Preferred sites of exocytosis and endocytosis colocalize during high- but not lower-frequency stimulation in mouse motor nerve terminals

Electrophysiological effects of BDNF and TrkB signaling at type-identified diaphragm neuromuscular junctions

Previous studies show that synaptic quantal release decreases during repetitive stimulation, i.e., synaptic depression. Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). We hypothesized that BDNF mitigates synaptic depression at the neuromuscular junction and that the effect is more pronounced at type IIx and/or IIb fibers compared to type I or IIa fibers given the more rapid reduction in docked synaptic vesicles with repetitive stimulation. Rat phrenic nerve-diaphragm muscle preparations were used to determine the effect of BDNF on synaptic quantal release during repetitive stimulation at 50 Hz. An ∼40% decline in quantal release was observed during each 330-ms duration train of nerve stimulation (intratrain synaptic depression), and this intratrain decline was observed across repetitive trains (20 trains at 1/s repeated every 5 min for 30 min for 6 sets). BDNF treatment significantly enhanced quantal release at all fiber types (P < 0.001). BDNF treatment did not change release probability within a stimulation set but enhanced synaptic vesicle replenishment between sets. In agreement, synaptic vesicle cycling (measured using FM4-64 fluorescence uptake) was increased following BDNF [or neurotrophin-4 (NT-4)] treatment (∼40%; P < 0.05). Conversely, inhibiting BDNF/TrkB signaling with the tyrosine kinase inhibitor K252a and TrkB-IgG (which quenches endogenous BDNF or NT-4) decreased FM4-64 uptake (∼34% across fiber types; P < 0.05). The effects of BDNF were generally similar across all fiber types. We conclude that BDNF/TrkB signaling acutely enhances presynaptic quantal release and thereby may serve to mitigate synaptic depression and maintain neuromuscular transmission during repetitive activation.NEW & NOTEWORTHY Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). Rat phrenic nerve-diaphragm muscle preparations were used to determine the rapid effect of BDNF on synaptic quantal release during repetitive stimulation. BDNF treatment significantly enhanced quantal release at all fiber types. BDNF increased synaptic vesicle cycling (measured using FM4-64 fluorescence uptake); conversely, inhibiting BDNF/TrkB signaling decreased FM4-64 uptake.

Linking Protons to Homeostatic Plasticity

Editorial on Extracellular protons mediate presynaptic homeostatic potentiation at the mouse neuromuscular junction.

Bicaudal C is required for the function of the follicular epithelium during oogenesis in Rhodnius prolixus

The morphology and physiology of the oogenesis have been well studied in the vector of Chagas disease Rhodnius prolixus. However, the molecular interactions that regulate the process of egg formation, key for the reproductive cycle of the vector, is still largely unknown. In order to understand the molecular and cellular basis of the oogenesis, we examined the function of the gene Bicaudal C (BicC) during oogenesis and early development of R. prolixus. We show that R. prolixus BicC (Rp-BicC) gene is expressed in the germarium, with cytoplasmic distribution, as well as in the follicular epithelium of the developing oocytes. RNAi silencing of Rp-BicC resulted in sterile females that lay few, small, non-viable eggs. The ovaries are reduced in size and show a disarray of the follicular epithelium. This indicates that Rp-BicC has a central role in the regulation of oogenesis. Although the follicular cells are able to form the chorion, the uptake of vitelline by the oocytes is compromised. We show evidence that the polarity of the follicular epithelium and the endocytic pathway, which are crucial for the proper yolk deposition, are affected. This study provides insights into the molecular mechanisms underlying oocyte development and show that Rp-BicC is important for de developmental of the egg and, therefore, a key player in the reproduction of this insect.

Nanoscale Organization of Vesicle Release at Central Synapses

Presynaptic boutons support neurotransmitter release with nanoscale precision at sub-millisecond timescales. Studies over the past two decades have revealed a rich tapestry of molecular players governing synaptic vesicle fusion at highly specialized release sites in the active zone (AZ). However, the spatiotemporal organization of release at active synapses remains elusive, in part owing to the extremely small size of the AZ and the limited resolution of conventional approaches. Recent advances in fluorescence nanoscopy have revolutionized direct investigation of presynaptic release organization and dynamics. We discuss here recent nanoscopy-based studies of the molecular architecture, the spatial organization and dynamic regulation of release sites, and the mechanisms of release site replenishment. These findings have uncovered previously unknown levels of structural and functional organization at central synapses, with important implications for synaptic transmission and plasticity.

Chemically Functionalized Water-Soluble Single-Walled Carbon Nanotubes Obstruct Vesicular/Plasmalemmal Recycling in Astrocytes Down-Stream of Calcium Ions

We used single-walled carbon nanotubes chemically functionalized with polyethylene glycol (SWCNT-PEG) to assess the effects of this nanomaterial on astrocytic endocytosis and exocytosis. We observed that the SWCNT-PEG do not affect the adenosine triphosphate (ATP)-evoked Ca²⁺ elevations in astrocytes but significantly reduce the Ca²⁺-dependent glutamate release. There was a significant decrease in the endocytic load of the recycling dye during constitutive and ATP-evoked recycling. Furthermore, SWCNT-PEG hampered ATP-evoked exocytotic release of the loaded recycling dye. Thus, by functionally obstructing evoked vesicular recycling, SWCNT-PEG reduced glutamate release from astrocytes via regulated exocytosis. These effects implicate SWCNT-PEG as a modulator of Ca²⁺-dependent exocytosis in astrocytes downstream of Ca²⁺, likely at the level of vesicle fusion with/pinching off the plasma membrane.

Zebrafish neuromuscular junction: The power of N

In the early 1950s, Katz and his colleagues capitalized on the newly developed intracellular microelectrode recording technique to investigate synaptic transmission. For study they chose frog neuromuscular junction (NMJ), which was ideally suited due to the accessibility and large size of the muscle cells. Paradoxically, the large size precluded the use of next generation patch clamp technology. Consequently, electrophysiological study of synaptic function shifted to small central synapses made amenable by patch clamp. Recently, however, the unique features offered by zebrafish have rekindled interest in the NMJ as a model for electrophysiological study of synaptic transmission. The small muscle size and synaptic simplicity provide the singular opportunity to perform in vivo spinal motoneuron-target muscle patch clamp recordings. Additional incentive is provided by zebrafish lines harboring mutations in key synaptic proteins, many of which are embryonic lethal in mammals, but all of which are able to survive well past synapse maturation in zebrafish. This mini-review will highlight features that set zebrafish NMJs apart from traditional NMJs. We also draw into focus findings that offer the promise of identifying features that define release sites, which serve to set the upper limit of transmitter release. Since its conception several candidates representing release sites have been proposed, most of which are based on distinctions among vesicle pools in their state of readiness for release. However, models based on distinctions among vesicles have become enormously complicated and none adequately account for setting an upper limit for exocytosis in response to an action potential (AP). Specifically, findings from zebrafish NMJ point to an alternative model, positing that elements other than vesicles per se set the upper limits of release.

Presynaptic mechanisms controlling calcium-triggered transmitter release at the neuromuscular junction

Calcium-triggered neurotransmission underlies most communication in the nervous system. Yet, despite the conserved and essential nature of this process, the molecular underpinnings of calcium-triggered neurotransmission have been difficult to study directly and our understanding to this date remains incomplete. Here we frame more recent efforts to understand this process with a historical perspective of the study of neurotransmitter release at the neuromuscular junction. We focus on the role of calcium channel distribution and organization relative to synaptic vesicles, as well as the nature of the calcium sensors that trigger release. Importantly, we provide a framework for understanding how the function of neurotransmitter release sites, or active zones, contributes to the function of the synapse as a whole.

Impact of spatiotemporal calcium dynamics within presynaptic active zones on synaptic delay at the frog neuromuscular junction

The spatiotemporal calcium dynamics within presynaptic neurotransmitter release sites (active zones, AZs) at the time of synaptic vesicle fusion is critical for shaping the dynamics of neurotransmitter release. Specifically, the relative arrangement and density of voltage-gated calcium channels (VGCCs) as well as the concentration of calcium buffering proteins can play a large role in the timing, magnitude, and plasticity of release by shaping the AZ calcium profile. However, a high-resolution understanding of the role of AZ structure in spatiotemporal calcium dynamics and how it may contribute to functional heterogeneity at an adult synapse is currently lacking. We demonstrate that synaptic delay varies considerably across, but not within, individual synapses at the frog neuromuscular junction (NMJ). To determine how elements of the AZ could contribute to this variability, we performed a parameter search using a spatially realistic diffusion reaction-based computational model of a frog NMJ AZ (Dittrich M, Pattillo JM, King JD, Cho S, Stiles JR, Meriney SD. Biophys J 104: 2751-2763, 2013; Ma J, Kelly L, Ingram J, Price TJ, Meriney SD, Dittrich M. J Neurophysiol 113: 71-87, 2015). We demonstrate with our model that synaptic delay is sensitive to significant alterations in the spatiotemporal calcium dynamics within an AZ at the time of release caused by manipulations of the density and organization of VGCCs or by the concentration of calcium buffering proteins. Furthermore, our data provide a framework for understanding how AZ organization and structure are important for understanding presynaptic function and plasticity. NEW & NOTEWORTHY The structure of presynaptic active zones (AZs) can play a large role in determining the dynamics of neurotransmitter release across many model preparations by influencing the spatiotemporal calcium dynamics within the AZ at the time of vesicle fusion. However, less is known about how different AZ structural schemes may influence the timing of neurotransmitter release. We demonstrate that variations in AZ structure create different spatiotemporal calcium profiles that, in turn, lead to differences in synaptic delay.

There Is No Simple Model of the Plasma Membrane Organization

Ever since technologies enabled the characterization of eukaryotic plasma membranes, heterogeneities in the distributions of its constituents were observed. Over the years this led to the proposal of various models describing the plasma membrane organization such as lipid shells, picket-and-fences, lipid rafts, or protein islands, as addressed in numerous publications and reviews. Instead of emphasizing on one model we in this review give a brief overview over current models and highlight how current experimental work in one or the other way do not support the existence of a single overarching model. Instead, we highlight the vast variety of membrane properties and components, their influences and impacts. We believe that highlighting such controversial discoveries will stimulate unbiased research on plasma membrane organization and functionality, leading to a better understanding of this essential cellular structure.

Different dynamin blockers interfere with distinct phases of synaptic endocytosis during stimulation in motoneurones

KEY POINTS

Neurotransmitter release requires a tight coupling between synaptic vesicle exocytosis and endocytosis with dynamin being a key protein in that process. We used imaging techniques to examine the time course of endocytosis at mouse motor nerve terminals expressing synaptopHluorin, a genetically encoded reporter of the synaptic vesicle cycle. We separated two sequential phases of endocytosis taking place during the stimulation train: early and late endocytosis. Freshly released synaptic vesicle proteins are preferentially retrieved during the early phase, which is very sensitive to dynasore, an inhibitor of dynamin GTPase activity. Synaptic vesicle proteins pre-existing at the plasma membrane before the stimulation are preferentially retrieved during the late phase, which is very sensitive to myristyl trimethyl ammonium bromide (MitMAB), an inhibitor of the dynamin-phospholipid interaction.

ABSTRACT

Synaptic endocytosis is essential at nerve terminals to maintain neurotransmitter release by exocytosis. Here, at the neuromuscular junction of synaptopHluorin (spH) transgenic mice, we have used imaging to study exo- and endocytosis occurring simultaneously during nerve stimulation. We observed two endocytosis components, which occur sequentially during stimulation. The early component of endocytosis apparently internalizes spH molecules freshly exocytosed. This component was sensitive to dynasore, a blocker of dynamin 1 GTPase activity. In contrast, this early component was resistant to myristyl trimethyl ammonium bromide (MiTMAB), a competitive agent that blocks dynamin binding to phospholipid membranes. The late component of endocytosis is likely to internalize spH molecules that pre-exist at the plasma membrane before stimulation starts. This component was blocked by MiTMAB, perhaps by impairing the binding of dynamin or other key endocytic proteins to phospholipid membranes. Our study suggests the co-existence of two sequential synaptic endocytosis steps taking place during stimulation that are susceptible to pharmacological dissection: an initial step, preferentially sensitive to dynasore, that internalizes vesicular components immediately after they are released, and a MiTMAB-sensitive step that internalizes vesicular components pre-existing at the plasma membrane surface. In addition, we report that post-stimulus endocytosis also has several components with different sensitivities to dynasore and MiTMAB.

Why does endocytosis in single cells care which side up?

Eukaryotic cells display an asymmetric distribution of cellular compartments relying on their adhesion and the underlying anisotropy of the actin and microtubule cytoskeleton. Studies using a minimal cell culture system based on confined adhesion on micropatterns have illustrated that trafficking compartments are well organized at the single cell level in response to the geometry of cellular adhesion cues. Expanding our analysis on cellular uptake processes, we have found that cellular adhesion additionally defines the topology of endocytosis and signaling. During endocytosis, transferrin (Tfn) and epidermal growth factor (EGF) concentrate at distinct cellular sites in micropatterned cells. Tfn is enriched in adhesive sites during uptake, whereas EGF endocytosis is restricted to the dorsal cellular surface. This unexpected dorsal/ventral asymmetry is regulated by uptake mechanisms and actin dynamics. Interestingly, restricted EGF uptake leads to asymmetry of EGF receptor activation that is required to sustain downstream signaling. Based on our results, we propose that differential sorting begins at the plasma membrane leading to spatially distinct intracellular trafficking routes that are well defined in space. We speculate that the intracellular positioning of trafficking compartments sustains an important coupling between the endocytic and signaling systems that allows cells to sense their environment.

Synaptic vesicle recycling: steps and principles

Synaptic vesicle recycling is one of the best-studied cellular pathways. Many of the proteins involved are known, and their interactions are becoming increasingly clear. However, as for many other pathways, it is still difficult to understand synaptic vesicle recycling as a whole. While it is generally possible to point out how synaptic reactions take place, it is not always easy to understand what triggers or controls them. Also, it is often difficult to understand how the availability of the reaction partners is controlled: how the reaction partners manage to find each other in the right place, at the right time. I present here an overview of synaptic vesicle recycling, discussing the mechanisms that trigger different reactions, and those that ensure the availability of reaction partners. A central argument is that synaptic vesicles bind soluble cofactor proteins, with low affinity, and thus control their availability in the synapse, forming a buffer for cofactor proteins. The availability of cofactor proteins, in turn, regulates the different synaptic reactions. Similar mechanisms, in which one of the reaction partners buffers another, may apply to many other processes, from the biogenesis to the degradation of the synaptic vesicle.

Blocking endocytosis enhances short-term synaptic depression under conditions of normal availability of vesicles

It is commonly thought that clathrin-mediated endocytosis is the rate-limiting step of synaptic transmission in small CNS boutons with limited capacity for synaptic vesicles, causing short-term depression during high rates of synaptic transmission. Here, we show by analyzing synaptopHluorin fluorescence that 200 action potentials evoke the same cumulative amount of vesicle fusion, irrespective of the frequency of stimulation (5-40 Hz), implying the absence of vesicle reuse, since the method used (alkaline-trapping) measures only first-round exocytosis. After blocking all slow or specifically clathrin-mediated endocytosis, however, the same stimulation patterns cause a rapid stimulation-frequency-dependent release depression. This form of depression does not reflect insufficient vesicle supply, but appears to be the result of slow clearance of vesicular components from the release site. Our findings uncover an important yet overlooked role of endocytic proteins for release site clearance in addition to their well-characterized role in endocytosis itself.

Probabilistic density maps to study the spatial organization of endocytosis

Despite a large body of publications on endocytosis, only a few studies have focused on its spatial organization. To study how endocytosis is related to distinct cellular sites, we combine cell normalization by the "micropatterning technique" with the quantification of spatial organization by "probabilistic density mapping." Micropatterns of extracellular matrix proteins impose adhesive and non-adhesive areas to cultured cells and allow the control of adhesion geometry, shape, and cell organization. Probabilistic density maps provide a visual summary for 3D localization of the structures of interest and enable the extraction of robust statistics for quantification of cellular organization. Here, we provide a method to analyze and compare the spatial distribution of endocytosed ligands in micropatterned cells. This approach permits to establish the role of cellular adhesion on uptake mechanisms and to address the potential function of predefined sites of endocytosis.

Cell adhesion defines the topology of endocytosis and signaling

Preferred sites of endocytosis have been observed in various cell types, but whether they occur randomly or are linked to cellular cues is debated. Here, we quantified the sites of endocytosis of transferrin (Tfn) and epidermal growth factor (EGF) in cells whose adhesion geometry was defined by micropatterns. 3D probabilistic density maps revealed that Tfn was enriched in adhesive sites during uptake, whereas EGF endocytosis was restricted to the dorsal cellular surface. This spatial separation was not due to distributions of corresponding receptors but was regulated by uptake mechanisms. Asymmetric uptake of Tfn resulted from the enrichment of clathrin and adaptor protein 2 at adhesive areas. Asymmetry in EGF uptake was strongly dependent on the actin cytoskeleton and led to asymmetry in EGF receptor activation. Mild alteration of actin dynamics abolished asymmetry in EGF uptake and decreased EGF-induced downstream signaling, suggesting that cellular adhesion cues influence signal propagation. We propose that restriction of endocytosis at distinct sites allows cells to sense their environment in an "outside-in" mechanism.

A noncanonical postsynaptic transport route for a GPCR belonging to the serotonin receptor family

Postsynaptic receptor trafficking plays an essential role in tuning neurotransmission and signal plasticity and has emerged as a potential therapeutic target in neuropsychiatric disease. Using a novel application of fluorescence recovery after photobleaching in rat hippocampal neurons, we examined transport from the soma to dendrites of seven G-protein-coupled receptors (GPCRs) implicated in mood disorders. Most GPCRs were delivered to dendrites via lateral diffusion, but one GPCR, the serotonin 1B receptor (5-HT(1B)), was delivered to the dendrites in secretory vesicles. Within the dendrites, 5-HT(1B) were stored in a reservoir of accessible vesicles that were recruited to preferential sites in plasma membrane, as observed with superecliptic pHluorin labeling. After membrane recruitment, 5-HT(1B) transport via lateral diffusion and temporal confinement to inhibitory and excitatory synapses was monitored by single particle tracking. These results suggest an alternative mechanism for control of neuronal activity via a GPCR that has been implicated in mood regulation.

Pool-independent labelling of synaptic vesicle exocytosis with single vesicle resolution in rat hippocampal neurons

FM dyes are an established tool to analyze synaptic vesicle pools. However, quantitative measurements using FM dyes are typically based on the re-release properties of previously labelled vesicles, which might vary depending on the experimental setup. An FM dye protocol independent of the previous labelling of vesicle membrane has not been applied for quantitative measurements of individual synaptic vesicles before. We therefore analyzed the direct staining of newly exocytosed vesicle membrane with FM dyes in cultured rat hippocampal neurons. In the presence of FM 1-43, stimulation-induced synaptic activity led to a stable fluorescence increase. The quantal release of synaptic vesicles was preserved and its amplitude correlated highly with the exocytic dye loss induced by a subsequent stimulation. Thus, the method presented here provides a tool for the pool-independent measurement of synaptic vesicle exocytosis.

Live imaging of bulk endocytosis in frog motor nerve terminals using FM dyes

We observed endocytosis in real time in stimulated frog motor nerve terminals by imaging the growth of large membrane infoldings labeled with a low concentration of FM dye. The spatial and temporal information made available by these experiments allowed us to image several new aspects of this synaptic vesicle recycling pathway. Membrane infoldings appeared near synaptic vesicle clusters and grew rapidly during long-duration, high-frequency stimulation. In some cases, we observed large, elongated infoldings growing laterally into the terminal. We used these observations to calculate infolding growth rates. A decrease in stimulation frequency caused a decrease in growth rates, but the overall length of these structures was unaffected by frequency changes. Attempts to wash the dye from these infoldings after stimulation were unsuccessful, demonstrating that the fluorescent structures had been endocytosed. We also used this technique to trigger and image infoldings during repeated, short trains. We found that membrane uptake occurred repeatedly at individual endocytosis sites, but only during a portion of the total number of trains delivered to the terminal. Finally, we showed that phosphatidylinositol 3-kinase, but not actin, was involved in this endocytosis pathway. The ability to monitor many individual bulk endocytosis sites in real time should allow for new types of endocytosis measurements and could reveal novel and unexpected mechanisms for coordinating membrane recovery during synaptic activity.

Tetanic failure due to decreased endogenous adenosine A(2A) tonus operating neuronal Ca(v) 1 (L-type) influx in Myasthenia gravis

In healthy motor endplates, tetanic depression is overcome by tonic adenosine A(2A) -receptor-mediated facilitation of transmitter release. The A(2A) receptor operates a coordinated shift from fast-desensitizing Ca(v) 2.1 (P/Q) calcium influx to long-lasting Ca(V) 1 (L) channels on motor nerve terminals. This study aimed at investigating whether A(2A) receptors-operated Ca(2+) influx via Ca(V) 1 (L)-type channels contribute to sustain acetylcholine release evoked by 50 Hz-bursts in toxin-induced Myasthenia gravis (TIMG) rats. In contrast to control animals, inhibition of [(3) H]acetylcholine (ACh) release by the Ca(V) 2.1 (P/Q) channel blocker, ω-Agatoxin IVA (100 nM), in TIMG rats had a higher magnitude than that observed with the Ca(V) 1 (L) channel blocker, nifedipine (1 μM). Adenosine deaminase (0.5 U/mL) and the A(2A) receptor antagonist, ZM 241385 (50 nM), decreased [(3) H]ACh release by a similar amount in control rats, but their effects were smaller in magnitude in myasthenic animals. The adenosine precursor, AMP (100 μM), increased (~40%) ACh release in both control and TIMG animals. Blockade of A(2A) , but not of A(1) , receptors prevented AMP-induced facilitation of transmitter release; nifedipine (1 μM) mimicked the effect of the A(2A) receptor antagonist. Video-microscopy studies designed to measure real-time transmitter exocytosis using the FM4-64 fluorescent dye fully supported radiochemical data. Thus, impairment of the adaptive shift from Ca(V) 2.1 (P/Q) to Ca(V) 1 (L) channels may contribute to tetanic failure in myasthenic rats. This parallels the reduction of adenosine A(2A) receptor tonus in TIMG animals, which might be restored by exogenous application of AMP.

Active zones and the readily releasable pool of synaptic vesicles at the neuromuscular junction of the mouse

Synchronous neurotransmitter release is a highly regulated process that takes place at specializations at the presynaptic membrane called active zones (AZs). The relationships between AZs, quantal release, and vesicle replenishment are not well understood in a mature synapse. We have measured the number, distribution, and other properties of AZs in mouse motor nerve terminals and combined these observations with electrophysiological estimates of the size of the readily releasable pool (RRP) of synaptic vesicles. On average, we counted 850 AZs per terminal. Assuming two primary docked vesicles per AZ, we predict a total of ∼1700 vesicles optimally positioned for exocytosis. Electrophysiological estimates of the size of the RRP, using a simple kinetic model that assumes exponential depletion of the initial pool and refilling by recruitment, gave an average value of 1730 quanta during 100 Hz stimulation, in satisfying agreement with the morphology. At lower stimulus frequencies, however, the model revealed that the estimated RRP size is smaller, suggesting that not all AZs participate in release at low stimulation frequencies.

Frequency-dependent modes of synaptic vesicle endocytosis and exocytosis at adult mouse neuromuscular junctions

During locomotion, adult rodent lumbar motoneurons fire in high-frequency (80-100 Hz) 1-2 s bursts every several seconds, releasing between 10,000 and 20,000 vesicles per burst. The estimated total vesicle pool size indicates that all vesicles would be used within 30 s; thus, a mechanism for rapid endocytosis and vesicle recycling is necessary to maintain effective transmission and motor behavior. However, whether such rapid recycling exists at mouse neuromuscular junctions (NMJs) or how it is regulated has been unclear. Here, we show that much less FM1-43 dye is lost per stimulus with 100 Hz stimulation than with 10 Hz stimulation even when the same number of vesicles undergo exocytosis. Electrophysiological data using folimycin show this lesser amount of dye loss is caused in part by the rapid reuse of vesicles. We showed previously that a myosin light chain kinase (MLCK)-myosin II pathway was required for effective transmission at 100 Hz. Here, we confirm the activation of MLCK, based on increased nerve terminal phospho-MLC immunostaining, with 100 Hz but not with 10 Hz stimulation. We further demonstrate that activation of MLCK, by increased extracellular Ca(2+), by PKC (protein kinase C) activation, or by a MLCK agonist peptide, reduces the amount of dye lost even with 10 Hz stimulation. MLCK activation at 10 Hz also resulted in more vesicles being rapidly reused. Thus, MLCK activation by 100 Hz stimulation switches the mechanism of vesicle cycling to a rapid-reuse mode and is required to sustain effective transmission in adult mouse NMJs.

Vesicle pools: lessons from adrenal chromaffin cells

The adrenal chromaffin cell serves as a model system to study fast Ca2+-dependent exocytosis. Membrane capacitance measurements in combination with Ca2+ uncaging offers a temporal resolution in the millisecond range and reveals that catecholamine release occurs in three distinct phases. Release of a readily releasable (RRP) and a slowly releasable (SRP) pool are followed by sustained release, due to maturation, and release of vesicles which were not release-ready at the start of the stimulus. Trains of depolarizations, a more physiological stimulus, induce release from a small immediately releasable pool of vesicles residing adjacent to calcium channels, as well as from the RRP. The SRP is poorly activated by depolarization. A sequential model, in which non-releasable docked vesicles are primed to a slowly releasable state, and then further mature to the readily releasable state, has been proposed. The docked state, dependent on membrane proximity, requires SNAP-25, synaptotagmin, and syntaxin. The ablation or modification of SNAP-25 and syntaxin, components of the SNARE complex, as well as of synaptotagmin, the calcium sensor, and modulators such complexins and Snapin alter the properties and/or magnitudes of different phases of release, and in particular can ablate the RRP. These results indicate that the composition of the SNARE complex and its interaction with modulatory molecules drives priming and provides a molecular basis for different pools of releasable vesicles.

Multiple functions of the vesicular proton pump in nerve terminals

Synaptic vesicles are acidified by a proton pump (vATPase), which allows vesicular uptake of neurotransmitters. After vesicle exocytosis, continued operation of the vATPase would seem to serve no useful function. In this issue of Neuron, however, Zhang and colleagues show that continued pumping alkalinizes the cytoplasm, accelerating endocytosis.

Vesicular ATPase inserted into the plasma membrane of motor terminals by exocytosis alkalinizes cytosolic pH and facilitates endocytosis

Key components of vesicular neurotransmitter release, such as Ca(2+) influx and membrane recycling, are affected by cytosolic pH. We measured the pH-sensitive fluorescence of Yellow Fluorescent Protein transgenically expressed in mouse motor nerve terminals, and report that Ca(2+) influx elicited by action potential trains (12.5-100 Hz) evokes a biphasic pH change: a brief acidification (∼ 13 nM average peak increase in [H(+)]), followed by a prolonged alkalinization (∼ 30 nM peak decrease in [H(+)]) that outlasts the stimulation train. The alkalinization is selectively eliminated by blocking vesicular exocytosis with botulinum neurotoxins, and is prolonged by the endocytosis-inhibitor dynasore. Blocking H(+) pumping by vesicular H(+)-ATPase (with folimycin or bafilomycin) suppresses stimulation-induced alkalinization and reduces endocytotic uptake of FM1-43. These results suggest that H(+)-ATPase, known to transfer cytosolic H(+) into prefused vesicles, continues to extrude cytosolic H(+) after being exocytotically incorporated into the plasma membrane. The resulting cytosolic alkalinization may facilitate vesicular endocytosis.