binding protein, is required for the stabilization of the murine leukaemia virus Gag protein

We recently identified a CD63-interacting protein to understand the role of CD63 in virion production of the human immunodeficiency virus type 1, and we have found that Rab3a forms a complex with CD63. In this study, we analysed the effect of Rab3a on virion production of the murine leukaemia virus (MLV), which is another member of the retrovirus family. We found that Rab3a silencing induced lysosomal degradation of the MLV Gag protein, and recovery of the Rab3a expression restored the level of the Gag protein through a complex formation of MLV Gag and Rab3a, indicating that Rab3a is required for MLV Gag protein expression. In contrast, CD63 silencing decreased the infectivity of released virions but had no effect on virion production, indicating that CD63 facilitates the infectivity of released MLV particles. Although Rab3a induced CD63 degradation in uninfected cells, the complex of MLV Gag and Rab3a suppressed the Rab3a-mediated CD63 degradation in MLV-infected cells. Finally, we found that the MLV Gag protein interacts with Rab3a to stabilize its own protein and CD63 that facilitates the infectivity of released MLV particles. Considering the involvement of Rab3a in lysosome trafficking to the plasma membrane, it may also induce cell surface transport of the MLV Gag protein.

Temporal Profiling of the Cortical Synaptic Mitochondrial Proteome Identifies Ageing Associated Regulators of Stability

Synapses are particularly susceptible to the effects of advancing age, and mitochondria have long been implicated as organelles contributing to this compartmental vulnerability. Despite this, the mitochondrial molecular cascades promoting age-dependent synaptic demise remain to be elucidated. Here, we sought to examine how the synaptic mitochondrial proteome (including strongly mitochondrial associated proteins) was dynamically and temporally regulated throughout ageing to determine whether alterations in the expression of individual candidates can influence synaptic stability/morphology. Proteomic profiling of wild-type mouse cortical synaptic and non-synaptic mitochondria across the lifespan revealed significant age-dependent heterogeneity between mitochondrial subpopulations, with aged organelles exhibiting unique protein expression profiles. Recapitulation of aged synaptic mitochondrial protein expression at the Drosophila neuromuscular junction has the propensity to perturb the synaptic architecture, demonstrating that temporal regulation of the mitochondrial proteome may directly modulate the stability of the synapse in vivo.

Dysfunction of RAB39B-Mediated Vesicular Trafficking in Lewy Body Diseases

Intracellular vesicular trafficking is essential for neuronal development, function, and homeostasis and serves to process, direct, and sort proteins, lipids, and other cargo throughout the cell. This intricate system of membrane trafficking between different compartments is tightly orchestrated by Ras analog in brain (RAB) GTPases and their effectors. Of the 66 members of the RAB family in humans, many have been implicated in neurodegenerative diseases and impairment of their functions contributes to cellular stress, protein aggregation, and death. Critically, RAB39B loss-of-function mutations are known to be associated with X-linked intellectual disability and with rare early-onset Parkinson's disease. Moreover, recent studies have highlighted altered RAB39B expression in idiopathic cases of several Lewy body diseases (LBDs). This review contextualizes the role of RAB proteins in LBDs and highlights the consequences of RAB39B impairment in terms of endosomal trafficking, neurite outgrowth, synaptic maturation, autophagy, as well as alpha-synuclein homeostasis. Additionally, the potential for therapeutic intervention is examined via a discussion of the recent progress towards the development of specific RAB modulators. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Molecular Insights into the Roles of Rab Proteins in Intracellular Dynamics and Neurodegenerative Diseases

In eukaryotes, the cellular functions are segregated to membrane-bound organelles. This inherently requires sorting of metabolites to membrane-limited locations. Sorting the metabolites from ribosomes to various organelles along the intracellular trafficking pathways involves several integral cellular processes, including an energy-dependent step, in which the sorting of metabolites between organelles is catalyzed by membrane-anchoring protein Rab-GTPases (Rab). They contribute to relaying the switching of the secretory proteins between hydrophobic and hydrophilic environments. The intracellular trafficking routes include exocytic and endocytic pathways. In these pathways, numerous Rab-GTPases are participating in discrete shuttling of cargoes. Long-distance trafficking of cargoes is essential for neuronal functions, and Rabs are critical for these functions, including the transport of membranes and essential proteins for the development of axons and neurites. Rabs are also the key players in exocytosis of neurotransmitters and recycling of neurotransmitter receptors. Thus, Rabs are critical for maintaining neuronal communication, as well as for normal cellular physiology. Therefore, cellular defects of Rab components involved in neural functions, which severely affect normal brain functions, can produce neurological complications, including several neurodegenerative diseases. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways of Rab proteins and the impact of their defects on different neurodegenerative diseases. The insights gathered into the dynamics of Rabs that are described in this review provide new avenues for developing effective treatments for neurodegenerative diseases-associated with Rab defects.

Rab3a-Bound CD63 Is Degraded and Rab3a-Free CD63 Is Incorporated into HIV-1 Particles

CD63, a member of the tetraspanin family, is involved in virion production by human immunodeficiency virus type 1 (HIV-1), but its mechanism is unknown. In this study, we showed that a small GTP-binding protein, Rab3a, interacts with CD63. When Rab3a was exogenously expressed, the amounts of CD63 decreased in cells. The Rab3a-mediated reduction of CD63 was suppressed by lysosomal and proteasomal inhibitors. The amount of CD63 was increased by reducing the endogenous Rab3a level using a specific shRNA. These results indicate that Rab3a binds to CD63 to induce the degradation of CD63. Rab3a is thought to be involved in exocytosis, but we found that another function of Rab3a affects the fate of CD63 in lysosomes. CD63 interacted with Rab3a and was incorporated into HIV-1 particles. However, Rab3a was not detected in HIV-1 virions, thereby indicating that Rab3a-free CD63, but not Rab3a-bound CD63, is incorporated into HIV-1 particles. Overexpression or silencing of Rab3a moderately reduced HIV-1 virion formation. Overexpression of Rab3a decreased CD63 levels, but did not affect the incorporation of CD63 into HIV-1 particles. This study showed that Rab3a binds to CD63 to induce the degradation of CD63, and only Rab3a-free CD63 is incorporated into HIV-1 particles.

Dynamic Partitioning of Synaptic Vesicle Pools by the SNARE-Binding Protein Tomosyn

Neural networks engaged in high-frequency activity rely on sustained synaptic vesicle recycling and coordinated recruitment from functionally distinct synaptic vesicle (SV) pools. However, the molecular pathways matching neural activity to SV dynamics and release requirements remain unclear. Here we identify unique roles of SNARE-binding Tomosyn1 (Tomo1) proteins as activity-dependent substrates that regulate dynamics of SV pool partitioning at rat hippocampal synapses. Our analysis is based on monitoring changes in distinct functionally defined SV pools via V-Glut1-pHluorin fluorescence in cultured hippocampal neurons in response to alterations in presynaptic protein expression. Specifically, we find knockdown of Tomo1 facilitates release efficacy from the Readily Releasable Pool (RRP), and regulates SV distribution to the Total Recycling Pool (TRP), which is matched by a decrease in the SV Resting Pool. Notably, these effects were reversed by Tomo1 rescue and overexpression. Further, we identify that these actions of Tomo1 are regulated via activity-dependent phosphorylation by cyclin-dependent kinase 5 (Cdk5). Assessment of molecular interactions that may contribute to these actions identified Tomo1 interaction with the GTP-bound state of Rab3A, an SV GTPase involved in SV targeting and presynaptic membrane tethering. In addition, Tomo1 via Rab3A-GTP was also observed to interact with Synapsin 1a/b cytoskeletal interacting proteins. Finally, our data indicate that Tomo1 regulation of SV pool sizes serves to adapt presynaptic neurotransmitter release to chronic silencing of network activity. Overall, the results establish Tomo1 proteins as central mediators in neural activity-dependent changes in SV distribution among SV pools.

SIGNIFICANCE STATEMENT

Although information transfer at central synapses via sustained high-frequency neural activity requires coordinated synaptic vesicle (SV) recycling, the mechanism(s) by which synapses sense and dynamically modify SV pools to match network demands remains poorly defined. To advance understanding, we quantified SV pool sizes and their sensitivity to neural activity while altering Tomo1 expression, a putative regulator of the presynaptic Readily Releasable Pool. Remarkably, we find Tomo1 actions to extend beyond the Readily Releasable Pool to mediate the Total Recycling Pool and SV Resting Pool distribution, and this action is sensitive to neural activity through Cdk5 phosphorylation of Tomo1. Moreover, Tomo1 appears to exert these actions through interaction with Rab3A-GTP and synapsin proteins. Together, our results argue that Tomo1 is a central mediator of SV availability for neurotransmission.

Activity-Dependent Degradation of Synaptic Vesicle Proteins Requires Rab35 and the ESCRT Pathway

Synaptic vesicle (SV) pools must maintain a functional repertoire of proteins to efficiently release neurotransmitter. The accumulation of old or damaged proteins on SV membranes is linked to synaptic dysfunction and neurodegeneration. However, despite the importance of SV protein turnover for neuronal health, the molecular mechanisms underlying this process are largely unknown. Here, we have used dissociated rat hippocampal neurons to investigate the pathway for SV protein degradation. We find that neuronal activity drives the degradation of a subset of SV proteins and that the endosomal sorting complex required for transport (ESCRT) machinery and SV-associated GTPase Rab35 are key elements of this use-dependent degradative pathway. Specifically, neuronal activity induces Rab35 activation and binding to the ESCRT-0 protein Hrs, which we have identified as a novel Rab35 effector. These actions recruit the downstream ESCRT machinery to SV pools, thereby initiating SV protein degradation via the ESCRT pathway. Our findings show that the Rab35/ESCRT pathway facilitates the activity-dependent removal of specific proteins from SV pools, thereby maintaining presynaptic protein homeostasis.

SIGNIFICANCE STATEMENT

Synaptic transmission is mediated by the release of chemical neurotransmitters from synaptic vesicles (SVs). This tightly regulated process requires a functional pool of SVs, necessitating cellular mechanisms for removing old or damaged proteins that could impair SV cycling. Here, we show that a subset of SV proteins is degraded in an activity-dependent manner and that key steps in this degradative pathway are the activation of the small GTPase Rab35 and the subsequent recruitment of the endosomal sorting complex required for transport (ESCRT) machinery to SV pools. Further, we demonstrate that ESCRT-0 component Hrs is an effector of Rab35, thus providing novel mechanistic insight into the coupling of neuronal activity with SV protein degradation and the maintenance of functional SV pools.

Hypertension-induced synapse loss and impairment in synaptic plasticity in the mouse hippocampus mimics the aging phenotype: implications for the pathogenesis of vascular cognitive impairment

Strong epidemiological and experimental evidence indicates that hypertension has detrimental effects on the cerebral microcirculation and thereby promotes accelerated brain aging. Hypertension is an independent risk factor for both vascular cognitive impairment (VCI) and Alzheimer's disease (AD). However, the pathophysiological link between hypertension-induced cerebromicrovascular injury (e.g., blood-brain barrier disruption, increased microvascular oxidative stress, and inflammation) and cognitive decline remains elusive. The present study was designed to characterize neuronal functional and morphological alterations induced by chronic hypertension and compare them to those induced by aging. To achieve that goal, we induced hypertension in young C57BL/6 mice by chronic (4 weeks) infusion of angiotensin II. We found that long-term potentiation (LTP) of performant path synapses following high-frequency stimulation of afferent fibers was decreased in hippocampal slices obtained from hypertensive mice, mimicking the aging phenotype. Hypertension and advanced age were associated with comparable decline in synaptic density in the stratum radiatum of the mouse hippocampus. Hypertension, similar to aging, was associated with changes in mRNA expression of several genes involved in regulation of neuronal function, including down-regulation of Bdnf, Homer1, and Dlg4, which may have a role in impaired synaptic plasticity. Collectively, hypertension impairs synaptic plasticity, reduces synaptic density, and promotes dysregulation of genes involved in synaptic function in the mouse hippocampus mimicking the aging phenotype. These hypertension-induced neuronal alterations may impair establishment of memories in the hippocampus and contribute to the pathogenesis and clinical manifestation of both vascular cognitive impairment (VCI) and Alzheimer's disease (AD).

Critical importance of RAB proteins for synaptic function

Neurons are highly polarized cells that exhibit one of the more complex morphology and function. Neuronal intracellular trafficking plays a key role in dictating the directionality and specificity of vesicle formation, transport and fusion, allowing the transmission of information in sophisticate cellular network. Thus, the integrity of protein trafficking and spatial organization is especially important in neuronal cells. RAB proteins, small monomeric GTPases belonging to the RAS superfamily, spatially and temporally orchestrate specific vesicular trafficking steps.

In this review we summarise the known roles of RAB GTPases involved in the maintenance of neuronal vesicular trafficking in the central nervous system. In particular, we discriminate the axonal pre-synaptic trafficking and dendritic post-synaptic trafficking, to better underlie how a correct orchestration of vesicle movement is necessary to maintain neuronal polarity and then, to permit an accurate architecture and functionality of synaptic activity.

Molecular Machines Determining the Fate of Endocytosed Synaptic Vesicles in Nerve Terminals

The cycle of a synaptic vesicle (SV) within the nerve terminal is a step-by-step journey with the final goal of ensuring the proper synaptic strength under changing environmental conditions. The SV cycle is a precisely regulated membrane traffic event in cells and, because of this, a plethora of membrane-bound and cytosolic proteins are devoted to assist SVs in each step of the journey. The cycling fate of endocytosed SVs determines both the availability for subsequent rounds of release and the lifetime of SVs in the terminal and is therefore crucial for synaptic function and plasticity. Molecular players that determine the destiny of SVs in nerve terminals after a round of exo-endocytosis are largely unknown. Here we review the functional role in SV fate of phosphorylation/dephosphorylation of SV proteins and of small GTPases acting on membrane trafficking at the synapse, as they are emerging as key molecules in determining the recycling route of SVs within the nerve terminal. In particular, we focus on: (i) the cyclin-dependent kinase-5 (cdk5) and calcineurin (CN) control of the recycling pool of SVs; (ii) the role of small GTPases of the Rab and ADP-ribosylation factor (Arf) families in defining the route followed by SV in their nerve terminal cycle. These regulatory proteins together with their synaptic regulators and effectors, are molecular nanomachines mediating homeostatic responses in synaptic plasticity and potential targets of drugs modulating the efficiency of synaptic transmission.

Distinct yet overlapping roles of Rab GTPases on synaptic vesicles

Exo-endocytotic cycling of synaptic vesicles (SVs) is one of the most intensely studied membrane trafficking pathways. It is governed by sets of conserved proteins including Rab GTPases. Long considered to define the identity and composition of a subcellular organelle, it has become increasingly evident that multiple Rabs co-exist on intracellular compartments, each contributing to its membrane organization and specialised function. Indeed, we have recently demonstrated that at least 11 distinct Rab proteins co-exist on highly purified SVs. These include Rabs involved in exocytosis (Rab3a/b/c and Rab27b) and intermediates of SV recycling such as early endosomes (Rab4, Rab5, Rab10, Rab11b and Rab14). Interestingly, we found that while two of these proteins, namely Rab3a and Rab27b, exhibited differential cycling dynamics on SV membranes; they played complementary roles during Ca(2+)-triggered neurotransmitter release. The implications of these findings in the SV trafficking cycle are discussed.

The role of rab proteins in neuronal cells and in the trafficking of neurotrophin receptors

Neurotrophins are a family of proteins that are important for neuronal development, neuronal survival and neuronal functions. Neurotrophins exert their role by binding to their receptors, the Trk family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to receptors triggers a complex series of signal transduction events, which are able to induce neuronal differentiation but are also responsible for neuronal maintenance and neuronal functions. Rab proteins are small GTPases localized to the cytosolic surface of specific intracellular compartments and are involved in controlling vesicular transport. Rab proteins, acting as master regulators of the membrane trafficking network, play a central role in both trafficking and signaling pathways of neurotrophin receptors. Axonal transport represents the Achilles' heel of neurons, due to the long-range distance that molecules, organelles and, in particular, neurotrophin-receptor complexes have to cover. Indeed, alterations of axonal transport and, specifically, of axonal trafficking of neurotrophin receptors are responsible for several human neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis and some forms of Charcot-Marie-Tooth disease. In this review, we will discuss the link between Rab proteins and neurotrophin receptor trafficking and their influence on downstream signaling pathways.

Synaptic Vesicle Endocytosis and Endosomal Recycling in Central Nerve Terminals: Discrete Trafficking Routes?

Synaptic vesicle (SV) retrieval from the presynaptic plasma membrane occurs via a variety of different and complementary modes. The dominant retrieval mode during high-intensity stimulation is activity-dependent bulk endocytosis (ADBE). ADBE involves the generation of endosomes direct from the plasma membrane which then donate membrane and cargo to form SVs that replenish the reserve SV pool. Recent evidence has suggested that ADBE may involve an additional endosomal processing step to produce a mature, functional SV. This suggests that ADBE may utilize key molecules or indeed whole pathways from classical endocytic recycling routes that are ubiquitous across all cell types. This review will assess the current evidence for a contribution of endocytic recycling to the SV life cycle, with a particular focus on ADBE. In doing so it highlights points where both routes may either converge or exploit existing mechanisms to ensure efficient generation of SVs during high-intensity stimulation.

Munc18-1 redistributes in nerve terminals in an activity- and PKC-dependent manner

PKC-dependent dynamic control of Munc18-1 levels enables individual synapses to tune their output during periods of activity.

Munc18-1 is a soluble protein essential for synaptic transmission. To investigate the dynamics of endogenous Munc18-1 in neurons, we created a mouse model expressing fluorescently tagged Munc18-1 from the endogenous munc18-1 locus. We show using fluorescence recovery after photobleaching in hippocampal neurons that the majority of Munc18-1 trafficked through axons and targeted to synapses via lateral diffusion together with syntaxin-1. Munc18-1 was strongly expressed at presynaptic terminals, with individual synapses showing a large variation in expression. Axon–synapse exchange rates of Munc18-1 were high: during stimulation, Munc18-1 rapidly dispersed from synapses and reclustered within minutes. Munc18-1 reclustering was independent of syntaxin-1, but required calcium influx and protein kinase C (PKC) activity. Importantly, a PKC-insensitive Munc18-1 mutant did not recluster. We show that synaptic Munc18-1 levels correlate with synaptic strength, and that synapses that recruit more Munc18-1 after stimulation have a larger releasable vesicle pool. Hence, PKC-dependent dynamic control of Munc18-1 levels enables individual synapses to tune their output during periods of activity.

Presynaptic and Postsynaptic Scaffolds

The development of methods to follow the dynamics of synaptic molecules in living neurons has radically altered our view of the synapse, from that of a generally static structure to that of a dynamic molecular assembly at steady state. This view holds not only for relatively labile synaptic components, such as synaptic vesicles, cytoskeletal elements, and neurotransmitter receptors, but also for the numerous synaptic molecules known as scaffolding molecules, a generic name for a diverse class of molecules that organize synaptic function in time and space. Recent studies reveal that these molecules, which confer a degree of stability to synaptic assemblies over time scales of hours and days, are themselves subject to significant dynamics. Furthermore, these dynamics are probably not without effect; wherever studied, these seem to be associated with spontaneous changes in scaffold molecule content, synaptic size, and possibly synaptic function. This review describes the dynamics exhibited by synaptic scaffold molecules, their typical time scales, and the potential implications to our understanding of synaptic function.

Presynaptic membrane retrieval and endosome biology: defining molecularly heterogeneous synaptic vesicles

The release and uptake of neurotransmitters by synaptic vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of synaptic vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse synaptic vesicle populations and the adaptor-dependent sorting machineries capable of generating vesicle diversity. We argue that there are various synaptic vesicle recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in synaptic vesicle recycling.

Commissureless regulation of axon outgrowth across the midline is independent of Rab function

Nervous system function requires that neurons within neural circuits are connected together precisely. These connections form during the process of axon guidance whereby each neuron extends an axon that migrates, often large distances, through a complex environment to reach its synaptic target. This task can be simplified by utilising intermediate targets to divide the route into smaller sections. This requires that axons adapt their behaviour as they migrate towards and away from intermediate targets. In the central nervous system the midline acts as an intermediate target for commissural axons. In Drosophila commissural axons switch from attraction towards to extension away from the midline by regulating the levels of the Roundabout receptor on their cell surface. This is achieved by Commissureless which directs Roundabout to an intracellular compartment in the soma prior to reaching the midline. Once across the midline Roundabout is allowed to reach the surface and acts as a receptor for the repellent ligand Slit that is secreted by cells at the midline. Here we investigated candidate intracellular mechanisms that may facilitate the intracellular targeting of Commissureless and Roundabout within the soma of commissural neurons. Using modified forms of Commissureless or Rabs we show that neither ubiquitination nor Rab activity are necessary for the intracellular targeting of Commissureless. In addition we reveal that axon outgrowth of many populations of neurons within the Drosophila central nervous system is also independent of Rab activity.

Melittin initiates dopamine transporter internalization and recycling in transfected HEK-293 cells

The dopamine transporter removes the neurotransmitter from the synapse, regulating dopamine availability. The transporter can be internalized and its function is blocked by cocaine and other ligands. Melittin inhibits dopamine transporter function and causes internalization of the recombinant transporter in stably transfected HEK-293 cells, but the specific pathways for internalization and disposition of the transporter are unknown. Here we report that melittin treatment increased both transporter internalization and colocalization with clathrin, effects that were blocked by pretreatment with cocaine. Density gradient centrifugation revealed that melittin treatment caused the dopamine transporter to associate with a density fraction containing the early endosome marker Rab 5A. Confocal microscopy revealed that melittin treatment also increased transporter colocalization with Rab 5A and decreased colocalization with the late endosome marker Rab 7 and the recycling endosome marker Rab 11. Following 60 min of melittin treatment, the transporter was trafficked back to the membrane. By comparison, phorbol ester treatment increased transporter colocalization with early endosome antigen 1 and Rab 7 in a time-dependent manner. Cocaine treatment alone does not affect transporter trafficking in these cells. Results indicate multiple dopamine transporter internalization and recycling pathways that depend on transporter-ligand interactions and post-translational modifications.

Rabx-5 regulates RAB-5 early endosomal compartments and synaptic vesicles in C. elegans

Early endosomal membrane compartments are required for the formation and recycling of synaptic vesicles, but how these compartments are regulated is incompletely understood. We performed a forward genetic screen in C. elegans for mutations that affect RAB-5 labeled early endosomal compartments in GABAergic motoneurons. Here we report the isolation and characterization of one mutation, rabx-5. The rabx-5 mutation leads to decreased intensity of YFP::RAB-5 in the cell soma but increased intensity in the synaptic and intersynaptic regions of the axon. This effect is due to the bias of the cycling state of RAB-5, and results from a change in the organization of the early endosomal compartment as well as the membrane binding state of RAB-5. Synaptic vesicle accumulation is altered in rabx-5 mutants, and synaptic transmission from cholinergic neurons is decreased. Early endosomal membrane compartments show disorganization with ageing and rabx-5 mutant animals age faster. These results suggest that rabx-5 regulation of RAB-5 compartments is important for maintaining proper synaptic function throughout the lifetime.

Use dependence of presynaptic tenacity

Recent studies indicate that synaptic vesicles (SVs) are continuously interchanged among nearby synapses at very significant rates. These dynamics and the lack of obvious barriers confining synaptic vesicles to specific synapses would seem to challenge the ability of synapses to maintain a constant amount of synaptic vesicles over prolonged time scales. Moreover, the extensive mobilization of synaptic vesicles associated with presynaptic activity might be expected to intensify this challenge. Here we examined the ability of individual presynaptic boutons of rat hippocampal neurons to maintain their synaptic vesicle content, and the degree to which this ability is affected by continuous activity. We found that the synaptic vesicle content of individual boutons belonging to the same axons gradually changed over several hours, and that these changes occurred independently of activity. Intermittent stimulation for 1 h accelerated rates of vesicle pool size change. Interestingly, however, following stimulation cessation, vesicle pool size change rates gradually converged with basal change rates. Over similar time scales, active zones (AZs) exhibited substantial remodeling; yet, unlike synaptic vesicles, AZ remodeling was not affected by the stimulation paradigms used here. These findings indicate that enhanced activity levels can increase synaptic vesicle redistribution among nearby synapses, but also highlight the presence of forces that act to restore particular set points in terms of SV contents, and support a role for active zones in preserving such set points. These findings also indicate, however, that neither AZ size nor SV content set points are particularly stable, questioning the long-term tenacity of presynaptic specializations.

Extrasynaptic vesicle recycling in mature hippocampal neurons

Fast neuronal signalling relies on highly regulated vesicle fusion and recycling at specialized presynaptic terminals. Recently, examples of non-classical neurotransmission have also been reported, where fusion of vesicles can occur at sites remote from conventional synapses. This has potentially broad biological implications, but the underlying mechanisms are not well established. Here we show that a complete vesicle recycling pathway can occur at discrete axonal sites in mature hippocampal neurons and that extrasynaptic fusion is a robust feature of native tissue. We demonstrate that laterally mobile vesicle clusters trafficking between synaptic terminals become transiently stabilized by evoked action potentials and undergo complete but delayed Ca(2+)-dependent fusion along axons. This fusion is associated with dynamic actin accumulation and, subsequently, vesicles can be locally recycled, re-acidified and re-used. Immunofluorescence and ultrastructural work demonstrates that extrasynaptic fusion sites can have apposed postsynaptic specializations, suggesting that mobile vesicle recycling may underlie highly dynamic neuron-neuron communication.

Overlapping role of dynamin isoforms in synaptic vesicle endocytosis

The existence of neuron-specific endocytic protein isoforms raises questions about their importance for specialized neuronal functions. Dynamin, a GTPase implicated in the fission reaction of endocytosis, is encoded by three genes, two of which, dynamin 1 and 3, are highly expressed in neurons. We show that dynamin 3, thought to play a predominantly postsynaptic role, has a major presynaptic function. Although lack of dynamin 3 does not produce an overt phenotype in mice, it worsens the dynamin 1 KO phenotype, leading to perinatal lethality and a more severe defect in activity-dependent synaptic vesicle endocytosis. Thus, dynamin 1 and 3, which together account for the overwhelming majority of brain dynamin, cooperate in supporting optimal rates of synaptic vesicle endocytosis. Persistence of synaptic transmission in their absence indicates that if dynamin plays essential functions in neurons, such functions can be achieved by the very low levels of dynamin 2.

Myosin5a tail associates directly with Rab3A-containing compartments in neurons

Myosin-Va (Myo5a) is a motor protein associated with synaptic vesicles (SVs) but the mechanism by which it interacts has not yet been identified. A potential class of binding partners are Rab GTPases and Rab3A is known to associate with SVs and is involved in SV trafficking. We performed experiments to determine whether Rab3A interacts with Myo5a and whether it is required for transport of neuronal vesicles. In vitro motility assays performed with axoplasm from the squid giant axon showed a requirement for a Rab GTPase in Myo5a-dependent vesicle transport. Furthermore, mouse recombinant Myo5a tail revealed that it associated with Rab3A in rat brain synaptosomal preparations in vitro and the association was confirmed by immunofluorescence imaging of primary neurons isolated from the frontal cortex of mouse brains. Synaptosomal Rab3A was retained on recombinant GST-tagged Myo5a tail affinity columns in a GTP-dependent manner. Finally, the direct interaction of Myo5a and Rab3A was determined by sedimentation velocity analytical ultracentrifugation using recombinant mouse Myo5a tail and human Rab3A. When both proteins were incubated in the presence of 1 mm GTPγS, Myo5a tail and Rab3A formed a complex and a direct interaction was observed. Further analysis revealed that GTP-bound Rab3A interacts with both the monomeric and dimeric species of the Myo5a tail. However, the interaction between Myo5a tail and nucleotide-free Rab3A did not occur. Thus, our results show that Myo5a and Rab3A are direct binding partners and interact on SVs and that the Myo5a/Rab3A complex is involved in transport of neuronal vesicles.

Quantitative analysis of synaptic vesicle Rabs uncovers distinct yet overlapping roles for Rab3a and Rab27b in Ca2+-triggered exocytosis

Rab GTPases are molecular switches that orchestrate protein complexes before membrane fusion reactions. In synapses, Rab3 and Rab5 proteins have been implicated in the exo-endocytic cycling of synaptic vesicles (SVs), but an involvement of additional Rabs cannot be excluded. Here, combining high-resolution mass spectrometry and chemical labeling (iTRAQ) together with quantitative immunoblotting and fluorescence microscopy, we have determined the exocytotic (Rab3a, Rab3b, Rab3c, and Rab27b) and endocytic (Rab4b, Rab5a/b, Rab10, Rab11b, and Rab14) Rab machinery of SVs. Analysis of two closely related proteins, Rab3a and Rab27b, revealed colocalization in synaptic nerve terminals, where they reside on distinct but overlapping SV pools. Moreover, whereas Rab3a readily dissociates from SVs during Ca(2+)-triggered exocytosis, and is susceptible to membrane extraction by Rab-GDI, Rab27b persists on SV membranes upon stimulation and is resistant to GDI-coupled Rab retrieval. Finally, we demonstrate that selective modulation of the GTP/GDP switch mechanism of Rab27b impairs SV recycling, suggesting that Rab27b, probably in concert with Rab3s, is involved in SV exocytosis.

Sharing vesicles between central presynaptic terminals: implications for synaptic function

Presynaptic terminals in hippocampal neurons house functionally distinct vesicle pools, the size, structure and biochemical features of which are major determinants of presynaptic strength and performance. In classical models of synaptic function these vesicle pools are synapse-specific, but accumulating evidence is now demonstrating that some vesicles are laterally mobile along axons and readily shared in a functional manner across adjacent terminals. In effect then, these mobile vesicles represent a further class of synapse-spanning vesicle pool, or "superpool". Here we outline the characteristics of this additional pool type, discussing its structural organization within axons and presynaptic terminals as well as its relationship with conventional vesicle pools. We draw comparisons between extrasynaptic vesicle dynamics and the growing literature on extrasynaptic mobility of non-vesicular synaptic elements which, taken together, raise important questions about the operational independence of adjacent release sites. We also examine the functional implications of lateral vesicle sharing, from the notion that extrasynaptic vesicles can contribute to the release capabilities of individual terminals, to its potential role as a substrate for facilitating changes in synaptic weight as a basis for plasticity.

A vesicle superpool spans multiple presynaptic terminals in hippocampal neurons

Synapse-specific vesicle pools have been widely characterized at central terminals. Here, we demonstrate a vesicle pool that is not confined to a synapse but spans multiple terminals. Using fluorescence imaging, correlative electron microscopy, and modeling of vesicle dynamics, we show that some recycling pool vesicles at synapses form part of a larger vesicle "superpool." The vesicles within this superpool are highly mobile and are rapidly exchanged between terminals (turnover: approximately 4% of total pool/min), significantly changing vesicular composition at synapses over time. In acute hippocampal slices we show that the mobile vesicle pool is also a feature of native brain tissue. We also demonstrate that superpool vesicles are available to synapses during stimulation, providing an extension of the classical recycling pool. Experiments using focal BDNF application suggest the involvement of a local TrkB-receptor-dependent mechanism for synapse-specific regulation of presynaptic vesicle pools through control of vesicle release and capture to or from the extrasynaptic pool.

EphA4 is localized in clathrin-coated and synaptic vesicles in adult mouse brain

EphA4, a receptor tyrosine kinase, is expressed in various pre-, post- and peri-synaptic organelles and implicated in the regulation of morphological and physiological properties of synapses. It regulates synaptic plasticity by acting as a binding partner for glial ephrin-A3 and possibly other pre- or post-synaptic ephrins. Now, its trafficking mechanisms remain unknown. In this study, we examine the association of EphA4 with transport, clathrin-coated and synaptic vesicles using cell fractionation, vesicle immunoisolation and electron microscopy. EphA4 was found in highly purified fractions of clathrin-coated or synaptic vesicles. It was also detected in vesicles immuno-isolated with antibodies anti-synaptophysin, anti-vesicular glutamate transporter or anti-vesicular GABA transporter; demonstrating its presence in synaptic vesicles. However, it was not detected in immuno-isolated piccolo-bassoon transport vesicles. In vivo and in dissociated cultures, EphA4 was localized by immunoelectron microscopy in vesicular glutamate transporter 1-positive terminals of hippocampal neurons. Remarkably, the cell surface immunofluorescence of EphA4 increased markedly in cultured hippocampal neurons following KCl depolarization. These observations indicate that EphA4 is present in subsets of synaptic vesicles, can be externalized during depolarization, and internalized within clathrin-coated vesicles. This trafficking itinerary may serve to regulate the levels of EphA4 in the synaptic plasma membrane and thereby modulate signaling events that contribute to synaptic plasticity.

APP anterograde transport requires Rab3A GTPase activity for assembly of the transport vesicle

The amyloid precursor protein (APP) is anterogradely transported by conventional kinesin in a distinct transport vesicle, but both the biochemical composition of such a vesicle and the specific kinesin-1 motor responsible for transport are poorly defined. APP may be sequentially cleaved by beta- and gamma-secretases leading to accumulation of beta-amyloid (Abeta) peptides in brains of Alzheimer's disease patients, whereas cleavage of APP by alpha-secretases prevents Abeta generation. Here, we demonstrate by time-lapse analysis and immunoisolations that APP is a cargo of a vesicle containing the kinesin heavy chain isoform kinesin-1C, the small GTPase Rab3A, and a specific subset of presynaptic protein components. Moreover, we report that assembly of kinesin-1C and APP in this vesicle type requires Rab3A GTPase activity. Finally, we show cleavage of APP in transport vesicles by alpha-secretase activity, likely mediated by ADAM10. Together, these data indicate that maturation of APP transport vesicles, including recruitment of conventional kinesin, requires Rab3 GTPase activity.

UNC-108/RAB-2 and its effector RIC-19 are involved in dense core vesicle maturation in Caenorhabditis elegans

Uncoordinated movement in Rab2 mutants is caused by impaired retention of cargo on dense core vesicles, not by defective synaptic vesicle release. (Also see the companion article by Edwards et al. in this issue.)

Small guanosine triphosphatases of the Rab family regulate intracellular vesicular trafficking. Rab2 is highly expressed in the nervous system, yet its function in neurons is unknown. In Caenorhabditis elegans, unc-108/rab-2 mutants have been isolated based on their locomotory defects. We show that the locomotion defects of rab-2 mutants are not caused by defects in synaptic vesicle release but by defects in dense core vesicle (DCV) signaling. DCVs in rab-2 mutants are often enlarged and heterogeneous in size; however, their number and distribution are not affected. This implicates Rab2 in the biogenesis of DCVs at the Golgi complex. We demonstrate that Rab2 is required to prevent DCV cargo from inappropriately entering late endosomal compartments during DCV maturation. Finally, we show that RIC-19, the C. elegans orthologue of the human diabetes autoantigen ICA69, is also involved in DCV maturation and is recruited to Golgi membranes by activated RAB-2. Thus, we propose that RAB-2 and its effector RIC-19 are required for neuronal DCV maturation.

Roles of endosomal trafficking in neurite outgrowth and guidance

Membrane trafficking and cargo delivery are essential for axonal and dendritic growth and guidance. Neurons have numerous diverse post-Golgi vesicles and recent advances have clarified their identity and regulation. Combinatorial approaches using in vivo imaging of 'intracellular cargo address labels' and functional perturbation have provided insight into these processes. In particular, the UNC-51 kinase regulates the trafficking of early endosomes and their axon guidance molecular cargos in several types of neurons in multiple organisms. Vesicular compartments bearing features of recycling endosomes, late endosomes or lysosomes also contribute to membrane addition and protein trafficking during neurite outgrowth and extension. New work shows that ubiquitylation of cargos and Rab effectors further specifies the trafficking routes of post-Golgi vesicles. These findings have begun to provide a more detailed view of the molecular mechanisms involved in neurite outgrowth and guidance. Additionally, high-resolution light microscopy imaging promises greater temporal and spatial understanding of vesicular exchange and maturation in neurons in the near future.

The JIP3 scaffold protein UNC-16 regulates RAB-5 dependent membrane trafficking at C. elegans synapses

How endosomes contribute to the maintenance of vesicular structures at presynaptic terminals remains controversial and poorly understood. Here, we have investigated synaptic endosomal compartments in the presynaptic terminals of C. elegans GABAergic motor neurons. Using RAB reporters, we find that several subsynaptic compartments reside in, or near, presynaptic regions. Loss of function in the C. elegans JIP3 protein, UNC-16, causes a RAB-5-containing compartment to accumulate abnormally at presynaptic terminals. Ultrastructural analysis shows that synapses in unc-16 mutants contain reduced number of synaptic vesicles, accompanied by an increase in the size and number of cisternae. FRAP analysis revealed a slow recovery of RAB-5 in unc-16 mutants, suggestive of an impairment of RAB-5 activity state and local vesicular trafficking. Overexpression of RAB-5:GDP partially suppresses, whereas overexpression of RAB-5:GTP enhances, the synaptic defects of unc-16 mutants. Our data demonstrate a novel function of UNC-16 in the regulation of synaptic membrane trafficking and suggest that the synaptic RAB-5 compartment contributes to synaptic vesicle biogenesis or maintenance.

Exchange and redistribution dynamics of the cytoskeleton of the active zone molecule bassoon

Presynaptic sites typically appear as varicosities (boutons) distributed along axons. Ultrastructurally, presynaptic boutons lack obvious physical barriers that separate them from the axon proper, yet activity-related and constitutive dynamics continuously promote the "reshuffling" of presynaptic components and even their dispersal into flanking axonal segments. How presynaptic sites manage to maintain their organization and individual characteristics over long durations is thus unclear. Conceivably, presynaptic tenacity might depend on the active zone (AZ), an electron-dense specialization of the presynaptic membrane, and particularly on the cytoskeletal matrix associated with the AZ (CAZ) that could act as a relatively stable "core scaffold" that conserves and dictates presynaptic organization. At present, however, little is known on the molecular dynamics of CAZ molecules, and thus, the factual basis for this hypothesis remains unclear. To examine the stability of the CAZ, we studied the molecular dynamics of the major CAZ molecule Bassoon in cultured hippocampal neurons. Fluorescence recovery after photobleaching and photoactivation experiments revealed that exchange rates of green fluorescent protein and photoactivatable green fluorescent protein-tagged Bassoon at individual presynaptic sites are very low (tau > 8 h). Exchange rates varied between boutons and were only slightly accelerated by stimulation. Interestingly, photoactivation experiments revealed that Bassoon lost from one synapse was occasionally assimilated into neighboring presynaptic sites. Our findings indicate that Bassoon is engaged in relatively stable associations within the CAZ and thus support the notion that the CAZ or some of its components might constitute a relatively stable presynaptic core scaffold.

ApRab3, a biosynthetic Rab protein, accumulates on the maturing phagosomes and symbiosomes in the tropical sea anemone, Aiptasia pulchella

Symbiosome biogenesis and function are central to the endosymbiotic interaction between symbiotic dinoflagellates and their host cnidarians. To understand these important organelles, we have been conducting studies to identify and characterize symbiosome-associated proteins of the Rab family, key regulatory components of vesicular trafficking and membrane fusion in eukaryotic cells. Our prior studies have implicated three endocytic Rab proteins in the regulation of symbiosome biogenesis. Here, we show that ApRab3 is a new member of the Rab3 subfamily, associating with symbiosomes and accumulating on the maturing phagosomes in the A. pulchella digestive cells. ApRab3 is 78% identical to human Rab3C, and contains all Rab 3-specific signature motifs. EGFP-ApRab3-labeled vesicular structures tended to either align along the cell peripheral, or aggregate at one side of the nucleus. ApRab3 specifically co-distributed with the TGN marker, WGA, but not other organelle-specific markers tested. Immunofluorescence staining with a specific peptide antibody showed similar results. Significantly, an expression of a constitutively active mutant caused the enlargement and random dispersion of EGFP-ApRab3-decorated compartments in PC12 cells. Together, these data suggest that ApRab3 is a new member of the Rab3 subfamily, participating in the biosynthetic trafficking pathway, and symbiosome biogenesis involves an interaction with ApRab3-positive vesicles.

Cognitive impairment in Gdi1-deficient mice is associated with altered synaptic vesicle pools and short-term synaptic plasticity, and can be corrected by appropriate learning training

The GDI1 gene, responsible in human for X-linked non-specific mental retardation, encodes alphaGDI, a regulatory protein common to all GTPases of the Rab family. Its alteration, leading to membrane accumulation of different Rab GTPases, may affect multiple steps in neuronal intracellular traffic. Using electron microscopy and electrophysiology, we now report that lack of alphaGDI impairs several steps in synaptic vesicle (SV) biogenesis and recycling in the hippocampus. Alteration of the SV reserve pool (RP) and a 50% reduction in the total number of SV in adult synapses may be dependent on a defective endosomal-dependent recycling and may lead to the observed alterations in short-term plasticity. As predicted by the synaptic characteristics of the mutant mice, the short-term memory deficit, observed when using fear-conditioning protocols with short intervals between trials, disappeared when the Gdi1 mutants were allowed to have longer intervals between sessions. Likewise, previously observed deficits in radial maze learning could be corrected by providing less challenging pre-training. This implies that an intact RP of SVs is necessary for memory processing under challenging conditions in mice. The possibility to correct the learning deficit in mice may have clinical implication for future studies in human.

Loss of RAB-3/A in Caenorhabditis elegans and the mouse affects behavioral response to ethanol

The mechanisms by which ethanol induces changes in behavior are not well understood. Here, we show that Caenorhabditis elegans loss-of-function mutations in the synaptic vesicle-associated RAB-3 protein and its guanosine triphosphate exchange factor AEX-3 confer resistance to the acute locomotor effects of ethanol. Similarly, mice lacking one or both copies of Rab3A are resistant to the ataxic and sedative effects of ethanol, and Rab3A haploinsufficiency increases voluntary ethanol consumption. These data suggest a conserved role of RAB-3-/RAB3A-regulated neurotransmitter release in ethanol-related behaviors.

Rab3a deletion reduces vesicle docking and transmitter release at the mouse diaphragm synapse

Rab3a is a small GTP binding protein associated with presynaptic vesicles that is thought to regulate vesicle targeting to active zones. Although this rab3a function implies that vesicle docking and action potential-evoked release might be inhibited in rab3a gene-deleted synapses, such inhibition has never been demonstrated. To investigate vesicle docking at the neuromuscular junction of rab3a gene-deleted (rab3a(-)) mice, we performed electron microscopy analysis of the diaphragm slow-fatigue (type I) synapses. We found a significant (26%) reduction in the number of vesicles docked to the presynaptic membrane in rab3a(-) terminals, although intraterminal vesicles were not affected. Aiming to detect possible changes in quantal release due to rab3a gene deletion, we minimized the variability between preparations employing focal recordings of synaptic responses from visualized type I endplates. We found a significant decrease in both evoked (27% reduction in quantal content) and spontaneous (28% reduction in mini frequency) quantal release. The decrease in the evoked release produced by rab3a deletion was most pronounced at reduced extracellular Ca(2+) concentrations (over 50% decrease at 0.5 and 0.2 mM Ca(2+)). By manipulating extracellular calcium, we demonstrated that calcium cooperativity is not altered in rab3a(-) synapses, however calcium sensitivity of quantal release is affected. Thus, we demonstrated that rab3a positively regulates docking and basal quantal release at the mouse neuromuscular junction. This result is consistent with the proposed role of rab3a in trafficking and targeting vesicles to the active zones.

The role of Rab3a in secretory vesicle docking requires association/dissociation of guanidine phosphates and Munc18-1

Rab3a is a small GTPase that binds selectively to secretory vesicles and switches between active, GTP-bound and inactive, GDP-bound conformations. In yeast, Rab and SM-genes interact genetically to promote vesicle targeting/fusion. We tested different Rab3a conformations and genetic interactions with the SM-gene munc18-1 on the docking function of Rab3a in mammalian chromaffin cells. We expressed Rab3a mutants locked in the GTP- or GDP-bound form in wild-type and munc18-1 null mutant cells and analyzed secretory vesicle distribution. We confirmed that wild-type Rab3a promotes vesicle docking in wild-type cells. Unexpectedly, both GTP- and GDP-locked Rab3a mutants did not promote docking. Furthermore, wild-type Rab3a did not promote docking in munc18-1 null cells and GTP- and GDP-Rab3a both decreased the amount of docked vesicles. The results show that GTP- and GDP-locked conformations do not support a Munc18-1 dependent role of Rab3a in docking. This suggests that nucleotide cycling is required to support docking and that this action of Rab3a is upstream of Munc18-1.

Share and share alike: trading of presynaptic elements between central synapses

Central presynaptic terminals harbour synaptic vesicles (SVs) and synapse-specific proteins necessary for neurotransmission. Classically, these elements were thought to reside more or less stably at individual mature synapses, giving rise to the idea that each terminal was essentially an independent functional unit. However, emerging evidence from fluorescence imaging studies in hippocampal cultured neurons is now challenging this view, suggesting that neighbouring synapses along axons share vesicles, and also other synaptic elements, at high levels. This raises the possibility that control of import and export might be an important regulatory target for the maintenance of release sites, modulation of synaptic efficacy and formation of new synaptic contacts. Here, temporal synaptic stability and the functional consequences for presynaptic operation will be considered.

Differential dynamics of Rab3A and Rab27A on secretory granules

We have assessed the dynamics of the association of Rab3A and Rab27A with secretory granules at various stages of their life in PC12 cells. Endogenous Rab3A colocalised with the secretory granule marker secretogranin II (SGII) and expressed EGFP-Rab3A and ECFP-Rab27A colocalised with one another. The extent of colocalisation between EGFP-Rab3A or EGFP-Rab27 and SGII increased after longer times post transfection suggesting that these Rab proteins are preferentially recruited to newly synthesised granules. Following the release of immature secretory granules from the trans-Golgi network, Rab3A and Rab27A became associated with the immature granules after a lag period of around 20 minutes. Rab dynamics on granules were analysed in fluorescence recovery after photobleaching (FRAP) experiments. The recovery profile of EGFP-Rab27A was comparable to that of ppANF-EGFP, whereas the recovery profile of EGFP-Rab3A was significantly faster, indicating that Rab3A but not Rab27A might be rapidly exchanged between granules and cytosol. Inhibition of heat-shock protein 90 with 10 muM geldanamycin did not affect the exchange process or regulated exocytosis. Rab dynamics during stimulation with 300 muM ATP were analysed in live cells. Loss of granular ppANF-EGFP fluorescence was seen at the cell periphery after stimulation but only limited changes in EGFP-Rab3A and EGFP-Rab27A fluorescence was observed, indicating that the Rab proteins do not immediately dissociate or disperse on stimulation. The data suggest potentially distinct roles for Rab3A and Rab27A and we suggest that the finding that young secretory granules have a higher capacity for binding Rab3A and Rab27A is functionally important for preferential exocytosis from these granules.

Local sharing as a predominant determinant of synaptic matrix molecular dynamics

Recent studies suggest that central nervous system synapses can persist for weeks, months, perhaps lifetimes, yet little is known as to how synapses maintain their structural and functional characteristics for so long. As a step toward a better understanding of synaptic maintenance we examined the loss, redistribution, reincorporation, and replenishment dynamics of Synapsin I and ProSAP2/Shank3, prominent presynaptic and postsynaptic matrix molecules, respectively. Fluorescence recovery after photobleaching and photoactivation experiments revealed that both molecules are continuously lost from, redistributed among, and reincorporated into synaptic structures at time-scales of minutes to hours. Exchange rates were not affected by inhibiting protein synthesis or proteasome-mediated protein degradation, were accelerated by stimulation, and greatly exceeded rates of replenishment from somatic sources. These findings indicate that the dynamics of key synaptic matrix molecules may be dominated by local protein exchange and redistribution, whereas protein synthesis and degradation serve to maintain and regulate the sizes of local, shared pools of these proteins.

To understand processes involved in synaptic maintenance, the authors examine the loss, redistribution, reincorporation and replenishment dynamics of two key synaptic proteins, Synapsin I and ProSAP2/Shank3.

Molecular dynamics of a presynaptic active zone protein studied in Munc13-1-enhanced yellow fluorescent protein knock-in mutant mice

GFP (green fluorescent protein) fusion proteins have revolutionized research on protein dynamics at synapses. However, corresponding analyses usually involve protein expression methods that override endogenous regulatory mechanisms, and therefore cause overexpression and temporal or spatial misexpression of exogenous fusion proteins, which may seriously compromise the physiological validity of such experiments. These problems can be circumvented by using knock-in mutagenesis of the endogenous genomic locus to tag the protein of interest with a fluorescent protein. We generated knock-in mice expressing a fusion protein of the presynaptic active zone protein Munc13-1 and enhanced yellow fluorescent protein (EYFP) from the Munc13-1 locus. Munc13-1-EYFP-containing nerve cells and synapses are functionally identical to those of wild-type mice. However, their presynaptic active zones are distinctly fluorescent and readily amenable for imaging. We demonstrated the usefulness of these mice by studying the molecular dynamics of Munc13-1-EYFP at individual presynaptic sites. Fluorescence recovery after photobleaching (FRAP) experiments revealed that Munc13-1-EYFP is rapidly and continuously lost from and incorporated into active zones (tau1 approximately 3 min; tau2 approximately 80 min). Munc13-1-EYFP steady-state levels and exchange kinetics were not affected by proteasome inhibitors or acute synaptic stimulation, but exchange kinetics were reduced by chronic suppression of spontaneous activity. These experiments, performed in a minimally perturbed system, provide evidence that presynaptic active zones of mammalian CNS synapses are highly dynamic structures. They demonstrate the usefulness of the knock-in approach in general and of Munc13-1-EYFP knock-in mice in particular for imaging synaptic protein dynamics.

Evidence for early endosome-like fusion of recently endocytosed synaptic vesicles

Early endosomes are well-established acceptor compartments of endocytic vesicles in many cell types. Little evidence of their existence or function has been obtained in synapses, and it is generally believed that synaptic vesicles recycle without passing through an endosomal intermediate. We show here that the early endosomal SNARE proteins are enriched in synaptic vesicles. To investigate their function in the synapse, we isolated synaptic nerve terminals (synaptosomes), stimulated them in presence of different fluorescent markers to label the recycling vesicles and used these vesicles in in vitro fusion assays. The recently endocytosed vesicles underwent homotypic fusion. They also fused with endosomes from PC12 and BHK cells. The fusion process was dependent upon NSF activity. Moreover, fusion was dependent upon the early endosomal SNAREs but not upon the SNAREs involved in exocytosis. Our results thus show that at least a fraction of the vesicles endocytosed during synaptic activity are capable of fusing with early endosomes and lend support to an involvement of endosomal intermediates during recycling of synaptic vesicles.

Inhibition of dynamin completely blocks compensatory synaptic vesicle endocytosis

The ability of synapses to sustain signal propagation relies on rapid recycling of transmitter-containing presynaptic vesicles. Clathrin- and dynamin-mediated retrieval of vesicular membrane has an undisputed role in synaptic vesicle recycling. There is also evidence for other modes of vesicle retrieval, including bulk retrieval and the so-called kiss-and-run recycling. Whether dynamin in required for these other modes of synaptic vesicle endocytosis remains unclear. Here, we have tested the role of dynamin in synaptic vesicle endocytosis by using a small molecule called dynasore, which rapidly inhibits the GTPase activity of dynamin with high specificity. Endocytosis after sustained or brief stimuli was completely and reversibly blocked by dynasore in cultured hippocampal neurons expressing the fluorescent tracer synaptopHluorin. By contrast, dynasore had no effect on exocytosis. In the presence of dynasore, low-frequency stimulation led to sustained accumulation of synaptopHluorin and other vesicular proteins on the surface membrane at a rate predicted from net exocytosis. These vesicular components remained on surface membranes even after the stimulus was terminated, suggesting that all endocytic events rely on dynamin during low-frequency activity as well as in the period after it. Ultrastructural analysis revealed a reduction in the density of synaptic vesicles and the presence of endocytic structures only at synapses that were stimulated in the presence of dynasore. In sum, our data indicate that dynamin is essential for all forms of compensatory synaptic vesicle endocytosis including any kiss-and-run events.