Activity-driven synaptic translocation of LGI1 controls excitatory neurotransmission

The fine control of synaptic function requires robust trans-synaptic molecular interactions. However, it remains poorly understood how trans-synaptic bridges change to reflect the functional states of the synapse. Here, we develop optical tools to visualize in firing synapses the molecular behavior of two trans-synaptic proteins, LGI1 and ADAM23, and find that neuronal activity acutely rearranges their abundance at the synaptic cleft. Surprisingly, synaptic LGI1 is primarily not secreted, as described elsewhere, but exo- and endocytosed through its interaction with ADAM23. Activity-driven translocation of LGI1 facilitates the formation of trans-synaptic connections proportionally to the history of activity of the synapse, adjusting excitatory transmission to synaptic firing rates. Accordingly, we find that patient-derived autoantibodies against LGI1 reduce its surface fraction and cause increased glutamate release. Our findings suggest that LGI1 abundance at the synaptic cleft can be acutely remodeled and serves as a critical control point for synaptic function.

Frequency of Spontaneous Neurotransmission at Individual Boutons Corresponds to the Size of the Readily Releasable Pool of Vesicles

Synapses maintain two forms of neurotransmitter release to support communication in the brain. First, evoked neurotransmitter release is triggered by the invasion of an action potential (AP) across en passant boutons that form along axons. The probability of evoked release (Pr) varies substantially across boutons, even within a single axon. Such heterogeneity is the result of differences in the probability of a single synaptic vesicle (SV) fusing (Pv) and in the number of vesicles available for immediate release, known as the readily releasable pool (RRP). Spontaneous release (also known as a mini) is an important form of neurotransmission that occurs in the absence of APs. Because it cannot be triggered with electrical stimulation, much less is known about potential heterogeneity in the frequency of spontaneous release between boutons. We utilized a photostable and bright fluorescent indicator of glutamate release (iGluSnFR3) to quantify both spontaneous and evoked release at individual glutamatergic boutons. We found that the rate of spontaneous release is quite heterogenous at the level of individual boutons. Interestingly, when measuring both evoked and spontaneous release at single synapses, we found that boutons with the highest rates of spontaneous release also displayed the largest evoked responses. Using a new optical method to measure RRP at individual boutons, we found that this heterogeneity in spontaneous release was strongly correlated with the size of the RRP, but not related to Pv. We conclude that the RRP is a critical and dynamic aspect of synaptic strength that contributes to both evoked and spontaneous vesicle release.

A ratiometric ER calcium sensor for quantitative comparisons across cell types and subcellular regions

The endoplasmic reticulum (ER) is an important regulator of Ca 2 + in cells and dysregulation of ER calcium homeostasis can lead to numerous pathologies. Understanding how various pharmacological and genetic perturbations of ER Ca 2 + homeostasis impacts cellular physiology would likely be facilitated by more quantitative measurements of ER Ca 2 + levels that allow easier comparisons across conditions. Here, we developed a ratiometric version of our original ER-GCaMP probe that allows for more quantitative comparisons of the concentration of Ca 2 + in the ER across cell types and sub-cellular compartments. Using this approach we show that the resting concentration of ER Ca2+ in primary dissociated neurons is substantially lower than that in measured in embryonic fibroblasts.

Glutamate indicators with improved activation kinetics and localization for imaging synaptic transmission

The fluorescent glutamate indicator iGluSnFR enables imaging of neurotransmission with genetic and molecular specificity. However, existing iGluSnFR variants exhibit low in vivo signal-to-noise ratios, saturating activation kinetics and exclusion from postsynaptic densities. Using a multiassay screen in bacteria, soluble protein and cultured neurons, we generated variants with improved signal-to-noise ratios and kinetics. We developed surface display constructs that improve iGluSnFR's nanoscopic localization to postsynapses. The resulting indicator iGluSnFR3 exhibits rapid nonsaturating activation kinetics and reports synaptic glutamate release with decreased saturation and increased specificity versus extrasynaptic signals in cultured neurons. Simultaneous imaging and electrophysiology at individual boutons in mouse visual cortex showed that iGluSnFR3 transients report single action potentials with high specificity. In vibrissal sensory cortex layer 4, we used iGluSnFR3 to characterize distinct patterns of touch-evoked feedforward input from thalamocortical boutons and both feedforward and recurrent input onto L4 cortical neuron dendritic spines.

NMDAR-dependent presynaptic homeostasis in adult hippocampus: Synapse growth and cross-modal inhibitory plasticity

Homeostatic plasticity (HP) encompasses a suite of compensatory physiological processes that counteract neuronal perturbations, enabling brain resilience. Currently, we lack a complete description of the homeostatic processes that operate within the mammalian brain. Here, we demonstrate that acute, partial AMPAR-specific antagonism induces potentiation of presynaptic neurotransmitter release in adult hippocampus, a form of compensatory plasticity that is consistent with the expression of presynaptic homeostatic plasticity (PHP) documented at peripheral synapses. We show that this compensatory plasticity can be induced within minutes, requires postsynaptic NMDARs, and is expressed via correlated increases in dendritic spine volume, active zone area, and docked vesicle number. Further, simultaneous postsynaptic genetic reduction of GluA1, GluA2, and GluA3 in triple heterozygous knockouts induces potentiation of presynaptic release. Finally, induction of compensatory plasticity at excitatory synapses induces a parallel, NMDAR-dependent potentiation of inhibitory transmission, a cross-modal effect consistent with the anti-epileptic activity of AMPAR-specific antagonists used in humans.

Condensing our understanding of endocytosis

In this issue of Neuron, Imoto et al. report that a splice variant of dynamin (Dyn1xA) interacts with syndapin to form a molecular condensate at the edge of the presynaptic active zone. This enables rapid recruitment of proteins to endocytic sites essential for powering ultrafast endocytosis.

Dividing communication, at the nanoscale

Fluorescent glutamate sensors shed light on the microscopic organization underlining spontaneous neurotransmission.

Focused Ultrasound Stimulation of an ex-vivo Aplysia Abdominal Ganglion Preparation

BACKGROUND

A growing body of research demonstrates that focused ultrasound stimulates activity in human and other mammalian nervous systems. However, there is no consensus on which sonication parameters are optimal. Furthermore, the mechanism of action behind ultrasound neurostimulation remains poorly understood. An invertebrate model greatly reduces biological complexity, permitting a systematic evaluation of sonication parameters suitable for ultrasound neurostimulation.

NEW METHOD

Here, we describe the use of focused ultrasound stimulation with an ex-vivo abdominal ganglion preparation of the California sea hare, Aplysia californica, a long-standing model system in neurobiology. We developed a system for stimulating an isolated ganglion preparation while obtaining extracellular recordings from nerves. The focused ultrasound stimulation uses one of two single-element transducers, enabling stimulation at four distinct carrier frequencies (0.515 MHz, 1.l MHz, 1.61 MHz, 3.41 MHz).

RESULTS

Using continuous wave ultrasound, we stimulated the ganglion at all four frequencies, and we present quantitative evaluation of elicited activation at four different sonication durations and three peak pressure levels, eliciting up to a 57-fold increase in spiking frequency.

COMPARISON WITH ELECTRICAL STIMULATION

We demonstrated that ultrasound-induced activation is repeatable, and the response consistency is comparable to electrical stimulation.

CONCLUSIONS

Due to the relative ease of long-term recordings for many hours, this ex-vivo ganglion preparation is suitable for investigating sonication parameters and the effects of focused ultrasound stimulation on neurons.

Freeze-frame imaging of synaptic activity using SynTagMA

Information within the brain travels from neuron to neuron across billions of synapses. At any given moment, only a small subset of neurons and synapses are active, but finding the active synapses in brain tissue has been a technical challenge. Here we introduce SynTagMA to tag active synapses in a user-defined time window. Upon 395-405 nm illumination, this genetically encoded marker of activity converts from green to red fluorescence if, and only if, it is bound to calcium. Targeted to presynaptic terminals, preSynTagMA allows discrimination between active and silent axons. Targeted to excitatory postsynapses, postSynTagMA creates a snapshot of synapses active just before photoconversion. To analyze large datasets, we show how to identify and track the fluorescence of thousands of individual synapses in an automated fashion. Together, these tools provide an efficient method for repeatedly mapping active neurons and synapses in cell culture, slice preparations, and in vivo during behavior.

The Photoconvertible Fluorescent Probe, CaMPARI, Labels Active Neurons in Freely-Moving Intact Adult Fruit Flies

Linking neural circuitry to behavior by mapping active neurons in vivo is a challenge. Both genetically encoded calcium indicators (GECIs) and intermediate early genes (IEGs) have been used to pinpoint active neurons during a stimulus or behavior but have drawbacks such as limiting the movement of the organism, requiring a priori knowledge of the active region or having poor temporal resolution. Calcium-modulated photoactivatable ratiometric integrator (CaMPARI) was engineered to overcome these spatial-temporal challenges. CaMPARI is a photoconvertible protein that only converts from green to red fluorescence in the presence of high calcium concentration and 405 nm light. This allows the experimenter to precisely mark active neurons within defined temporal windows. The photoconversion can then be quantified by taking the ratio of the red fluorescence to the green. CaMPARI promises the ability to trace active neurons during a specific stimulus; however, CaMPARI's uses in adult Drosophila have been limited to photoconversion during fly immobilization. Here, we demonstrate a method that allows photoconversion of multiple freely-moving intact adult flies during a stimulus. Flies were placed in a dish with filter paper wet with acetic acid (pH = 2) or neutralized acetic acid (pH = 7) and exposed to photoconvertible light (60 mW) for 30 min (500 ms on, 200 ms off). Immediately following photoconversion, whole flies were fixed and imaged by confocal microscopy. The red:green ratio was quantified for the DC4 glomerulus, a bundle of neurons expressing Ir64a, an ionotropic receptor that senses acids in the Drosophila antennal lobe. Flies exposed to acetic acid showed 1.3-fold greater photoconversion than flies exposed to neutralized acetic acid. This finding was recapitulated using a more physiological stimulus of apple cider vinegar. These results indicate that CaMPARI can be used to label neurons in intact, freely-moving adult flies and will be useful for identifying the circuitry underlying complex behaviors.

Genetically Encoded Voltage Indicators Are Illuminating Subcellular Physiology of the Axon

Everything we see and do is regulated by electrical signals in our nerves and muscle. Ion channels are crucial for sensing and generating electrical signals. Two voltage-dependent conductances, Na+ and K+, form the bedrock of the electrical impulse in the brain known as the action potential. Several classes of mammalian neurons express combinations of nearly 100 different varieties of these two voltage-dependent channels and their subunits. Not surprisingly, this variability orchestrates a diversity of action potential shapes and firing patterns that have been studied in detail at neural somata. A remarkably understudied phenomena exists in subcellular compartments of the axon, where action potentials initiate synaptic transmission. Ion channel research was catalyzed by the invention of glass electrodes to measure electrical signals in cell membranes, however, progress in the field of neurobiology has been stymied by the fact that most axons in the mammalian CNS are far too small and delicate for measuring ion channel function with electrodes. These quantitative measurements of membrane voltage can be achieved within the axon using light. A revolution of optical voltage sensors has enabled exploring important questions of how ion channels regulate axon physiology and synaptic transmission. In this review we will consider advantages and disadvantages of different fluorescent voltage indicators and discuss particularly relevant questions that these indicators can elucidate for understanding the crucial relationship between action potentials and synaptic transmission.

Subcellular control of membrane excitability in the axon

Ion channels are microscopic pore proteins in the membrane that open and close in response to chemical and electrical stimuli. This simple concept underlies rapid electrical signaling in the brain as well as several important aspects of neural plasticity. Although the soma accounts for less than 1% of many neurons by membrane area, it has been the major site of measuring ion channel function. However, the axon is one of the longest processes found in cellular biology and hosts a multitude of critical signaling functions in the brain. Not only does the axon initiate and rapidly propagate action potentials (APs) across the brain but it also forms the presynaptic terminals that convert these electrical inputs into chemical outputs. Here, we review recent advances in the physiological role of ion channels within the diverse landscape of the axon and presynaptic terminals.

Loss of Neurofascin-186 Disrupts Alignment of AnkyrinG Relative to Its Binding Partners in the Axon Initial Segment

The axon initial segment (AIS) is a specialized region within the proximal portion of the axon that initiates action potentials thanks in large part to an enrichment of sodium channels. The scaffolding protein ankyrinG (AnkG) is essential for the recruitment of sodium channels as well as several other intracellular and extracellular proteins to the AIS. In the present study, we explore the role of the cell adhesion molecule (CAM) neurofascin-186 (NF-186) in arranging the individual molecular components of the AIS in cultured rat hippocampal neurons. Using a CRISPR depletion strategy to ablate NF expression, we found that the loss of NF selectively perturbed AnkG accumulation and its relative proximal distribution within the AIS. We found that the overexpression of sodium channels could restore AnkG accumulation, but not its altered distribution within the AIS without NF present. We go on to show that although the loss of NF altered AnkG distribution, sodium channel function within the AIS remained normal. Taken together, these results demonstrate that the regulation of AnkG and sodium channel accumulation within the AIS can occur independently of one another, potentially mediated by other binding partners such as NF.

Adaptor Protein 2 (AP-2) complex is essential for functional axogenesis in hippocampal neurons

The complexity and diversity of a neural network requires regulated elongation and branching of axons, as well as the formation of synapses between neurons. In the present study we explore the role of AP-2, a key endocytic adaptor protein complex, in the development of rat hippocampal neurons. We found that the loss of AP-2 during the early stage of development resulted in impaired axon extension and failed maturation of the axon initial segment (AIS). Normally the AIS performs two tasks in concert, stabilizing neural polarity and generating action potentials. In AP-2 silenced axons polarity is established, however there is a failure to establish action potential firing. Consequently, this impairs activity-driven Ca²⁺ influx and exocytosis at nerve terminals. In contrast, removal of AP-2 from older neurons does not impair axonal growth or signaling and synaptic function. Our data reveal that AP-2 has important roles in functional axogenesis by proper extension of axon as well as the formation of AIS during the early step of neurodevelopment.

Intrinsic variability in Pv, RRP size, Ca(2+) channel repertoire, and presynaptic potentiation in individual synaptic boutons

The strength of individual synaptic contacts is considered a key modulator of information flow across circuits. Presynaptically the strength can be parsed into two key parameters: the size of the readily releasable pool (RRP) and the probability that a vesicle in that pool will undergo exocytosis when an action potential fires (Pv). How these variables are controlled and the degree to which they vary across individual nerve terminals is crucial to understand synaptic plasticity within neural circuits. Here we report robust measurements of these parameters in rat hippocampal neurons and their variability across populations of individual synapses. We explore the diversity of presynaptic Ca²⁺ channel repertoires and evaluate their effect on synaptic strength at single boutons. Finally, we study the degree to which synapses can be differentially modified by a known potentiator of presynaptic function, forskolin. Our experiments revealed that both Pv and RRP spanned a large range, even for synapses made by the same axon, demonstrating that presynaptic efficacy is governed locally at the single synapse level. Synapses varied greatly in their dependence on N or P/Q type Ca²⁺ channels for neurotransmission, but there was no association between specific channel repertoires and synaptic efficacy. Increasing cAMP concentration using forskolin enhanced synaptic transmission in a Ca²⁺-independent manner that was inversely related with a synapse's initial Pv, and independent of its RRP size. We propose a simple model based on the relationship between Pv and calcium entry that can account for the variable potentiation of synapses based on initial probability of vesicle fusion.

Progression of diet-induced diabetes in C57BL6J mice involves functional dissociation of Ca2(+) channels from secretory vesicles

OBJECTIVE

The aim of the study was to elucidate the cellular mechanism underlying the suppression of glucose-induced insulin secretion in mice fed a high-fat diet (HFD) for 15 weeks.

RESEARCH DESIGN AND METHODS

C57BL6J mice were fed a HFD or a normal diet (ND) for 3 or 15 weeks. Plasma insulin and glucose levels in vivo were assessed by intraperitoneal glucose tolerance test. Insulin secretion in vitro was studied using static incubations and a perfused pancreas preparation. Membrane currents, electrical activity, and exocytosis were examined by patch-clamp technique measurements. Intracellular calcium concentration ([Ca(2+)](i)) was measured by microfluorimetry. Total internal reflection fluorescence microscope (TIRFM) was used for optical imaging of exocytosis and submembrane depolarization-evoked [Ca(2+)](i). The functional data were complemented by analyses of histology and gene transcription.

RESULTS

After 15 weeks, but not 3 weeks, mice on HFD exhibited hyperglycemia and hypoinsulinemia. Pancreatic islet content and beta-cell area increased 2- and 1.5-fold, respectively. These changes correlated with a 20-50% reduction of glucose-induced insulin secretion (normalized to insulin content). The latter effect was not associated with impaired electrical activity or [Ca(2+)](i) signaling. Single-cell capacitance and TIRFM measurements of exocytosis revealed a selective suppression (>70%) of exocytosis elicited by short (50 ms) depolarization, whereas the responses to longer depolarizations were (500 ms) less affected. The loss of rapid exocytosis correlated with dispersion of Ca(2+) entry in HFD beta-cells. No changes in gene transcription of key exocytotic protein were observed.

CONCLUSIONS

HFD results in reduced insulin secretion by causing the functional dissociation of voltage-gated Ca(2+) entry from exocytosis. These observations suggest a novel explanation to the well-established link between obesity and diabetes.

The long lifespan and low turnover of human islet beta cells estimated by mathematical modelling of lipofuscin accumulation

AIMS/HYPOTHESIS

Defects in pancreatic beta cell turnover are implicated in the pathogenesis of type 2 diabetes by genetic markers for diabetes. Decreased beta cell neogenesis could contribute to diabetes. The longevity and turnover of human beta cells is unknown; in rodents <1 year old, a half-life of 30 days is estimated. Intracellular lipofuscin body (LB) accumulation is a hallmark of ageing in neurons. To estimate the lifespan of human beta cells, we measured beta cell LB accumulation in individuals aged 1-81 years.

METHODS

LB content was determined by electron microscopical morphometry in sections of beta cells from human (non-diabetic, n = 45; type 2 diabetic, n = 10) and non-human primates (n = 10; 5-30 years) and from 15 mice aged 10-99 weeks. Total cellular LB content was estimated by three-dimensional (3D) mathematical modelling.

RESULTS

LB area proportion was significantly correlated with age in human and non-human primates. The proportion of human LB-positive beta cells was significantly related to age, with no apparent differences in type 2 diabetes or obesity. LB content was low in human insulinomas (n = 5) and alpha cells and in mouse beta cells (LB content in mouse <10% human). Using 3D electron microscopy and 3D mathematical modelling, the LB-positive human beta cells (representing aged cells) increased from >or=90% (<10 years) to >or=97% (>20 years) and remained constant thereafter.

CONCLUSIONS/INTERPRETATION

Human beta cells, unlike those of young rodents, are long-lived. LB proportions in type 2 diabetes and obesity suggest that little adaptive change occurs in the adult human beta cell population, which is largely established by age 20 years.

De novo lipogenesis and stearoyl-CoA desaturase are coordinately regulated in the human adipocyte and protect against palmitate-induced cell injury

De novo lipogenesis (DNL) is paradoxically up-regulated by its end product, saturated fatty acids (SAFAs). We tested the hypothesis that SAFA-induced up-regulation of DNL reflects coordinate up-regulation of elongation and desaturation pathways for disposal of SAFAs and production of monounsaturated fatty acids to protect cells from SAFA toxicity. Human preadipocytes were differentiated in vitro for 14 days with [U-(13)C]palmitate (0-200 microM) to distinguish exogenous fatty acids from those synthesized by DNL. Exogenous palmitate up-regulated DNL (p < 0.001) concomitantly with SCD and elongation (each p < 0.001). Adipocytes from some donors were intolerant to high palmitate concentrations (400 microM). Palmitate-intolerant cells showed lower TG accumulation. They had lower expression of SCD mRNA and less monounsaturated fatty acids in TG, emphasizing the importance of desaturation for dealing with exogenous SAFAs. There was greater [U-(13)C]palmitate incorporation in phospholipids. SCD knockdown with small interfering RNA caused down-regulation of DNL and of expression of DNL-related genes, with reduced membrane fluidity (p < 0.02) and insulin sensitivity (p < 0.01), compared with scrambled small interfering RNA controls. There was preferential channeling of DNL-derived versus exogenous palmitate into elongation and of DNL-derived versus exogenous stearate into desaturation. DNL may not act primarily to increase fat stores but may serve as a key regulator, in tandem with elongation and desaturation, to maintain cell membrane fluidity and insulin sensitivity within the human adipocyte.

Chronic palmitate exposure inhibits insulin secretion by dissociation of Ca(2+) channels from secretory granules

Long-term (72 hr) exposure of pancreatic islets to palmitate inhibited glucose-induced insulin secretion by >50% with first- and second-phase secretion being equally suppressed. This inhibition correlated with the selective impairment of exocytosis evoked by brief (action potential-like) depolarizations, whereas that evoked by long ( approximately 250 ms) stimuli was unaffected. Under normal conditions, Ca(2+) influx elicited by brief membrane depolarizations increases [Ca(2+)](i) to high levels within discrete microdomains and triggers the exocytosis of closely associated insulin granules. We found that these domains of localized Ca(2+) entry become dispersed by long-term (72 hr), but not by acute (2 hr), exposure to palmitate. Importantly, the release competence of the granules was not affected by palmitate. Thus, the location rather than the magnitude of the Ca(2+) increase determines its capacity to evoke exocytosis. In both mouse and human islets, the palmitate-induced secretion defect was reversed when the beta cell action potential was pharmacologically prolonged.

Suppression of sulfonylurea- and glucose-induced insulin secretion in vitro and in vivo in mice lacking the chloride transport protein ClC-3

Priming of insulin secretory granules for release requires intragranular acidification and depends on vesicular Cl(-)-fluxes, but the identity of the chloride transporter/ion channel involved is unknown. We tested the hypothesis that the chloride transport protein ClC-3 fulfills these actions in pancreatic beta cells. In ClC-3(-/-) mice, insulin secretion evoked by membrane depolarization (high extracellular K(+), sulfonylureas), or glucose was >60% reduced compared to WT animals. This effect was mirrored by a approximately 80% reduction in depolarization-evoked beta cell exocytosis (monitored as increases in cell capacitance) in single ClC-3(-/-) beta cells, as well as a 44% reduction in proton transport across the granule membrane. ClC-3 expression in the insulin granule was demonstrated by immunoblotting, immunostaining, and negative immuno-EM in a high-purification fraction of large dense-core vesicles (LDCVs) obtained by phogrin-EGFP labeling. The data establish the importance of granular Cl(-) fluxes in granule priming and provide direct evidence for the involvement of ClC-3 in the process.

Insulin granule recruitment and exocytosis is dependent on p110gamma in insulinoma and human beta-cells

OBJECTIVE

Phosphatidylinositol 3-OH kinase (PI3K) has a long-recognized role in beta-cell mass regulation and gene transcription and is implicated in the modulation of insulin secretion. The role of nontyrosine kinase receptor-activated PI3K isoforms is largely unexplored. We therefore investigated the role of the G-protein-coupled PI3Kgamma and its catalytic subunit p110gamma in the regulation of insulin granule recruitment and exocytosis.

RESEARCH DESIGN AND METHODS

The expression of p110gamma was knocked down by small-interfering RNA, and p110gamma activity was selectively inhibited with AS605240 (40 nmol/l). Exocytosis and granule recruitment was monitored by islet perifusion, whole-cell capacitance, total internal reflection fluorescence microscopy, and electron microscopy in INS-1 and human beta-cells. Cortical F-actin was examined in INS-1 cells and human islets and in mouse beta-cells lacking the phosphatase and tensin homolog (PTEN).

RESULTS

Knockdown or inhibition of p110gamma markedly blunted depolarization-induced insulin secretion and exocytosis and ablated the exocytotic response to direct Ca(2+) infusion. This resulted from reduced granule localization to the plasma membrane and was associated with increased cortical F-actin. Inhibition of p110gamma had no effect on F-actin in beta-cells lacking PTEN. Finally, the effect of p110gamma inhibition on granule localization and exocytosis could be rapidly reversed by agents that promote actin depolymerization.

CONCLUSIONS

The G-protein-coupled PI3Kgamma is an important determinant of secretory granule trafficking to the plasma membrane, at least in part through the negative regulation of cortical F-actin. Thus, p110gamma activity plays an important role in maintaining a membrane-docked, readily releasable pool of secretory granules in insulinoma and human beta-cells.

Novel aspects of the molecular mechanisms controlling insulin secretion

Pancreatic beta-cells secrete insulin by Ca(2+)-dependent exocytosis of secretory granules. beta-cell exocytosis involves SNARE (soluble NSF-attachment protein receptor) proteins similar to those controlling neurotransmitter release and depends on the close association of L-type Ca(2+) channels and granules. In most cases, the secretory granules fuse individually but there is ultrastructural and biophysical evidence of multivesicular exocytosis. Estimates of the secretory rate in beta-cells in intact islets indicate a release rate of approximately 15 granules per beta-cell per second, 100-fold higher than that observed in biochemical assays. Single-vesicle capacitance measurements reveal that the diameter of the fusion pore connecting the granule lumen with the exterior is approximately 1.4 nm. This is considerably smaller than the size of insulin and membrane fusion is therefore not obligatorily associated with release of the cargo, a feature that may contribute to the different rates of secretion detected by the biochemical and biophysical measurements. However, small molecules like ATP and GABA, which are stored together with insulin in the granules, are small enough to be released via the narrow fusion pore, which accordingly functions as a molecular sieve. We finally consider the possibility that defective fusion pore expansion accounts for the decrease in insulin secretion observed in pathophysiological states including long-term exposure to lipids.