Submaximal responses in calcium-triggered exocytosis are explained by differences in the calcium sensitivity of individual secretory vesicles

Unbiased Thiol-Labeling and Top-Down Proteomic Analyses Implicate Multiple Proteins in the Late Steps of Regulated Secretion

Regulated exocytosis enables temporal and spatial control over the secretion of biologically active compounds; however, the mechanism by which Ca²⁺ modulates different stages of exocytosis is still poorly understood. For an unbiased, top-down proteomic approach, select thiol- reactive reagents were used to investigate this process in release-ready native secretory vesicles. We previously characterized a biphasic effect of these reagents on Ca²⁺-triggered exocytosis: low doses potentiated Ca²⁺ sensitivity, whereas high doses inhibited Ca²⁺ sensitivity and extent of vesicle fusion. Capitalizing on this novel potentiating effect, we have now identified fluorescent thiol- reactive reagents producing the same effects: Lucifer yellow iodoacetamide, monobromobimane, and dibromobimane. Top-down proteomic analyses of fluorescently labeled proteins from total and cholesterol-enriched vesicle membrane fractions using two-dimensional gel electrophoresis coupled with mass spectrometry identified several candidate targets, some of which have been previously linked to the late steps of regulated exocytosis and some of which are novel. Initial validation studies indicate that Rab proteins are involved in the modulation of Ca²⁺ sensitivity, and thus the efficiency of membrane fusion, which may, in part, be linked to their previously identified upstream roles in vesicle docking.

Unraveling the mechanisms of calcium-dependent secretion

Ca2+-dependent secretion is a process by which important signaling molecules that are produced within a cell-including proteins and neurotransmitters-are expelled to the extracellular environment. The cellular mechanism that underlies secretion is referred to as exocytosis. Many years of work have revealed that exocytosis in neurons and neuroendocrine cells is tightly coupled to Ca2+ and orchestrated by a series of protein-protein/protein-lipid interactions. Here, we highlight landmark discoveries that have informed our current understanding of the process. We focus principally on reductionist studies performed using powerful model secretory systems and cell-free reconstitution assays. In recent years, molecular cloning and genetics have implicated the involvement of a sizeable number of proteins in exocytosis. We expect reductionist approaches will be central to attempts to resolve their roles. The Journal of General Physiology will continue to be an outlet for much of this work, befitting its tradition of publishing strongly mechanistic, basic research.

Sphingolipids modulate docking, Ca2+ sensitivity and membrane fusion of native cortical vesicles

Docking, priming, and membrane fusion of secretory vesicles (i.e. regulated exocytosis) requires lipids and proteins. Sphingolipids, in particular, sphingosine and sphingosine-1-phosphate, have been implicated in the modulation of exocytosis. However, the specific exocytotic steps that sphingolipids modulate and the enzymes that regulate sphingolipid concentrations on native secretory vesicle membranes remain unknown. Here we use tightly coupled functional and molecular analyses of fusion-ready cell surface complexes and cortical vesicles isolated from oocytes to assess the role of sphingolipids in the late, Ca²⁺-triggered steps of exocytosis. The molecular changes resulting from treatments with sphingolipid modifying compounds coupled with immunoblotting analysis revealed the presence of sphingosine kinase on native vesicles; the presence of a sphingosine-1-phosphate phosphatase is also indicated. Changes in sphingolipid concentrations on vesicles altered their docking/priming, Ca²⁺-sensitivity, and ability to fuse, indicating that sphingolipid concentrations are tightly regulated and maintained at optimal levels and ratios to ensure efficient exocytosis.

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

The Role of Phospholipase D in Regulated Exocytosis

There are a diversity of interpretations concerning the possible roles of phospholipase D and its biologically active product phosphatidic acid in the late, Ca(2+)-triggered steps of regulated exocytosis. To quantitatively address functional and molecular aspects of the involvement of phospholipase D-derived phosphatidic acid in regulated exocytosis, we used an array of phospholipase D inhibitors for ex vivo and in vitro treatments of sea urchin eggs and isolated cortices and cortical vesicles, respectively, to study late steps of exocytosis, including docking/priming and fusion. The experiments with fluorescent phosphatidylcholine reveal a low level of phospholipase D activity associated with cortical vesicles but a significantly higher activity on the plasma membrane. The effects of phospholipase D activity and its product phosphatidic acid on the Ca(2+) sensitivity and rate of fusion correlate with modulatory upstream roles in docking and priming rather than to direct effects on fusion per se.

Quantification of exocytosis kinetics by DIC image analysis of cortical lawns

Cortical lawns prepared from sea urchin eggs have offered a robust in vitro system for study of regulated exocytosis and membrane fusion events since their introduction by Vacquier almost 40 years ago (Vacquier in Dev Biol 43:62-74, 1975). Lawns have been imaged by phase contrast, darkfield, differential interference contrast, and electron microscopy. Quantification of exocytosis kinetics has been achieved primarily by light scattering assays. We present simple differential interference contrast image analysis procedures for quantifying the kinetics and extent of exocytosis in cortical lawns using an open vessel that allows rapid solvent equilibration and modification. These preparations maintain the architecture of the original cortices, allow for cytological and immunocytochemical analyses, and permit quantification of variation within and between lawns. When combined, these methods can shed light on factors controlling the rate of secretion in a spatially relevant cellular context. We additionally provide a subroutine for IGOR Pro® that converts raw data from line scans of cortical lawns into kinetic profiles of exocytosis. Rapid image acquisition reveals spatial variations in time of initiation of individual granule fusion events with the plasma membrane not previously reported.

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.

Observations of calcium dynamics in cortical secretory vesicles

Calcium (Ca(2+)) dynamics were evaluated in fluorescently labeled sea urchin secretory vesicles using confocal microscopy. 71% of the vesicles examined exhibited one or more transient increases in the fluorescence signal that was damped in time. The detection of transient increases in signal was dependent upon the affinity of the fluorescence indicator; the free Ca(2+) concentration in the secretory vesicles was estimated to be in the range of ∼10 to 100 μM. Non-linear stochastic analysis revealed the presence of extra variance in the Ca(2+) dependent fluorescence signal. This noise process increased linearly with the amplitude of the Ca(2+) signal. Both the magnitude and spatial properties of this noise process were dependent upon the activity of vesicle p-type (Ca(v)2.1) Ca(2+) channels. Blocking the p-type Ca(2+) channels with ω-agatoxin decreased signal variance, and altered the spatial noise pattern within the vesicle. These fluorescence signal properties are consistent with vesicle Ca(2+) dynamics and not simply due to obvious physical properties such as gross movement artifacts or pH driven changes in Ca(2+) indicator fluorescence. The results suggest that the free Ca(2+) content of cortical secretory vesicles is dynamic; this property may modulate the exocytotic fusion process.

Anionic lipids in Ca(2+)-triggered fusion

Anionic lipids are native membrane components that have a profound impact on many cellular processes, including regulated exocytosis. Nonetheless, the full nature of their contribution to the fast, Ca(2+)-triggered fusion pathway remains poorly defined. Here we utilize the tightly coupled quantitative molecular and functional analyses enabled by the cortical vesicle model system to elucidate the roles of specific anionic lipids in the docking, priming and fusion steps of regulated release. Studies with cholesterol sulfate established that effectively localized anionic lipids could contribute to Ca(2+)-sensing and even bind Ca(2+) directly as effectors of necessary membrane rearrangements. The data thus support a role for phosphatidylserine in Ca(2+) sensing. In contrast, phosphatidylinositol would appear to serve regulatory functions in the physiological fusion machine, contributing to priming and thus the modulation and tuning of the fusion process. We note the complexities associated with establishing the specific roles of (anionic) lipids in the native fusion mechanism, including their localization and interactions with other critical components that also remain to be more clearly and quantitatively defined.

A new approach to the molecular analysis of docking, priming, and regulated membrane fusion

Studies using isolated sea urchin cortical vesicles have proven invaluable in dissecting mechanisms of Ca(2+)-triggered membrane fusion. However, only acute molecular manipulations are possible in vitro. Here, using selective pharmacological manipulations of sea urchin eggs ex vivo, we test the hypothesis that specific lipidic components of the membrane matrix selectively affect defined late stages of exocytosis, particularly the Ca(2+)-triggered steps of fast membrane fusion. Egg treatments with cholesterol-lowering drugs resulted in the inhibition of vesicle fusion. Exogenous cholesterol recovered fusion extent and efficiency in cholesterol-depleted membranes; α-tocopherol, a structurally dissimilar curvature analogue, selectively restored fusion extent. Inhibition of phospholipase C reduced vesicle phosphatidylethanolamine and suppressed both the extent and kinetics of fusion. Although phosphatidylinositol-3-kinase inhibition altered levels of polyphosphoinositide species and reduced all fusion parameters, sequestering polyphosphoinositides selectively inhibited fusion kinetics. Thus, cholesterol and phosphatidylethanolamine play direct roles in the fusion pathway, contributing negative curvature. Cholesterol also organizes the physiological fusion site, defining fusion efficiency. A selective influence of phosphatidylethanolamine on fusion kinetics sheds light on the local microdomain structure at the site of docking/fusion. Polyphosphoinositides have modulatory upstream roles in priming: alterations in specific polyphosphoinositides likely represent the terminal priming steps defining fully docked, release-ready vesicles. Thus, this pharmacological approach has the potential to be a robust high-throughput platform to identify molecular components of the physiological fusion machine critical to docking, priming, and triggered fusion.

Nutrient regulation of enteroendocrine cellular activity linked to cholecystokinin gene expression and secretion

The hormone cholecystokinin is produced by the enteroendocrine I cells in the intestine, and it plays an important role in a number of physiological processes including digestion and food intake. Recent data suggest that cholecystokinin gene expression and protein secretion are regulated by macronutrients. The mechanism involves a change in intracellular levels of cAMP and Ca(+2), brought about by the activity of a number of nutrient-responsive G protein-coupled receptors, nutrient transporters, ion channels and intracellular enzymes. How these intracellular responses could lead to gene expression and protein secretion are discussed along with new directions for future investigation.

Dissecting the mechanism of Ca2+-triggered membrane fusion: probing protein function using thiol reactivity

1. Ca(2+)-triggered membrane fusion involves the coordinated actions of both lipids and proteins, but the specific mechanisms remain poorly understood. The urchin cortical vesicle model is a stage-specific native preparation fully enabling the directly coupled functional-molecular analyses necessary to identify critical components of fast triggered membrane fusion. 2. Recent work on lipidic components has established a direct role for cholesterol in the fusion mechanism via local contribution of negative curvature to readily enable the formation of transient lipidic fusion intermediates. In addition, cholesterol- and sphingomyelin-enriched domains regulate the efficiency of fusion by focally organizing other components to ensure an optimized response to the triggering Ca(2+) transient. 3. There is less known about the identity of proteins involved in the Ca(2+)-triggering steps of membrane fusion. Thiol reagents can be used as unbiased tools to probe protein functions. Comparisons of several thiol-reactive reagents have identified different effects on Ca(2+) sensitivity and the extent of fusion, suggesting that there are at least two distinct thiol sites that participate in the fusion mechanism: one that regulates the efficiency of Ca(2+) sensing/triggering and one that may function during the membrane merger event itself. 4. To identify the proteins that regulate Ca(2+) sensitivity, the fluorescent thiol reagent Lucifer yellow iodoacetamide was used to potentiate fusion and simultaneously tag the proteins involved. Ongoing work involves the isolation of cholesterol-enriched membrane fractions to reduce the complexity of the labelled proteome, narrowing the number of candidate proteins.

Identifying critical components of native Ca2+-triggered membrane fusion. Integrating studies of proteins and lipids

Ca(2+)-triggered membrane fusion is the defining step of exocytosis. Despite realization that the fusion machinery must include lipids and proteins working in concert, only of late has work in the field focused more equally on both these components. Here we use isolated sea urchin egg cortical vesicles (CV), a stage-specific preparation of Ca(2+)-sensitive release-ready vesicles that enables the tight coupling of molecular and functional analyses necessary to dissect molecular mechanisms. The stalk-pore hypothesis proposes that bilayer merger proceeds rapidly via transient, high-negative curvature, intermediate membrane structures. Consistent with this, cholesterol, a major component of the CV membrane, contributes to a critical local negative curvature that supports formation of lipidic fusion intermediates. Following cholesterol depletion, structurally dissimilar lipids having intrinsic negative curvature greater than or equal to cholesterol recover the ability of CV to fuse but do not recover fusion efficiency (Ca(2+) sensitivity and kinetics). Conversely, cholesterol- and sphingomyelin-enriched microdomains regulate the efficiency of the fusion mechanism, presumably by contributing spatial and functional organization of other critical lipids and proteins at the fusion site. Critical proteins are thought to participate in Ca(2+) sensing, initiating membrane deformations, and facilitating fusion pore expansion. Capitalizing on a novel effect of the thiol-reactive reagent iodoacetamide (IA), potentiation of the Ca(2+) sensitivity and kinetics, a fluorescently tagged IA has been used to enhance fusion efficiency and simultaneously label the proteins involved. Isolation of cholesterol-enriched CV membrane fractions, using density gradient centrifugation, is being used to narrow the list of protein candidates potentially critical to the mechanism of fast Ca(2+)-triggered membrane fusion.

Enhancement of the Ca(2+)-triggering steps of native membrane fusion via thiol-reactivity

Ca(2+)-triggered membrane fusion is the defining step of exocytosis. Isolated urchin cortical vesicles (CV) provide a stage-specific preparation to study the mechanisms by which Ca(2+) triggers the merger of two apposed native membranes. Thiol-reactive reagents that alkylate free sulfhydryl groups on proteins have been consistently shown to inhibit triggered fusion. Here, we characterize a novel effect of the alkylating reagent iodoacetamide (IA). IA was found to enhance the kinetics and Ca(2+) sensitivity of both CV-plasma membrane and CV-CV fusion. If Sr(2+), a weak Ca(2+) mimetic, was used to trigger fusion, the potentiation was even greater than that observed for Ca(2+), suggesting that IA acts at the Ca(2+)-sensing step of triggered fusion. Comparison of IA to other reagents indicates that there are at least two distinct thiol sites involved in the underlying fusion mechanism: one that regulates the efficiency of fusion and one that interferes with fusion competency.

Specific lipids supply critical negative spontaneous curvature--an essential component of native Ca2+-triggered membrane fusion

The Ca(2+)-triggered merger of two apposed membranes is the defining step of regulated exocytosis. CHOL is required at critical levels in secretory vesicle membranes to enable efficient, native membrane fusion: CHOL-sphingomyelin enriched microdomains organize the site and regulate fusion efficiency, and CHOL directly supports the capacity for membrane merger by virtue of its negative spontaneous curvature. Specific, structurally dissimilar lipids substitute for CHOL in supporting the ability of vesicles to fuse: diacylglycerol, alphaT, and phosphatidylethanolamine support triggered fusion in CHOL-depleted vesicles, and this correlates quantitatively with the amount of curvature each imparts to the membrane. Lipids of lesser negative curvature than cholesterol do not support fusion. The fundamental mechanism of regulated bilayer merger requires not only a defined amount of membrane-negative curvature, but this curvature must be provided by molecules having a specific, critical spontaneous curvature. Such a local lipid composition is energetically favorable, ensuring the necessary "spontaneous" lipid rearrangements that must occur during native membrane fusion-Ca(2+)-triggered fusion pore formation and expansion. Thus, different fusion sites or vesicle types can use specific alternate lipidic components, or combinations thereof, to facilitate and modulate the fusion pore.

The MEK inhibitor, U0126, alters fertilization-induced [Ca2+]i oscillation parameters and secretion: differential effects associated with in vivo and in vitro meiotic maturation

Although mitogen-activated protein kinase (MAPK) is a well-known cell cycle regulator, emerging studies have also implicated its activity in the regulation of intracellular calcium concentration ([Ca2+](i)) and secretion. Those studies raise the hypothesis that MAPK activity during oocyte maturation and early fertilization is required for normal egg Ca2+ oscillations and cortical granule (CG) secretion. We extend the findings of [Lee, B., Vermassen, E., Yoon, S.-Y., Vanderheyden, V., Ito, J., Alfandari, D., De Smedt, H., Parys, J.B., Fissore, R.A., 2006. Phosphorylation of IP(3)R1 and the regulation of [Ca2+](i) responses at fertilization: a role for the MAP kinase pathway. Development 133, 4355-4365] by demonstrating acute effects on Ca2+ oscillation frequency, amplitude, and duration in fertilized mouse eggs matured in vitro with the MAPK inhibitor, U0126. Frequency was increased, whereas amplitude and duration were greatly decreased. These effects were significantly reduced in eggs matured in vivo and fertilized in the presence of the inhibitor. Ionomycin studies indicated that intracellular Ca2+ stores were differentially affected in eggs matured in vitro with U0126. Consistent with these effects on [Ca2+](i) elevation, fertilization-induced CG exocytosis and metaphase II exit were also reduced in in vitro-matured eggs with U0126, but not in those similarly treated after in vivo maturation. These results indicate that MAPK targets Ca2+ regulatory proteins during both maturation and fertilization, as well as provide a new hypothesis for MAPK function, which is to indirectly regulate events of early development by controlling Ca2+ oscillation parameters.

A mechanism intrinsic to the vesicle fusion machinery determines fast and slow transmitter release at a large CNS synapse

Heterogeneity of release probability p between vesicles in the readily releasable pool (RRP) is expected to strongly influence the kinetics of depression at synapses, but the underlying mechanism(s) are not well understood. To test whether differences in the intrinsic Ca2+ sensitivity of vesicle fusion might cause heterogeneity of p, we made presynaptic Ca2+-uncaging measurements at the calyx of Held and analyzed the time course of transmitter release by EPSC deconvolution. Ca2+ uncaging, which produced spatially homogeneous elevations of [Ca2+]i, evoked a fast and a slow component of release over a wide range of [Ca2+]i, showing that mechanism(s) intrinsic to the vesicle fusion machinery cause fast and slow transmitter release. Surprisingly, the number of vesicles released in the fast component increased with Ca2+-uncaging stimuli of larger amplitudes, a finding that was most obvious below approximately 10 microM [Ca2+]i and that we call "submaximal release" of fast-releasable vesicles. During trains of action potential-like presynaptic depolarizations, submaximal release was also observed as an increase in the cumulative fast release at enhanced release probabilities. A model that assumes two separate subpools of RRP vesicles with different intrinsic Ca2+ sensitivities predicted the observed Ca2+ dependencies of fast and slow transmitter release but could not fully account for submaximal release. Thus, fast and slow transmitter release in response to prolonged [Ca2+]i elevations is caused by intrinsic differences between RRP vesicles, and an "a posteriori" reduction of the Ca2+ sensitivity of vesicle fusion after the onset of the stimulus might cause submaximal release of fast-releasable vesicles and contribute to short-term synaptic depression.

Actin is not an essential component in the mechanism of calcium-triggered vesicle fusion

Actin has been suggested as an essential component in the membrane fusion stage of exocytosis. In some model systems disruption of the actin filament network associated with exocytotic membranes results in a decrease in secretion. Here we analyze the fast Ca2+-triggered membrane fusion steps of regulated exocytosis using a stage-specific preparation of native secretory vesicles (SV) to directly test whether actin plays an essential role in this mechanism. Although present on secretory vesicles, selective pharmacological inhibition of actin did not affect the Ca2+-sensitivity, extent, or kinetics of membrane fusion, nor did the addition of exogenous actin or an anti-actin antibody. There was also no discernable affect on inter-vesicle contact (docking). Overall, the results do not support a direct role for actin in the fast, Ca2+-triggered steps of regulated membrane fusion. It would appear that actin acts elsewhere within the exocytotic cycle.

Cholesterol facilitates the native mechanism of Ca2+-triggered membrane fusion

The process of regulated exocytosis is defined by the Ca2+-triggered fusion of two apposed membranes, enabling the release of vesicular contents. This fusion step involves a number of energetically complex steps and requires both protein and lipid membrane components. The role of cholesterol has been investigated using isolated release-ready native cortical secretory vesicles to analyze the Ca2+-triggered fusion step of exocytosis. Cholesterol is a major component of vesicle membranes and we show here that selective removal from membranes, selective sequestering within membranes, or enzymatic modification causes a significant inhibition of the extent, Ca2+ sensitivity and kinetics of fusion. Depending upon the amount incorporated, addition of exogenous cholesterol to cholesterol-depleted membranes consistently recovers the extent, but not the Ca2+ sensitivity or kinetics of fusion. Membrane components of comparable negative curvature selectively recover the ability to fuse, but are unable to recover the kinetics and Ca2+ sensitivity of vesicle fusion. This indicates at least two specific positive roles for cholesterol in the process of membrane fusion: as a local membrane organizer contributing to the efficiency of fusion, and, by virtue of its intrinsic negative curvature, as a specific molecule working in concert with protein factors to facilitate the minimal molecular machinery for fast Ca2+-triggered fusion.

Membrane fusion of secretory vesicles of the sea urchin egg in the absence of NSF

The role of cytosolic ATPases such as N-ethylmaleimide (NEM)-sensitive fusion protein (NSF) in membrane fusion is controversial. We examined the physiology and biochemistry of ATP and NSF in the cortical system of the echinoderm egg to determine if NSF is an essential factor in membrane fusion during Ca(2+)-triggered exocytosis. Neither exocytosis in vitro, nor homotypic cortical vesicle (CV) fusion required soluble proteins or nucleotides, and both occurred in the presence of non-hydrolyzable analogs of ATP. While sensitive to thiol-specific reagents, CV exocytosis is not restored by the addition of cytosolic NSF, and fusion and NSF function are differentially sensitive to thiol-specific agents. To test participation of tightly bound, non-exchangeable NSF in CV-CV fusion, we cloned the sea urchin homolog and developed a species-specific antibody for western blots and physiological analysis. This antibody was without effect on CV exocytosis or homotypic fusion, despite being functionally inhibitory. NSF is detectable in intact cortices, cortices from which CVs had been removed and isolated CVs treated with ATP-gamma-S and egg cytosol to reveal NSF binding sites. In contrast, isolated CVs, though all capable of Ca(2+)-triggered homotypic fusion, contain less than one hexamer of NSF per CV. Thus NSF is not a required component of the CV fusion machinery.

βγ subunits of heterotrimeric G-proteins contribute to Ca2+ release at fertilization in the sea urchin

A cytoplasmic Ca2+ transient is required for egg activation at fertilization in all animals. The pathway leading to release of Ca2+ from the endoplasmic reticulum in echinoderms includes activation of a SRC homolog, followed by phospholipase Cγ activation, and formation of inositol trisphosphate. However, the upstream activators or modulators of this signaling pathway are not known. We recently identified four Gα subunits of heterotrimeric G-proteins present in the sea urchin egg, and here we find that activation of G-proteins of the Gαs and Gαq type, but not Gαi or Gα12 type, is required for normal Ca2+ dynamics at fertilization. The effects of these G-proteins are mediated by the Gβγ subunits, occur upstream of the cytoplasmic Ca2+ release, and influence both the amplitude of Ca2+ release and the duration of the lag phase. We propose integration of the G-protein input into the framework of signaling at sea urchin fertilization.

Evidence that rabies virus forms different kinds of fusion machines with different pH thresholds for fusion

Fusion of rabies virus with membranes is triggered at a low pH and is mediated by a viral glycoprotein (G). Fusion of rabies virus with liposomes was monitored by using a lipid mixing assay based on fluorescence resonance energy transfer. Fusion was detected below pH 6.4, and its extent increased with H(+) concentrations to be maximal around pH 6.15. The origin of the partial fusion activity of rabies virus under suboptimal pH conditions (i.e., between pH 6.15 and 6.4) was investigated. We demonstrate unambiguously that fusion at a suboptimal pH is distinct from the phenomenon of low-pH-induced inactivation and that it is not due to heterogeneity of the virus population. We also show that viruses that do not fuse under suboptimal pH conditions are indeed bound to the target liposomes and that the fusion complexes they have formed are blocked at an early stage of the fusion pathway. Our conclusion is that along the fusion reaction, different kinds of fusion machines with different pH thresholds for fusion can be formed. Possible explanations of this difference of pH sensitivity are discussed.

Regulated secretion: SNARE density, vesicle fusion and calcium dependence

SNAREs such as VAMP, SNAP-25 and syntaxin are essential for intracellular trafficking, but what are their exact molecular roles and how are their interactions with other proteins manifest? Capitalizing on the differential sensitivity of SNAREs to exogenous proteases, we quantified the selective removal of identified SNAREs from native secretory vesicles without loss of fusion competence. Using previously established fusion assays and a high sensitivity immunoblotting protocol, we analyzed the relationship between these SNARE proteins and Ca2+-triggered membrane fusion. Neither the extent of fusion nor the number of intermembrane fusion complexes per vesicle were correlated with the measured density of identified egg cortical vesicle (CV) SNAREs. Without syntaxin, CVs remained fusion competent. Surprisingly, for one (but not another) protease the Ca2+ dependence of fusion was correlated with CV SNARE density, suggesting a native protein complex that associates with SNAREs, the architecture of which ensures high Ca2+ sensitivity. As SNAREs may function during CV docking in vivo, and as further proteolysis after SNARE removal eventually ablates fusion, we hypothesize that the triggered steps of regulated fusion (Ca2+ sensitivity and the catalysis and execution of fusion) require additional proteins that function downstream of SNAREs.

Calcium requirements for exocytosis do not delimit the releasable neuropeptide pool

Recently, it was proposed that secretory vesicles have widely varying Ca(2+) thresholds for exocytosis. This model can explain adaptation of secretory responses and predicts that incomplete release is a consequence of insufficient Ca(2+). However, membrane capacitance-based measurements have not supported varying Ca(2+) thresholds. Here, Green Fluorescent Protein (GFP) imaging is used to test whether a Ca(2+) limitation determines the size of the releasable neuropeptide pool in differentiated PC12 cells. We show that depolarization-evoked release correlates with failure to sustain fully elevated [Ca(2+)](i). However, this is coincidental because release remains incomplete when [Ca(2+)](i) is maintained at a relatively high level by application of an ionophore or by dialysis with a buffered Ca(2+) solution. Furthermore, in contradiction with the existence of high threshold vesicles, stimulating maximal release with moderate [Ca(2+)](i) prevents secretory responses to large increases in [Ca(2+)](i) induced by photolysis of the caged dimethoxynitrophenyl-EGTA-4 (DMNPE-4). Thus, optical measurements show that limited capacity for neuropeptide release in response to depolarization is not caused by an insufficient duration of [Ca(2+)](i) elevation or by variation among vesicles in Ca(2+) sensitivity for exocytosis.

Local routes revisited: the space and time dependence of the Ca2+ signal for phasic transmitter release at the rat calyx of Held

During the last decade, advances in experimental techniques and quantitative modelling have resulted in the development of the calyx of Held as one of the best preparations in which to study synaptic transmission. Here we review some of these advances, including simultaneous recording of pre- and postsynaptic currents, measuring the Ca2+ sensitivity of transmitter release, reconstructing the 3-D anatomy at the electron microscope (EM) level, and modelling the buffered diffusion of Ca2+ in the nerve terminal. An important outcome of these studies is an improved understanding of the Ca2+ signal that controls phasic transmitter release. This article illustrates the spatial and temporal aspects of the three main steps in the presynaptic signalling cascade: Ca2+ influx through voltage-gated calcium channels, buffered Ca2+ diffusion from the channels to releasable vesicles, and activation of the Ca2+ sensor for release. Particular emphasis is placed on how presynaptic Ca2+ buffers affect the Ca2+ signal and thus the amplitude and time course of the release probability. Since many aspects of the signalling cascade were first conceived with reference to the squid giant presynaptic terminal, we include comparisons with the squid model and revisit some of its implications. Whilst the characteristics of buffered Ca2+ diffusion presented here are based on the calyx of Held, we demonstrate the circumstances under which they may be valid for other nerve terminals at mammalian CNS synapses.

A highly Ca2+-sensitive pool of vesicles is regulated by protein kinase C in adrenal chromaffin cells

We have used flash photolysis of caged Ca2+ and membrane capacitance measurements to probe exocytosis in chromaffin cells at low concentrations of intracellular Ca2+ ([Ca2+]i) (<10 microM). We observed a small pool of granules that is more sensitive to [Ca2+]i than the previously described "readily releasable pool." Upon activation of PKC, this "highly Ca2+-sensitive pool" is enhanced in size to a greater extent than the readily releasable pool but is eliminated upon expression of a C-terminal deletion mutant (Delta9) of synaptosome-associated protein of 25 kDa (SNAP-25). Thus, in chromaffin cells, PKC enhances exocytosis both by increasing the number of readily releasable vesicles and by shifting vesicles to a highly Ca2+-sensitive state, enabling exocytosis at sites relatively distant from Ca2+ channels.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Calcium secretion coupling at calyx of Held governed by nonuniform channel-vesicle topography

Phasic transmitter release at synapses in the mammalian CNS is regulated by local [Ca2+] transients, which control the fusion of readily releasable vesicles docked at active zones (AZs) in the presynaptic membrane. The time course and amplitude of these [Ca2+] transients critically determine the time course and amplitude of the release and thus the frequency and amplitude tuning of the synaptic connection. As yet, the spatiotemporal nature of the [Ca2+] transients and the number and location of release-controlling Ca2+ channels relative to the vesicles, the "topography" of the release sites, have remained elusive. We used a time-dependent model to simulate Ca2+ influx, three-dimensional buffered Ca2+ diffusion, and the binding of Ca2+ to the release sensor. The parameters of the model were constrained by recent anatomical and biophysical data of the calyx of Held. Comparing the predictions of the model with previously measured release probabilities under a variety of experimental conditions, we inferred which release site topography is likely to operate at the calyx: At each AZ one or a few clusters of Ca2+ channels control the release of the vesicles. The distance of a vesicle to the cluster(s) varies across the multiple release sites of a single calyx (ranging from 30 to 300 nm; average approximately 100 nm). Assuming this topography, vesicles in different locations are exposed to different [Ca2+] transients, with peak amplitudes ranging from 0.5 to 40 microm (half-width approximately 400 microsec) during an action potential. Consequently the vesicles have different release probabilities ranging from <0.01 to 1. We demonstrate how this spatially heterogeneous release probability creates functional advantages for synaptic transmission.

Calcium secretion coupling at calyx of Held governed by nonuniform channel-vesicle topography

Phasic transmitter release at synapses in the mammalian CNS is regulated by local [Ca2+] transients, which control the fusion of readily releasable vesicles docked at active zones (AZs) in the presynaptic membrane. The time course and amplitude of these [Ca2+] transients critically determine the time course and amplitude of the release and thus the frequency and amplitude tuning of the synaptic connection. As yet, the spatiotemporal nature of the [Ca2+] transients and the number and location of release-controlling Ca2+ channels relative to the vesicles, the "topography" of the release sites, have remained elusive. We used a time-dependent model to simulate Ca2+ influx, three-dimensional buffered Ca2+ diffusion, and the binding of Ca2+ to the release sensor. The parameters of the model were constrained by recent anatomical and biophysical data of the calyx of Held. Comparing the predictions of the model with previously measured release probabilities under a variety of experimental conditions, we inferred which release site topography is likely to operate at the calyx: At each AZ one or a few clusters of Ca2+ channels control the release of the vesicles. The distance of a vesicle to the cluster(s) varies across the multiple release sites of a single calyx (ranging from 30 to 300 nm; average approximately 100 nm). Assuming this topography, vesicles in different locations are exposed to different [Ca2+] transients, with peak amplitudes ranging from 0.5 to 40 microm (half-width approximately 400 microsec) during an action potential. Consequently the vesicles have different release probabilities ranging from <0.01 to 1. We demonstrate how this spatially heterogeneous release probability creates functional advantages for synaptic transmission.

The actions of barium and strontium on exocytosis and endocytosis in the synaptic terminal of goldfish bipolar cells

1. We investigated the properties of Ca2+-sensitive steps in the cycling of synaptic vesicles by comparing the actions of Ca2+, Ba2+ and Sr2+ in the synaptic terminal of depolarizing bipolar cells isolated from the retina of goldfish. FM1-43 fluorescence and capacitance measurements demonstrated that exocytosis, endocytosis and vesicle mobilization were maintained when external Ca2+ was replaced by either Ba2+ or Sr2+. 2. The rapidly releasable pool of vesicles (RRP) was equivalent to 1.5 % of the membrane surface area when measured in the presence of 2.5 mM Ca2+, but only 0.4 % in 2.5 mM Sr2+. The relative sizes of the RRP in Ca2+, Sr2+ and Ba2+ were 1.0, 0.28 and 0.1, respectively. We conclude that a smaller proportion of docked vesicles are available for fast exocytosis triggered by the influx of Sr2+ or Ba2+ compared to Ca2+. 3. The slow phase of exocytosis was not altered when Ca2+ was replaced by Ba2+, but it was accelerated 1.6-fold in Sr2+. The peak concentrations of Ca2+, Sr2+ and Ba2+ (measured using Mag-fura-5) were approximately 4, approximately 14 and approximately 60 microM, respectively. The order of efficiency for the stimulation of slow exocytosis was Ca2+ approximately Sr2+ > Ba2+. 4. Exocytosis was prolonged after the influx of Sr2+ and Ba2+. Sr2+ was cleared from the synaptic terminal with the same time constant as Ca2+ (1.3 s), but Ba2+ was cleared 10-100 times more slowly. Although Ba(2+) stimulates the slow release of a large number of vesicles, it did so less efficiently than Ca2+ or Sr2+. 5. The recovery of the membrane capacitance was equally rapid in Sr2+ and Ca2+, demonstrating that the fast mode of endocytosis could be triggered by either cation.

Plasma membrane resident 'fusion complexes' mediate reconstituted exocytosis

Calcium-triggered exocytosis is thought to be mediated by membrane-associated protein complexes. In sea urchin eggs, high concentrations of calcium activate multiple 'fusion complexes' per cortical vesicle-plasma membrane docking site. Some of these fusion complexes are known to reside in the vesicle membrane. It is not known if fusion complexes also reside in the plasma membrane, or if plasma membrane-resident fusion complexes require cognate partners in the vesicle membrane. Using reconstitution, we show that N-ethylmaleimide treatment of either vesicles or plasma membrane fragments prior to reconstitution does not completely inhibit exocytosis. Treatment of both components did result in complete inhibition. Upon reconstitution, cortical vesicles and the early endosomes formed by compensatory endocytosis both contributed, on average, two fusion complexes per reconstituted docking site. The plasma membrane contributed, on average, two fusion complexes per docking site when assembled with cortical vesicles, but only one complex when reconstituted with endosomes. We conclude that there are at least two types of plasma membrane-resident fusion complexes that participate in reconstituted cortical vesicle-plasma membrane fusion. The activity of one of these fusion complexes is target-specific for cortical vesicles, while the second type also supports fusion with endosomes.

A kinetic analysis of calcium-triggered exocytosis

Although the relationship between exocytosis and calcium is fundamental both to synaptic and nonneuronal secretory function, analysis is problematic because of the temporal and spatial properties of calcium, and the fact that vesicle transport, priming, retrieval, and recycling are coupled. By analyzing the kinetics of sea urchin egg secretory vesicle exocytosis in vitro, the final steps of exocytosis are resolved. These steps are modeled as a three-state system: activated, committed, and fused, where interstate transitions are given by the probabilities that an active fusion complex commits (alpha) and that a committed fusion complex results in fusion, p. The number of committed complexes per vesicle docking site is Poisson distributed with mean n. Experimentally, p and n increase with increasing calcium, whereas alpha and the pn ratio remain constant, reducing the kinetic description to only one calcium-dependent, controlling variable, n. On average, the calcium dependence of the maximum rate (R(max)) and the time to reach R(max) (T(peak)) are described by the calcium dependence of n. Thus, the nonlinear relationship between the free calcium concentration and the rate of exocytosis can be explained solely by the calcium dependence of the distribution of fusion complexes at vesicle docking sites.

Vesicular localization and characterization of a novel post-proline-cleaving aminodipeptidase, quiescent cell proline dipeptidase

A large number of chemokines, cytokines, and signal peptides share a highly conserved X-Pro motif on the N-terminus. The cleavage of this N-terminal X-Pro dipeptide results in functional alterations of chemokines such as RANTES, stroma-derived factor-1, and macrophage-derived chemokine. Until recently, CD26/DPPIV was the only known protease with the ability to cleave N-terminal X-Pro motifs at neutral pH. We have isolated and cloned a novel serine protease, quiescent cell proline dipeptidase (QPP), with substrate specificity similar to that of CD26/DPPIV. In this paper we show that QPP, like CD26/DPPIV, is synthesized with a propeptide and undergoes N:-glycosylation. Interestingly, this glycosylation is required for QPP enzymatic activity, but not for its localization. Unlike the cell surface molecule, CD26/DPPIV, QPP is targeted to intracellular vesicles that are distinct from lysosomes. Proteinase K treatment of intact vesicles indicates that QPP is located within the vesicles. These vesicles appear to have a secretory component, as QPP is secreted in a functionally active form in response to calcium release. The presence of QPP in the vesicular compartment suggests that molecules bearing the N-terminal X-Pro motif can be cleaved at multiple sites within and outside the cell. These results expand the potential site(s) and scope of a process that appears to be an important mechanism of post-translational regulation.

Dissection of three Ca2+-dependent steps leading to secretion in chromaffin cells from mouse adrenal slices

In neurosecretory cells, intracellular Ca2+ ([Ca2+]i) not only acts as the trigger for secretion but also regulates earlier steps in the secretory pathway. Here, a novel approach was developed to control [Ca2+]i over a broad concentration range, which allowed the quantification of three distinct actions of [Ca2+]i on large dense-core vesicle (LDCV) fusion in chromaffin cells from mouse adrenal slices. Basal [Ca2+]i regulated the transfer of vesicles toward a slowly releasable state, whereas further maturation to the readily releasable state was Ca2+ independent. [Ca2+]i levels above 3 microM triggered exocytosis of all readily and slowly releasable vesicles in two parallel, kinetically distinct fusion reactions. In a molecular context, these results suggest that Ca2+ acts both before and after trans-SNARE complex formation to regulate fusion competence and fusion kinetics of LDCVs.

Synaptic depression and the kinetics of exocytosis in retinal bipolar cells

The capacitance technique was used to investigate exocytosis at the ribbon synapse of depolarizing bipolar cells from the goldfish retina. When the Ca(2+) current was activated strongly, the rapidly releasable pool of vesicles (RRP) was released with a single rate-constant of approximately 300-500 sec(-1). However, when the Ca(2+) current was activated weakly by depolarization in the physiological range (-45 to -25 mV), exocytosis from the RRP occurred in two phases. After the release of 20% or more of the RRP, the rate-constant of exocytosis fell by a factor of 4-10. Thus, synaptic depression was caused by a reduced sensitivity to Ca(2+) influx, as well as simple depletion of the RRP. In the resting state, the rate of exocytosis varied with the amplitude of the Ca(2+) current raised to the power of 2. In the depressed state, the sensitivity to Ca(2+) influx was reduced approximately fourfold. The initial phase of exocytosis accelerated e-fold for every 2.1 mV depolarization over the physiological range and averaged 120 sec(-1) at -25 mV. The synapse of depolarizing bipolar cells therefore responds to a step depolarization in a manner similar to a high-pass filter. This transformation appears to be determined by the presence of rapidly releasable vesicles with differing sensitivities to Ca(2+) influx. This might occur if vesicles were docked to the plasma membrane at different distances from Ca(2+) channels. These results suggest that the ribbon synapse of depolarizing bipolar cells may be a site of adaptation in the retina.

Synaptic depression and the kinetics of exocytosis in retinal bipolar cells

The capacitance technique was used to investigate exocytosis at the ribbon synapse of depolarizing bipolar cells from the goldfish retina. When the Ca(2+) current was activated strongly, the rapidly releasable pool of vesicles (RRP) was released with a single rate-constant of approximately 300-500 sec(-1). However, when the Ca(2+) current was activated weakly by depolarization in the physiological range (-45 to -25 mV), exocytosis from the RRP occurred in two phases. After the release of 20% or more of the RRP, the rate-constant of exocytosis fell by a factor of 4-10. Thus, synaptic depression was caused by a reduced sensitivity to Ca(2+) influx, as well as simple depletion of the RRP. In the resting state, the rate of exocytosis varied with the amplitude of the Ca(2+) current raised to the power of 2. In the depressed state, the sensitivity to Ca(2+) influx was reduced approximately fourfold. The initial phase of exocytosis accelerated e-fold for every 2.1 mV depolarization over the physiological range and averaged 120 sec(-1) at -25 mV. The synapse of depolarizing bipolar cells therefore responds to a step depolarization in a manner similar to a high-pass filter. This transformation appears to be determined by the presence of rapidly releasable vesicles with differing sensitivities to Ca(2+) influx. This might occur if vesicles were docked to the plasma membrane at different distances from Ca(2+) channels. These results suggest that the ribbon synapse of depolarizing bipolar cells may be a site of adaptation in the retina.

Establishment of ionic channels and signalling cascades in the embryonic stem cell-derived primitive endoderm and cardiovascular system

The first organ system to be established in early embryogenesis is the cardiovascular system which develops upon interaction with hypoblastic cells of the primitive endoderm. Here we focus on recent work on embryoid bodies derived from pluripotent embryonic stem (ES) cells. Ca(2+) oscillations and Ca(2+) signalling pathways during the differentiation of primitive endodermal cell layers are reported. Furthermore, the development-dependent expression of ion channels and the buildup of signalling cascades involved in the modulation of voltage-dependent L-type Ca(2+) channels during early cardiomyogenesis and the formation of functional vascular structures in the process of vasculogenesis and angiogenesis are reviewed. We also report on the use of green fluorescent protein reporter gene expression under the control of cardiac-specific promoters, e.g. the human cardiac alpha-actin promoter, which enables the identification and in vivo characterization of cardiomyocytes at very early stages of cardiomyogenesis.

Sea urchin egg preparations as systems for the study of calcium-triggered exocytosis

This paper reviews recent work in our laboratory on the mechanism of calcium-triggered exocytosis. Upon echinoderm egg fertilization, cortical secretory vesicle exocytosis is massive and synchronous. By combining physiological and molecular analyses with a variety of purified membrane isolates containing secretory vesicles that fuse to the plasma membrane or each other, we have characterized the final steps of this calcium-triggered exocytosis. Our kinetic analysis led to a functional definition of the fusion complex whose activation by calcium follows Poisson statistics. The properties of this complex are compared with the properties of the heterotrimeric SNARE protein complex that is present in the cortical vesicle system. Our data do not support the hypothesis that this particular heterotrimeric complex is by itself the biological fusogen.

The reduced release probability of releasable vesicles during recovery from short-term synaptic depression

Recovery from synaptic depression is believed to depend mainly on replenishment of the releasable pool of vesicles. We observed that during recovery from depression in a calyx-type synapse, part of the releasable pool was replenished rapidly. Half recovery occurred within 1 s, even in the absence of residual calcium. Vesicles that had recently entered the releasable pool had a 7- to 8-fold lower release probability than those that had been in the pool for more than 30 s. These results suggest that the reduction in the release probability of releasable vesicles contributes greatly to the level of depression. How synapses maintain transmission during repetitive firing is in debate. We propose that during repetitive firing, accumulation of intracellular Ca2+ may facilitate release of the rapidly replenished but reluctant vesicles, making them available for sustaining synaptic transmission.

SNARE complex formation is triggered by Ca2+ and drives membrane fusion

Neurotransmitter exocytosis, a process mediated by a core complex of syntaxin, SNAP-25, and VAMP (SNAREs), is inhibited by SNARE-cleaving neurotoxins. Botulinum neurotoxin E inhibition of norepinephrine release in permeabilized PC12 cells can be rescued by adding a 65 aa C-terminal fragment of SNAP-25 (S25-C). Mutations along the hydrophobic face of the S25-C helix result in SNARE complexes with different thermostabilities, and these mutants rescue exocytosis to different extents. Rescue depends on the continued presence of both S25-C and Ca2+ and correlates with complex formation. The data suggest that Ca2+ triggers S25-C binding to a low-affinity site, initiating trans-complex formation. Pairing of SNARE proteins on apposing membranes leads to bilayer fusion and results in a high-affinity cis-SNARE complex.

Biochemical and functional studies of cortical vesicle fusion: the SNARE complex and Ca2+ sensitivity

Cortical vesicles (CV) possess components critical to the mechanism of exocytosis. The homotypic fusion of CV centrifuged or settled into contact has a sigmoidal Ca2+ activity curve comparable to exocytosis (CV-PM fusion). Here we show that Sr2+ and Ba2+ also trigger CV-CV fusion, and agents affecting different steps of exocytotic fusion block Ca2+, Sr2+, and Ba2+-triggered CV-CV fusion. The maximal number of active fusion complexes per vesicle, <n\>Max, was quantified by NEM inhibition of fusion, showing that CV-CV fusion satisfies many criteria of a mathematical analysis developed for exocytosis. Both <n\>Max and the Ca2+ sensitivity of fusion complex activation were comparable to that determined for CV-PM fusion. Using Ca2+-induced SNARE complex disruption, we have analyzed the relationship between membrane fusion (CV-CV and CV-PM) and the SNARE complex. Fusion and complex disruption have different sensitivities to Ca2+, Sr2+, and Ba2+, the complex remains Ca2+- sensitive on fusion-incompetent CV, and disruption does not correlate with the quantified activation of fusion complexes. Under conditions which disrupt the SNARE complex, CV on the PM remain docked and fusion competent, and isolated CV still dock and fuse, but with a markedly reduced Ca2+ sensitivity. Thus, in this system, neither the formation, presence, nor disruption of the SNARE complex is essential to the Ca2+-triggered fusion of exocytotic membranes. Therefore the SNARE complex alone cannot be the universal minimal fusion machine for intracellular fusion. We suggest that this complex modulates the Ca2+ sensitivity of fusion.

The calcium sensitivity of individual secretory vesicles is invariant with the rate of calcium delivery

Differences in the calcium sensitivity of individual secretory vesicles can explain a defining feature of calcium-regulated exocytosis, a graded response to calcium. The role of the time dependence of calcium delivery in defining the observed differences in the calcium sensitivity of sea urchin egg secretory vesicles in vitro was examined. The calcium sensitivity of individual secretory vesicles (i.e., the distribution of calcium thresholds) is invariant over a range of calcium delivery rates from faster than micromolar per millisecond to slower than micromolar per second. Any specific calcium concentration above threshold triggers subpopulations of vesicles to fuse, and the size of these subpopulations is independent of the time course required to reach that calcium concentration. All evidence supports the hypothesis that the magnitude of the free calcium is the single controlling variable that determines the fraction of vesicles that fuse, and that this fraction is established before the application of calcium. Submaximal responses to calcium cannot be attributed to alterations in the calcium sensitivity of individual secretory vesicles arising from the temporal properties of the calcium delivery. Models that attempt to explain the cessation of fusion using changes in the distribution of calcium thresholds arising from the rate of calcium delivery and/or adaptation are not applicable to this system, and thus cannot be general.