Physiological stimuli evoke two forms of endocytosis in bovine chromaffin cells

'Full fusion' is not ineluctable during vesicular exocytosis of neurotransmitters by endocrine cells

Vesicular exocytosis is an essential and ubiquitous process in neurons and endocrine cells by which neurotransmitters are released in synaptic clefts or extracellular fluids. It involves the fusion of a vesicle loaded with chemical messengers with the cell membrane through a nanometric fusion pore. In endocrine cells, unless it closes after some flickering ('Kiss-and-Run' events), this initial pore is supposed to expand exponentially, leading to a full integration of the vesicle membrane into the cell membrane-a stage called 'full fusion'. We report here a compact analytical formulation that allows precise measurements of the fusion pore expansion extent and rate to be extracted from individual amperometric spike time courses. These data definitively establish that, during release of catecholamines, fusion pores enlarge at most to approximately one-fifth of the radius of their parent vesicle, hence ruling out the ineluctability of 'full fusion'.

Sustained Exocytosis after Action Potential-Like Stimulation at Low Frequencies in Mouse Chromaffin Cells Depends on a Dynamin-Dependent Fast Endocytotic Process

Under basal conditions the action potential firing rate of adrenal chromaffin cells is lower than 0.5 Hz. The maintenance of the secretory response at such frequencies requires a continuous replenishment of releasable vesicles. However, the mechanism that allows such vesicle replenishment remains unclear. Here, using membrane capacitance measurements on mouse chromaffin cells, we studied the mechanism of replenishment of a group of vesicles released by a single action potential-like stimulus (AP_ls). The exocytosis triggered by AP_ls (ETAP) represents a fraction (40%) of the immediately releasable pool, a group of vesicles highly coupled to voltage dependent calcium channels. ETAP was replenished with a time constant of 0.73 ± 0.11 s, fast enough to maintain synchronous exocytosis at 0.2–0.5 Hz stimulation. Regarding the mechanism involved in rapid ETAP replenishment, we found that it depends on the ready releasable pool; indeed depletion of this vesicle pool significantly delays ETAP replenishment. On the other hand, ETAP replenishment also correlates with a dynamin-dependent fast endocytosis process (τ = 0.53 ± 0.01 s). In this regard, disruption of dynamin function markedly inhibits the fast endocytosis and delays ETAP replenishment, but also significantly decreases the synchronous exocytosis during repetitive AP_ls stimulation at low frequencies (0.2 and 0.5 Hz). Considering these findings, we propose a model in where both the transfer of vesicles from ready releasable pool and fast endocytosis allow rapid ETAP replenishment during low stimulation frequencies.

Myosin light chain kinase facilitates endocytosis of synaptic vesicles at hippocampal boutons

At nerve terminals, endocytosis efficiently recycles vesicle membrane to maintain synaptic transmission under different levels of neuronal activity. Ca(2+) and its downstream signal pathways are critical for the activity-dependent regulation of endocytosis. An activity- and Ca(2+) -dependent kinase, myosin light chain kinase (MLCK) has been reported to regulate vesicle mobilization, vesicle cycling, and motility in different synapses, but whether it has a general contribution to regulation of endocytosis at nerve terminals remains unknown. We investigated this issue at rat hippocampal boutons by imaging vesicle endocytosis as the real-time retrieval of vesicular synaptophysin tagged with a pH-sensitive green fluorescence protein. We found that endocytosis induced by 200 action potentials (5-40 Hz) was slowed by acute inhibition of MLCK and down-regulation of MLCK with RNA interference, while the total amount of vesicle exocytosis and somatic Ca(2+) channel current did not change with MLCK down-regulation. Acute inhibition of myosin II similarly impaired endocytosis. Furthermore, down-regulation of MLCK prevented depolarization-induced phosphorylation of myosin light chain, an effect shared by blockers of Ca(2+) channels and calmodulin. These results suggest that MLCK facilitates vesicle endocytosis through activity-dependent phosphorylation of myosin downstream of Ca(2+) /calmodulin, probably as a widely existing mechanism among synapses. Our study suggests that MLCK is an important activity-dependent regulator of vesicle recycling in hippocampal neurons, which are critical for learning and memory. The kinetics of vesicle membrane endocytosis at nerve terminals has long been known to depend on activity and Ca(2+) . This study provides evidence suggesting that myosin light chain kinase increases endocytosis efficiency at hippocampal neurons by mediating Ca(2+) /calmodulin-dependent phosphorylation of myosin. The authors propose that this signal cascade may serve as a common pathway contributing to the activity-dependent regulation of vesicle endocytosis at synapses.

Catecholamine exocytosis during low frequency stimulation in mouse adrenal chromaffin cells is primarily asynchronous and controlled by the novel mechanism of Ca2+ syntilla suppression

Adrenal chromaffin cells (ACCs), stimulated by the splanchnic nerve, generate action potentials (APs) at a frequency near 0.5 Hz in the resting physiological state, at times described as 'rest and digest'. How such low frequency stimulation in turn elicits sufficient catecholamine exocytosis to set basal sympathetic tone is not readily explained by the classical mechanism of stimulus-secretion coupling, where exocytosis is synchronized to AP-induced Ca(2+) influx. By using simulated action potentials (sAPs) at 0.5 Hz in isolated patch-clamped mouse ACCs, we show here that less than 10% of all catecholaminergic exocytosis, measured by carbon fibre amperometry, is synchronized to an AP. The asynchronous phase, the dominant phase, of exocytosis does not require Ca(2+) influx. Furthermore, increased asynchronous exocytosis is accompanied by an AP-dependent decrease in frequency of Ca(2+) syntillas (i.e. transient, focal Ca(2+) release from internal stores) and is ryanodine sensitive. We propose a mechanism of disinhibition, wherein APs suppress Ca(2+) syntillas, which themselves inhibit exocytosis as they do in the case of spontaneous catecholaminergic exocytosis.

Calcineurin is universally involved in vesicle endocytosis at neuronal and nonneuronal secretory cells

Calcium influx triggers and accelerates endocytosis in nerve terminals and nonneuronal secretory cells. Whether calcium/calmodulin-activated calcineurin, which dephosphorylates endocytic proteins, mediates this process is highly controversial for different cell types, developmental stages, and endocytic forms. Using three preparations that previously produced discrepant results (i.e., large calyx-type synapses, conventional cerebellar synapses, and neuroendocrine chromaffin cells containing large dense-core vesicles), we found that calcineurin gene knockout consistently slowed down endocytosis, regardless of cell type, developmental stage, or endocytic form (rapid or slow). In contrast, calcineurin and calmodulin blockers slowed down endocytosis at a relatively small calcium influx, but did not inhibit endocytosis at a large calcium influx, resulting in false-negative results. These results suggest that calcineurin is universally involved in endocytosis. They may also help explain the discrepancies among previous pharmacological studies. We therefore suggest that calcineurin should be included as a key player in mediating calcium-triggered and -accelerated vesicle endocytosis.

Syndapin 3 modulates fusion pore expansion in mouse neuroendocrine chromaffin cells

Adrenal neuroendocrine chromaffin cells receive excitatory synaptic input from the sympathetic nervous system and secrete hormones into the peripheral circulation. Under basal sympathetic tone, modest amounts of freely soluble catecholamine are selectively released through a restricted fusion pore formed between the secretory granule and the plasma membrane. Upon activation of the sympathoadrenal stress reflex, elevated stimulation drives fusion pore expansion, resulting in increased catecholamine secretion and facilitating release of copackaged peptide hormones. Thus regulated expansion of the secretory fusion pore is a control point for differential hormone release of the sympathoadrenal stress response. Previous work has shown that syndapin 1 deletion alters transmitter release and that the dynamin 1-syndapin 1 interaction is necessary for coupled endocytosis in neurons. Dynamin has also been shown to be involved in regulation of fusion pore expansion in neuroendocrine chromaffin cells through an activity-dependent association with syndapin. However, it is not known which syndapin isoform(s) contributes to pore dynamics in neuroendocrine cells. Nor is it known at what stage of the secretion process dynamin and syndapin associate to modulate pore expansion. Here we investigate the expression and localization of syndapin isoforms and determine which are involved in mediating fusion pore expansion. We show that all syndapin isoforms are expressed in the adrenal medulla. Mutation of the SH3 dynamin-binding domain of all syndapin isoforms shows that fusion pore expansion and catecholamine release are limited specifically by mutation of syndapin 3. The mutation also disrupts targeting of syndapin 3 to the cell periphery. Syndapin 3 exists in a persistent colocalized state with dynamin 1.

Exocytosis and endocytosis: modes, functions, and coupling mechanisms

Vesicle exocytosis releases content to mediate many biological events, including synaptic transmission essential for brain functions. Following exocytosis, endocytosis is initiated to retrieve exocytosed vesicles within seconds to minutes. Decades of studies in secretory cells reveal three exocytosis modes coupled to three endocytosis modes: (a) full-collapse fusion, in which vesicles collapse into the plasma membrane, followed by classical endocytosis involving membrane invagination and vesicle reformation; (b) kiss-and-run, in which the fusion pore opens and closes; and (c) compound exocytosis, which involves exocytosis of giant vesicles formed via vesicle-vesicle fusion, followed by bulk endocytosis that retrieves giant vesicles. Here we review these exo- and endocytosis modes and their roles in regulating quantal size and synaptic strength, generating synaptic plasticity, maintaining exocytosis, and clearing release sites for vesicle replenishment. Furthermore, we highlight recent progress in understanding how vesicle endocytosis is initiated and is thus coupled to exocytosis. The emerging model is that calcium influx via voltage-dependent calcium channels at the calcium microdomain triggers endocytosis and controls endocytosis rate; calmodulin and synaptotagmin are the calcium sensors; and the exocytosis machinery, including SNARE proteins (synaptobrevin, SNAP25, and syntaxin), is needed to coinitiate endocytosis, likely to control the amount of endocytosis.

Most vesicles in a central nerve terminal participate in recycling

Studies over the last decade using FM dyes to label vesicles at many terminals, including the calyx-type nerve terminal, led to a well accepted "principle" that only a small fraction of vesicles (∼5-20%) participate in recycling under physiological conditions. This principle imposes a large challenge in maintaining synaptic transmission during repetitive firing, because the small recycling pool may limit the number of available vesicles for release and nerve terminals would have to distinguish the recycling pool from the reserve pool and keep reserve pool vesicles from being used. By recording the presynaptic capacitance changes and the postsynaptic EPSC at rat calyx of Held synapses in the absence or presence of transmitter glutamate in nerve terminals, we developed a new method to count functional recycling vesicles. We found that essentially all vesicles in calyces participated in recycling, challenging the small-recycling-pool principle established by FM dye labeling. Nerve terminals may use all available vesicles to maximize their ability in maintaining synaptic transmission during repetitive firing.

Activity-dependent fusion pore expansion regulated by a calcineurin-dependent dynamin-syndapin pathway in mouse adrenal chromaffin cells

Neuroendocrine chromaffin cells selectively secrete a variety of transmitter molecules into the circulation as a function of sympathetic activation. Activity-dependent release of transmitter species is controlled through regulation of the secretory fusion pore. Under sympathetic tone, basal synaptic excitation drives chromaffin cells to selectively secrete modest levels of catecholamine through a restricted secretory fusion pore. In contrast, elevated sympathetic activity, experienced under stress, results in fusion pore expansion to evoke maximal catecholamine release and to facilitate release of copackaged peptide transmitters. Therefore, fusion pore expansion is a key control point for the activation of the sympatho-adrenal stress response. Despite the physiological importance of this process, the molecular mechanism by which it is regulated remains unclear. Here we employ fluorescence imaging with electrophysiological and electrochemical-based approaches to investigate the role of dynamin I in the regulation of activity-mediated fusion pore expansion in mouse adrenal chromaffin cells. We show that under elevated stimulation, dynamin I is dephosphorylated at Ser-774 by calcineurin. We also demonstrate that disruption of dynamin I-syndapin binding, an association regulated by calcineurin-dependent dynamin dephosphorylation, limits fusion pore expansion. Last, we show that perturbation of N-WASP function (a syndapin substrate) limits activity-mediated fusion pore expansion. Our results suggest that fusion pore expansion is regulated by a calcineurin-dependent dephosphorylation of dynamin I. Dephosphorylated dynamin I acts via a syndapin/N-WASP signaling cascade to mediate pore expansion.

Functional chromaffin cell plasticity in response to stress: focus on nicotinic, gap junction, and voltage-gated Ca2+ channels

An increase in circulating catecholamines constitutes one of the mechanisms whereby human body responds to stress. In response to chronic stressful situations, the adrenal medullary tissue exhibits crucial morphological and functional changes that are consistent with an improvement of chromaffin cell stimulus-secretion coupling efficiency. Stimulus-secretion coupling encompasses multiple intracellular (chromaffin cell excitability, Ca(2+) signaling, exocytosis, endocytosis) and intercellular pathways (splanchnic nerve-mediated synaptic transmission, paracrine and endocrine communication, gap junctional coupling), each of them being potentially subjected to functional remodeling upon stress. This review focuses on three chromaffin cell incontrovertible actors, the cholinergic nicotinic receptors and the voltage-dependent T-type Ca(2+) channels that are directly involved in Ca(2+)-dependent events controlling catecholamine secretion and electrical activity, and the gap junctional communication involved in the modulation of catecholamine secretion. We show here that these three actors react differently to various stressors, sometimes independently, sometimes in concert or in opposition.

Dynamin and myosin regulate differential exocytosis from mouse adrenal chromaffin cells

Neuroendocrine chromaffin cells of the adrenal medulla represent a primary output for the sympathetic nervous system. Chromaffin cells release catecholamine as well as vaso- and neuro-active peptide transmitters into the circulation through exocytic fusion of large dense-core secretory granules. Under basal sympathetic activity, chromaffin cells selectively release modest levels of catecholamines, helping to set the "rest and digest" status of energy storage. Under stress activation, elevated sympathetic firing leads to increased catecholamine as well as peptide transmitter release to set the "fight or flight" status of energy expenditure. While the mechanism for catecholamine release has been widely investigated, relatively little is known of how peptide transmitter release is regulated to occur selectively under elevated stimulation. Recent studies have shown selective catecholamine release under basal stimulation is accomplished through a transient, restricted exocytic fusion pore between granule and plasma membrane, releasing a soluble fraction of the small, diffusible molecules. Elevated cell firing leads to the active dilation of the fusion pore, leading to the release of both catecholamine and the less diffusible peptide transmitters. Here we propose a molecular mechanism regulating the activity-dependent dilation of the fusion pore. We review the immediate literature and provide new data to formulate a working mechanistic hypothesis whereby calcium-mediated dephosphorylation of dynamin I at Ser-774 leads to the recruitment of the molecular motor myosin II to actively dilate the fusion pore to facilitate release of peptide transmitters. Thus, activity-dependent dephosphorylation of dynamin is hypothesized to represent a key molecular step in the sympatho-adrenal stress response.

Massive calcium-activated endocytosis without involvement of classical endocytic proteins

We describe rapid massive endocytosis (MEND) of >50% of the plasmalemma in baby hamster kidney (BHK) and HEK293 cells in response to large Ca transients. Constitutively expressed Na/Ca exchangers (NCX1) are used to generate Ca transients, whereas capacitance recording and a membrane tracer dye, FM 4–64, are used to monitor endocytosis. With high cytoplasmic adenosine triphosphate (ATP; >5 mM), Ca influx causes exocytosis followed by MEND. Without ATP, Ca transients cause only exocytosis. MEND can then be initiated by pipette perfusion of ATP, and multiple results indicate that ATP acts via phosphatidylinositol-bis 4,5-phosphate (PIP₂) synthesis: PIP₂ substitutes for ATP to induce MEND. ATP-activated MEND is blocked by an inositol 5-phosphatase and by guanosine 5′-[γ-thio]triphosphate (GTPγS). Block by GTPγS is overcome by the phospholipase C inhibitor, U73122, and PIP₂ induces MEND in the presence of GTPγS. MEND can occur in the absence of ATP and PIP₂ when cytoplasmic free Ca is clamped to 10 µM or more by Ca-buffered solutions. ATP-independent MEND occurs within seconds during Ca transients when cytoplasmic solutions contain polyamines (e.g., spermidine) or the membrane is enriched in cholesterol. Although PIP₂ and cholesterol can induce MEND minutes after Ca transients have subsided, polyamines must be present during Ca transients. MEND can reverse over minutes in an ATP-dependent fashion. It is blocked by brief β-methylcyclodextrin treatments, and tests for involvement of clathrin, dynamins, calcineurin, and actin cytoskeleton were negative. Therefore, we turned to the roles of lipids. Bacterial sphingomyelinases (SMases) cause similar MEND responses within seconds, suggesting that ceramide may be important. However, Ca-activated MEND is not blocked by reagents that inhibit SMases. MEND is abolished by the alkylating phospholipase A₂ inhibitor, bromoenol lactone, whereas exocytosis remains robust, and Ca influx causes MEND in cardiac myocytes without preceding exocytosis. Thus, exocytosis is not prerequisite for MEND. From these results and two companion studies, we suggest that Ca promotes the formation of membrane domains that spontaneously vesiculate to the cytoplasmic side.

Mechanistic analysis of massive endocytosis in relation to functionally defined surface membrane domains

A large fraction of endocytosis in eukaryotic cells occurs without adaptors or dynamins. Here, we present evidence for the involvement of lipid domains in massive endocytosis (MEND) activated by both large Ca transients and amphipathic compounds in baby hamster kidney and HEK293 cells. First, we demonstrate functional coupling of the two MEND types. Ca transients can strongly facilitate detergent-activated MEND. Conversely, an amphipath with dual alkyl chains, ditridecylphthalate, is without effect in the absence of Ca transients but induces MEND to occur within seconds during Ca transients. Ca transients, like amphipaths, enhance the extraction of lipids from cells by β-cyclodextrins. Second, we demonstrate that electrical and/or optical signals generated by selected membrane probes are nearly insensitive to MEND, suggesting that those probes segregate into membrane domains that are not taken up by MEND. Triphenylphosphoniums are increasingly excluded from domains that internalize as the carbon chain length increases from 4 to 12. The small cationic membrane dye, FM 4–64, binds well to domains that internalize, whereas a closely related dye with a larger hydrophobic moiety, di-4-ANEPPDHQ (ANEPPDHQ) is excluded. Multiple carrier-type ionophores and a small amphipathic anion, niflumic acid, are also excluded. Probes with modest MEND sensitivity include the hydrophobic anion, dipicrylamine, carbonyl cyanide m-chlorophenylhydrazone, and NBD-phosphatidylethanolamine. Third, we demonstrate that large Ca transients can strongly enhance the extracellular binding of several membrane probes, monitored electrically or optically, consistent with a more disordered membrane with more amphipath-binding sites. Fluorescence shifts of ANEPPDHQ report increased disorder of the extracellular monolayer after large Ca transients, consistent with an increased propensity of the membrane to phase separate and vesiculate. Collectively, the results indicate that >50% of the outer monolayer is ordered and can be selectively internalized during MEND responses initiated by two very different cell perturbations.

Rapid endocytosis and vesicle recycling in neuroendocrine cells

Endocytosis is a crucial process for neuroendocrine cells that ensures membrane homeostasis, vesicle recycling, and hormone release reliability. Different endocytic mechanisms have been described in chromaffin cells, such as clathrin-dependent slow endocytosis and clathrin-independent rapid endocytosis. Rapid endocytosis, classically measured in terms of a fast decrease in membrane capacitance, exhibits two different forms, "rapid compensatory endocytosis" and "excess retrieval." While excess retrieval seems to be associated with formation of long-lasting endosomes, rapid compensatory endocytosis is well correlated with exocytotic activity, and it is regarded as a mechanism associated to rapid vesicle recycling during normal secretory activity. It has been suggested that rapid compensatory endocytosis may be related to the prevalence of a transient fusion mode of exo-endocytosis. In the latter mode, the fusion pore, a nanometric-sized channel formed at the onset of exocytosis, remains open for a few hundred milliseconds and later abruptly closes, releasing a small amount of transmitters. By this mechanism, endocrine cell selectively releases low molecular weight transmitters, and rapidly recycles the secretory vesicles. In this article, we discuss the cellular and molecular mechanisms that define the different forms of exocytosis and endocytosis and their impact on vesicle recycling pathways.

Developmental shift to a mechanism of synaptic vesicle endocytosis requiring nanodomain Ca2+

Ca(2+) is thought to be essential for the exocytosis and endocytosis of synaptic vesicles. However, the manner in which Ca(2+) coordinates these processes remains unclear, particularly at mature synapses. Using membrane capacitance measurements from calyx of Held nerve terminals in rats, we found that vesicle endocytosis is initiated primarily in Ca(2+) nanodomains around Ca(2+) channels, where exocytosis is triggered. Bulk Ca(2+) outside of the domain could also be involved in endocytosis at immature synapses, although only after extensive exocytosis at more mature synapses. This bulk Ca(2+)-dependent endocytosis required calmodulin and calcineurin activation at immature synapses, but not at more mature synapses. Similarly, GTP-independent endocytosis, which occurred after extensive exocytosis at immature synapses, became negligible after maturation. We propose that nanodomain Ca(2+) simultaneously triggers exocytosis and endocytosis of synaptic vesicles and that the molecular mechanisms underlying Ca(2+)-dependent endocytosis undergo major developmental changes at this fast central synapse.

The molecular physiology of activity-dependent bulk endocytosis of synaptic vesicles

Central nerve terminals release neurotransmitter in response to a wide variety of stimuli. Because maintenance of neurotransmitter release is dependent on the continual supply of synaptic vesicles (SVs), nerve terminals possess an array of endocytosis modes to retrieve and recycle SV membrane and proteins. During mild stimulation conditions, single SV retrieval modes such as clathrin-mediated endocytosis predominate. However, during increased neuronal activity, additional SV retrieval capacity is required, which is provided by activity-dependent bulk endocytosis (ADBE). ADBE is the dominant SV retrieval mechanism during elevated neuronal activity. It is a high capacity SV retrieval mode that is immediately triggered during such stimulation conditions. This review will summarize the current knowledge regarding the molecular mechanism of ADBE, including molecules required for its triggering and subsequent steps, including SV budding from bulk endosomes. The molecular relationship between ADBE and the SV reserve pool will also be discussed. It is becoming clear that an understanding of the molecular physiology of ADBE will be of critical importance in attempts to modulate both normal and abnormal synaptic function during intense neuronal activity.

Loose coupling between calcium channels and sites of exocytosis in chromaffin cells

Calcium microdomains generated by tight clusters of calcium channels regulate fusion of small vesicles at the synaptic terminal and have also been suggested to trigger exocytosis of large dense-core vesicles from neuroendocrine cells. To test this idea, we have compared sites of exocytosis and the spatial distribution of calcium channels in chromaffin cells. Fusion of individual vesicles was visualized using interference reflection microscopy and the submembranous calcium signal was assessed using total internal reflection fluorescence microscopy. Depolarization triggered a burst of exocytosis from up to seven sites in a membrane area of 11 microm(2), but these sites did not colocalize with calcium microdomains. Instead, calcium influx occurred in large patches (averaging 34 microm(2)) containing a mixture of P/Q- and N-type channels. About 20% of fusion events occurred outside calcium channel patches. Further, the delay between the onset of stimulation and a burst of exocytosis was prolonged for several seconds by increasing the concentration of the slow calcium chelator EGTA from 1.5 to 5 mM. These results demonstrate that while calcium channels and release sites tend to congregate in specialized regions of the surface membrane, these have dimensions of several micrometres. The dominant calcium signal regulating release in chromaffin cells is generated by the cooperative action of many channels operating over distances of many micrometres rather than discrete clusters of calcium channels generating localized microdomains.

Enhanced dense core granule function and adrenal hypersecretion in a mouse model of Rett syndrome

Rett syndrome (RTT) is a progressive developmental disorder resulting from loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2), a transcription regulatory protein. The RTT phenotype is complex and includes severe cardiorespiratory abnormalities, dysautonomia and behavioral symptoms of elevated stress. These findings have been attributed to an apparent hyperactivity of the sympathetic nervous system due to defects in brainstem development; however, the possibility that the peripheral sympathoadrenal axis itself is abnormal has not been explored. The present study demonstrates that the adrenal medulla and sympathetic ganglia of Mecp2 null mice exhibit markedly reduced catecholamine content compared with wild-type controls. Despite this, null animals exhibit significantly higher plasma epinephrine levels, suggesting enhanced secretory granule function in adrenal chromaffin cells. Indeed, we find that Mecp2 null chromaffin cells exhibit a cell autonomous hypersecretory phenotype characterized by significant increases in the speed and size of individual secretory granule fusion events in response to electrical stimulation. These findings appear to indicate accelerated formation and enhanced dilation of the secretory granule fusion pore, resulting in elevated catecholamine release. Our data therefore highlight abnormal catecholamine function in the sympathoadrenal axis as a potential source of autonomic dysfunction in RTT. These findings may help to explain the apparent 'overactivity' of the sympathetic nervous system reported in patients with RTT.

The phospho-dependent dynamin-syndapin interaction triggers activity-dependent bulk endocytosis of synaptic vesicles

Synaptic vesicles (SVs) are retrieved by more than one mode in central nerve terminals. During mild stimulation, the dominant SV retrieval pathway is classical clathrin-mediated endocytosis (CME). During elevated neuronal activity, activity-dependent bulk endocytosis (ADBE) predominates, which requires activation of the calcium-dependent protein phosphatase calcineurin. We now report that calcineurin dephosphorylates dynamin I in nerve terminals only above the same activity threshold that triggers ADBE. ADBE was arrested when the two major phospho-sites on dynamin I were perturbed, suggesting that dynamin I dephosphorylation is a key step in its activation. Dynamin I dephosphorylation stimulates a specific dynamin I-syndapin I interaction. Inhibition of this interaction by competitive peptides or by site-directed mutagenesis exclusively inhibited ADBE but did not affect CME. The results reveal that the phospho-dependent dynamin-syndapin interaction recruits ADBE to massively increase SV endocytosis under conditions of elevated neuronal activity.

Activity-dependent bulk synaptic vesicle endocytosis--a fast, high capacity membrane retrieval mechanism

Central nerve terminals are placed under considerable stress during intense stimulation due to large numbers of synaptic vesicles (SVs) fusing with the plasma membrane. Classical clathrin-dependent SV endocytosis cannot correct for the large increase in nerve terminal surface area in the short term, due to its slow kinetics and low capacity. During such intense stimulation, an additional SV retrieval pathway is recruited called bulk endocytosis. Recent studies have shown that bulk endocytosis fulfils all of the physiological requirements to remedy the acute changes in nerve terminal surface area to allow the nerve terminal to continue to function. This review will summarise the recent developments in the field that characterise the physiology of bulk endocytosis which show that it is a fast, activity-dependent and high capacity mechanism that is essential for the function of central nerve terminals.

Cortical F-actin, the exocytic mode, and neuropeptide release in mouse chromaffin cells is regulated by myristoylated alanine-rich C-kinase substrate and myosin II

Adrenal medullary chromaffin cells are innervated by the sympathetic splanchnic nerve and translate graded sympathetic firing into a differential hormonal exocytosis. Basal sympathetic firing elicits a transient kiss-and-run mode of exocytosis and modest catecholamine release, whereas elevated firing under the sympathetic stress response results in full granule collapse to release catecholamine and peptide transmitters into the circulation. Previous studies have shown that rearrangement of the cell actin cortex regulates the mode of exocytosis. An intact cortex favors kiss-and-run exocytosis, whereas disrupting the cortex favors the full granule collapse mode. Here, we investigate the specific roles of two actin-associated proteins, myosin II and myristoylated alanine-rich C-kinase substrate (MARCKS) in this process. Our data demonstrate that MARCKS phosphorylation under elevated cell firing is required for cortical actin disruption but is not sufficient to elicit peptide transmitter exocytosis. Our data also demonstrate that myosin II is phospho-activated under high stimulation conditions. Inhibiting myosin II activity prevented disruption of the actin cortex, full granule collapse, and peptide transmitter release. These results suggest that phosphorylation of both MARCKS and myosin II lead to disruption of the actin cortex. However, myosin II, but not MARCKS, is required for the activity-dependent exocytosis of the peptide transmitters.

The dynamic control of kiss-and-run and vesicular reuse probed with single nanoparticles

Vesicular secretion of neurotransmitter is essential for neuronal communication. Kiss-and-run is a mode of membrane fusion and retrieval without the full collapse of the vesicle into the plasma membrane and de novo regeneration. The importance of kiss-and-run during efficient neurotransmission has remained in doubt. We developed an approach for loading individual synaptic vesicles with single quantum dots. Their size and pH-dependent photoluminescence change allowed us to distinguish kiss-and-run from full-collapse fusion and to track single vesicles through multiple rounds of kiss-and-run and reuse, without perturbing vesicle cycling. Kiss-and-run dominated at the beginning of stimulus trains, reflecting the preference of vesicles with high release probability. Its incidence was increased by rapid firing, a response appropriate to shape the kinetics of neurotransmission during a wide range of firing patterns.

Myosin II activation and actin reorganization regulate the mode of quantal exocytosis in mouse adrenal chromaffin cells

Chromaffin cells of the adrenal medulla are innervated by the sympathetic nervous system. Stimulation causes chromaffin cells to fire action potentials, leading to the exocytosis of various classes of transmitters into the circulation. Low-frequency electrical stimulation (action potentials delivered at 0.5 Hz) causes adrenal chromaffin cells to selectively release catecholamines through a kiss-and-run fusion event. Elevated electrical stimulation (action potentials at 15 Hz) evokes fusion pore dilation, full granule collapse, and additional release of the neuropeptide-containing proteinaceous granule core. Here we apply single-cell electrophysiological, electrochemical, and fluorescence measurements to investigate the cellular mechanism for this shift in exocytic behavior. We show that at low-frequency stimulation, a filamentous-actin cell cortex plays a key role in stabilizing the kiss-and-run fusion event. Increased stimulation disrupts the actin cortex, driving full granule collapse. We show that pharmacological perturbation of the actin cortex supersedes stimulus frequency in controlling exocytic mode. Finally, we show that nonmuscle myosin II activation contributes to the cytoskeleton-dependent control of the fusion event. Inhibition of myosin II or myosin light chain kinase under elevated stimulation frequencies inhibits fusion pore dilation and maintains the granule in a kiss-and-run mode of exocytosis. These results demonstrate an essential role for activity-evoked cytoskeletal rearrangement and the action of myosin II in the regulation of catecholamine and neuropeptide exocytosis and represent an essential element of the sympathetic stress response.

Dynamin I plays dual roles in the activity-dependent shift in exocytic mode in mouse adrenal chromaffin cells

Under low stimulation, adrenal chromaffin cells release freely soluble catecholamines through a restricted granule fusion pore while retaining the large neuropeptide-containing proteinacious granule core. Elevated activity causes dilation of the pore and release of all granule contents. Thus, physiological differential transmitter release is achieved through regulation of fusion pore dilation. We examined the mechanism for pore dilation utilizing a combined approach of peptide transfection, electrophysiology, electrochemistry and quantitative imaging techniques. We report that disruption of dynamin I function alters both fusion modes. Under low stimulation, interference with dynamin I does not affect granule fusion but blocks its re-internalization. In full collapse mode, disruption of dynamin I limits fusion pore dilation, but does not block membrane re-internalization. These data suggest that dynamin I is involved in both modes of exocytosis by regulating contraction or dilation of the fusion pore and thus contributes to activity-dependent differential transmitter release from the adrenal medulla.

Two independent forms of endocytosis maintain embryonic cell surface homeostasis during early development

Eukaryotic cells have multiple forms of endocytosis which maintain cell surface homeostasis. One explanation for this apparent redundancy is to allow independent retrieval of surface membranes derived from different types of vesicles. Consistent with this hypothesis we find that sea urchin eggs have at least two types of compensatory endocytosis. One is associated with retrieving cortical vesicle membranes, and formed large endosomes by a mechanism that was inhibited by agatoxin, cadmium, staurosporine and FK506. The second type is thought to compensate for constitutive exocytosis, and formed small endosomes using a mechanism that was insensitive to the above mentioned reagents, but was inhibited by phenylarsine oxide (PAO), and by microinjection of mRNA encoding Src kinase. Both mechanisms could act concurrently, and account for all of the endocytosis occurring during early development. Inhibition of either form did not trigger compensation by the other form, and phorbol ester treatment rescued the endocytotic activity blocked by agatoxin, but not the retrieval blocked by PAO.

Vesicle priming and recruitment by ubMunc13-2 are differentially regulated by calcium and calmodulin

Ca2+ regulates multiple processes in nerve terminals, including synaptic vesicle recruitment, priming, and fusion. Munc13s, the mammalian homologs of Caenorhabditis elegans Unc13, are essential vesicle-priming proteins and contain multiple regulatory domains that bind second messengers such as diacylglycerol and Ca2+/calmodulin (Ca2+/CaM). Binding of Ca2+/CaM is necessary for the regulatory effect that allows Munc13-1 and ubMunc13-2 to promote short-term synaptic plasticity. However, the relative contributions of Ca2+ and Ca2+/CaM to vesicle priming and recruitment by Munc13 are not known. Here, we investigated the effect of Ca2+/CaM binding on ubMunc13-2 activity in chromaffin cells via membrane-capacitance measurements and a detailed simulation of the exocytotic machinery. Stimulating secretion under various basal Ca2+ concentrations from cells overexpressing either ubMunc13-2 or a ubMunc13-2 mutant deficient in CaM binding enabled a distinction between the effects of Ca2+ and Ca2+/CaM. We show that vesicle priming by ubMunc13-2 is Ca2+ dependent but independent of CaM binding to ubMunc13-2. However, Ca2+/CaM binding to ubMunc13-2 specifically promotes vesicle recruitment during ongoing stimulation. Based on the experimental data and our simulation, we propose that ubMunc13-2 is activated by two Ca2+-dependent processes: a slow activation mode operating at low Ca2+ concentrations, in which ubMunc13-2 acts as a priming switch, and a fast mode at high Ca2+ concentrations, in which ubMunc13-2 is activated in a Ca2+/CaM-dependent manner and accelerates vesicle recruitment and maturation during stimulation. These different Ca2+ activation steps determine the kinetic properties of exocytosis and vesicle recruitment and can thus alter plasticity and efficacy of transmitter release.

Activity-dependent control of slow synaptic vesicle endocytosis by cyclin-dependent kinase 5

The stimulated dephosphorylation of the dephosphin group of endocytic proteins by calcineurin and their subsequent rephosphorylation by cyclin-dependent kinase 5 (cdk5) is required for synaptic vesicle (SV) retrieval in central nerve terminals. However, the specific endocytic pathway(s) controlled by these enzymes is unknown. To address this issue, we combined functional and morphological assays of endocytosis in primary neuronal cultures with pharmacological and molecular ablation of calcineurin and cdk5 activity. During strong stimulation, inhibition of calcineurin or cdk5 blocked uptake of the activity-dependent membrane marker FM1-43, but not the more hydrophilic FM2-10. However, FM2-10 uptake-measured poststimulation was sensitive to cdk5 and calcineurin inhibition, indicating that a slow form of endocytosis persists after termination of stimulation. In parallel EM studies, inhibition of cdk5 during strong stimulation greatly reduced horseradish peroxidase labeling of plasma membrane-derived nerve terminal endosomes, but not SVs. Furthermore, during mild stimulation, FM1-43 uptake was unaffected by cdk5 inhibition and the SV membrane was exclusively retrieved via a single SV route, suggesting that recruitment of the endosomal route of membrane retrieval is activity dependent. Thus, we propose that the calcineurin/cdk5-dependent phosphorylation cycle of the dephosphins specifically controls a slow endocytic pathway that proceeds via endosomal intermediates and is activated by strong physiological stimulation in central nerve terminals.

Activity-dependent differential transmitter release in mouse adrenal chromaffin cells

Chromaffin cells of the adrenal medulla are a primary neuroendocrine output of the sympathetic nervous system. When stimulated, they secrete a host of transmitter molecules, including catecholamines and neuropeptides, through the fusion of dense core secretory granules with the cell surface. At basal firing rates, set by the sympathetic tone, chromaffin cells selectively release catecholamines at a modest rate. Stress-mediated sympathetic activation leads to elevated catecholamine secretion and also evokes neuropeptide release. Catecholamines and neuropeptides are copackaged in the same granules; thus, it is unclear how this activity-dependent differential transmitter release is achieved. In this report, we use electrophysiological, electrochemical, fluorescence, and immunocytochemical approaches to quantify transmitter release under physiological electrical stimulation at the single cell level. We provide data to show that chromaffin cells selectively release catecholamine under basal firing conditions but release both neuropeptides and catecholamines under conditions that match acute stress. We further show that this differential transmitter release is achieved through a regulated activity-dependent dilation of the granule fusion pore. Thus, chromaffin cells may regulate release of different transmitters through a simple size-exclusion mechanism.

Developmental change in the calcium sensor for synaptic vesicle endocytosis in central nerve terminals

Synaptic vesicle endocytosis is stimulated by calcium influx in mature central nerve terminals via activation of the calcium-dependent protein phosphatase, calcineurin. However, in different neuronal preparations calcineurin activity is either inhibitory, stimulatory or irrelevant to the process. We addressed this inconsistency by investigating the requirement for calcineurin activity in synaptic vesicle endocytosis during development, using vesicle recycling assays in isolated nerve terminals. We show that endocytosis occurs independently of calcineurin activity in immature nerve terminals, and that a calcineurin requirement develops 2-4 weeks after birth. Calcineurin-independent endocytosis is not due to the absence of calcineurin activity, since calcineurin is present in immature nerve terminals and its substrate, dynamin I, is dephosphorylated on depolarization. Calcineurin-independent endocytosis is calcium-dependent, since substitution of the divalent cation, barium, inhibits the process. Finally, we demonstrated that in primary neuronal cultures derived from neonatal rats, endocytosis that was initially calcineurin-independent developed a calcineurin requirement on maturation in culture. Our data account for the apparent inconsistencies regarding the role of calcineurin in synaptic vesicle endocytosis, and we propose that an unidentified calcium sensor exists to couple calcium influx to endocytosis in immature nerve terminals.

Control of secretion by temporal patterns of action potentials in adrenal chromaffin cells

Action potentials (APs) are the principal physiological stimuli for neurotransmitter secretion in neurons. Most studies on stimulus-secretion coupling have been performed under voltage clamp using artificial electrical stimuli. To investigate the modulatory effects of AP codes on neural secretion, we introduce a capacitance method to study AP-induced secretion in single cells. The action potential pattern was defined by a four-parameter "code function:" F(n, m, f, d). With this method, cell secretion evoked by stimulation with an AP code was quantified in real time by membrane capacitance (Cm) in adrenal chromaffin cells. We found, in addition to AP frequency (f), for a given number of APs, another parameter of the AP code, the number of AP bursts (m) in which the set of APs occurs, can effectively modulate cell secretion. Possible mechanisms of the m effect are depletion of the readily releasable pool and inactivation of Ca2+ channels during a burst of APs. The physiological m effect may play a key role in AP-mediated neural information transfer within a single neuron and among the elements of a neural network.

Control of secretion by temporal patterns of action potentials in adrenal chromaffin cells

Action potentials (APs) are the principal physiological stimuli for neurotransmitter secretion in neurons. Most studies on stimulus-secretion coupling have been performed under voltage clamp using artificial electrical stimuli. To investigate the modulatory effects of AP codes on neural secretion, we introduce a capacitance method to study AP-induced secretion in single cells. The action potential pattern was defined by a four-parameter "code function:" F(n, m, f, d). With this method, cell secretion evoked by stimulation with an AP code was quantified in real time by membrane capacitance (Cm) in adrenal chromaffin cells. We found, in addition to AP frequency (f), for a given number of APs, another parameter of the AP code, the number of AP bursts (m) in which the set of APs occurs, can effectively modulate cell secretion. Possible mechanisms of the m effect are depletion of the readily releasable pool and inactivation of Ca2+ channels during a burst of APs. The physiological m effect may play a key role in AP-mediated neural information transfer within a single neuron and among the elements of a neural network.

Low frequency stimulation of mouse adrenal slices reveals a clathrin-independent, protein kinase C-mediated endocytic mechanism

Evidence suggests that chromaffin cells employ separate mechanisms for evoked endocytosis and granule recycling when stimulated at basal (approximately 0.5 Hz) and stress-activated (approximately 15 Hz) rates. Previous studies have focused mainly on elucidating the cellular mechanisms responsible for membrane recycling under conditions similar to the stress-activated state and indicate a clathrin/dephosphin-mediated retrieval via coated pits. However, the mechanism for membrane internalisation at basal stimulus intensity remains largely unexplored. We electrically stimulated chromaffin cells in adrenal tissue slices at the sympathetic basal firing rate and measured cell capacitance in the perforated voltage clamp configuration. A new method for the separation of non-secretory from secretory cell capacitance signals is presented. Simultaneous catecholamine release was measured electrochemically to isolate the exocytic from endocytic components of the capacitance responses. Using this approach we demonstrate that firing patterns that mimic basal sympathetic input results in rapid and graded membrane retrieval. We show that block of the calcium-mediated protein phosphatase 2B, a common step in clathrin-mediated processes, did not alter endocytosis elicited at basal firing levels. We further blocked clathrin-mediated retrieval with a clathrin/dephosphin-disrupting peptide (PP-19) and found endocytosis to be blocked at 15 Hz stimulation but complete and indistinguishable from control cells at 0.5 Hz stimulation. Lastly, pharmacological treatments show that conventional isoforms of protein kinase C (cPKC) are required for the 0.5 Hz-evoked retrieval mechanism. From these data we conclude that unlike endocytosis evoked under stress conditions, basal firing activity results in a clathrin-independent rapid membrane retrieval mediated through conventional isoforms of PKC.