Search

"\"The precise subcellular organization of synaptic vesicles (SVs) at presynaptic sites allows for rapid and spatially restricted exocytotic release of neurotransmitter. The synapsins (Syns) are a family of presynaptic proteins that control the availability of SVs for exocytosis by reversibly tethering them to each other and to the actin cytoskeleton in a phosphorylation-dependent manner. Syn ablation leads to reduction in the density of SV proteins in nerve terminals and increased synaptic fatigue under high frequency stimulation, accompanied by the development of an epileptic phenotype. We analyzed cultured neurons from wild type and Syn I,II,III \\\/- triple knocked-out (TKO) mice and found that vesicular components were severely dispersed in the absence of Syns. Vesicle dispersion did not affect the readily releasable pool of SVs, whereas the total number of SVs was considerably reduced in TKO mice. Interestingly, the higher mobility observed in TKO apparently involved exocytosis competent SVs as well; it was not affected by stimulation but was reverted by chronic neuronal activity blockade. In summary, our data show that while a Syn-dependent mechanism reduces the mobility of a subpopulation of SVs, by reversibly tethering them to the synaptic region, an additional Syn-independent mechanism, whose molecular substrate remains to be clarified, targets SVs to synaptic boutons at rest, and might be outpaced by activity. Altogether these findings indicate that Syns are essential to maintain the dynamic structural organization of synapses and the size of the reserve pool of SVs during intense SV recycling.\""

Adhesion of monocytes to the extracellular matrix is mediated by a direct high affinity interaction between cell-surface urokinase-type plasminogen activator (uPA) receptor (uPAR) and the extracellular matrix protein vitronectin. We demonstrate a tight connection between uPA-regulated uPAR oligomerization and high affinity binding to immobilized vitronectin. We find that binding of soluble uPAR (suPAR) to immobilized vitronectin is strictly ligand-dependent with a linear relationship between the observed binding and the concentration of ligand added. Nevertheless, a comparison of experimentally obtained binding curves to those generated using a simple equilibrium model suggests that the high affinity vitronectin-binding pro-uPA.suPAR complex contains two molecules of suPAR. In co-immunoprecipitation experiments, using different epitope-tagged suPAR molecules, suPAR/suPAR co-immunoprecipitation displayed a similar uPA dose dependence as that observed for vitronectin binding, demonstrating that the high affinity vitronectin-binding complex indeed contains oligomeric suPAR. Structurally, the kringle domain of uPA was found to be critical for the formation of the vitronectin-binding competent complex because the amino-terminal fragment, but not the growth factor-like domain, behaved as a full-length uPA. Our data represent the first demonstration of functional, ligand-induced uPAR oligomerization having extensive implications for glycosylphosphatidylinositol-anchored receptors in general, and for the biology of the uPA/uPAR system in particular.

The synapsins are a family of synaptic vesicle phosphoproteins which play a key role in the regulation of neurotransmitter release and synapse formation. In the case of synapsin I, these biological properties have been attributed to its ability to interact with both synaptic vesicles and the actin-based cytoskeleton. Although synapsin II shares some of the biological properties of synapsin I, much less is known of its molecular properties. We have investigated the interactions of recombinant rat synapsin Ila with monomeric and filamentous actin and the sensitivity of those interactions to phosphorylation, and found that: i) dephosphorylated synapsin II stimulates actin polymerization by binding to actin monomers and forming actively elongating nuclei and by facilitating the spontaneous nucleation/elongation processes; ii) dephosphorylated synapsin II induces the formation of thick and ordered bundles of actin filaments with greater potency than synapsin I; iii) phosphorylation by protein kinase A markedly inhibits the ability of synapsin II to interact with both actin monomers and filaments. The results indicate that the interactions of synapsin II with actin are similar but not identical to those of synapsin I and suggest that synapsin II may play a major structural role in mature and developing nerve terminals, which is only partially overlapping with the role played by synapsin I.

The clear synaptic vesicles of neurons release their contents at the presynaptic membrane and are then quickly retrieved. However, it is unclear whether a complete cycle of exocytosis and endocytosis is always involved or whether neurotransmitter can be released by a transient interaction. Recent findings in chromaffin and mast cells suggest that exocytosis is preceded by the formation of a pore that has similar conductance properties to ion channels. The content of the secretory organelle partially escapes at this early step, but the pore can close before the vesicle fuses fully. This article looks at the evidence that quantal release of neurotransmitter from clear synaptic vesicles may occur by a similar ‘kiss-and-run' mechanism.

Human serum albumin (HSA) is best known for its extraordinary ligand binding capacity. HSA has a high affinity for heme and is responsible for the transport of medium and long chain fatty acids. Here, we report myristate binding to the N and B conformational states of Mn(III)heme–HSA (i.e. at pH 7.0 and 10.0, respectively) as investigated by optical absorbance and NMR spectroscopy. At pH 7.0, Mn(III)heme binds to HSA with lower affinity than Fe(III)heme, and displays a water molecule coordinated to the metal. Myristate binding to a secondary site FAx, allosterically coupled to the heme site, not only increases optical absorbance of Mn(III)heme-bound HSA by a factor of approximately three, but also increases the Mn(III)heme affinity for the fatty acid binding site FA1 by 10–500-fold. Cooperative binding appears to occur at FAx and accessory myristate binding sites. The conformational changes of the Mn(III)heme–HSA tertiary structure allosterically induced by myristate are associated with a noticeable change in both optical absorbance and NMR spectroscopic properties of Mn(III)heme–HSA, allowing the Mn(III)-coordinated water molecule to exchange with the solvent bulk. At pH = 10.0 both myristate affinity for FAx and allosteric modulation of FA1 are reduced, whereas cooperation of accessory sites and FAx is almost unaffected. Moreover, Mn(III)heme binds to HSA with higher affinity than at pH 7.0 even in the absence of myristate, and the metal-coordinated water molecule is displaced. As a whole, these results suggest that FA binding promotes conformational changes reminiscent of N to B state HSA transition, and appear of general significance for a deeper understanding of the allosteric modulation of ligand binding properties of HSA.

Slow oscillations of cytosolic calcium ion concentration - [Ca(2+)](c) - typically originate from release by intracellular stores, but in some cell types can be triggered and sustained by Ca(2+) influx as well. In this study we simultaneously monitored changes in [Ca(2+)](c) and in the electrical activity of the cell membrane by combining indo-1 and patch-clamp measurements in single rat chromaffin cells. By this approach we observed a novel type of spontaneous [Ca(2+)](c) oscillations, much faster than those previously described in these cells. These oscillations are triggered and sustained by complex electrical activity (slow action potentials and spike bursts), require Ca(2+) influx and do not involve release from intracellular stores. The possible physiological implications of this new pathway of intracellular signalling are discussed.

Human Ero1alpha is an endoplasmic reticulum (ER)-resident protein responsible for protein disulfide isomerase (PDI) oxidation. To clarify the molecular mechanisms underlying its function, we generated a panel of cysteine replacement mutants and analyzed their capability of: 1) complementing a temperature-sensitive yeast Ero1 mutant, 2) favoring oxidative folding in mammalian cells, 3) forming mixed disulfides with PDI and ERp44, and 4) adopting characteristic redox-dependent conformations. Our results reveal that two essential cysteine triads (Cys85-Cys94-Cys99 and Cys391-Cys394-Cys397) cooperate in electron transfer, with Cys94 likely forming mixed disulfides with PDI. Dominant negative phenotypes arise when critical residues within the triads are mutated (Cys394, Cys397, and to a lesser extent Cys99). Replacing the first cysteine in either triad (Cys85 or Cys391) generates mutants with weaker activity. In addition, mutating either Cys85 or Cys391, but not Cys397, reverts the dominant negative phenotype of the C394A mutant. These findings suggest that interactions between the two triads, dependent on Cys85 and Cys391, are important for Ero1alpha function, possibly stabilizing a platform for efficient PDI oxidation.

The effects of reducing external Cl- on the electrophysiological properties of the Na+/Cl(-)-dependent GABA transporter rGAT1 expressed in Xenopus oocytes were investigated. In agreement with a recently proposed kinetic scheme, the effects of Cl- are complex but preserve the mutual relationship that links the transport-associated current, I(tr), measured in saturating GABA concentration, and the transient current, I(pre), recorded in the absence of GABA following a voltage step from the holding potential Vh to V. In particular, I(tr) (V) - I(tr) (Vh) = r integral I(pre) (V) dt, where r is the relaxation rate of I(pre) at the same membrane potential and Cl- concentration. The model also predicts a relationship between charge relaxation rate and apparent affinity for GABA, which is also verified in the presence of lowered Na+ or Cl- concentrations. In these conditions, the binding rate of GABA to the transporter is increased. All these effects are consistent with the hypothesis that interaction of the organic substrate with rGAT1 induces a conversion from a capacitive to a conductive mode of operation without strongly altering either the amount or the rate of charge movement.

Signals travel along nerve cell membranes as electrical impulses but are transmitted from one neuron to other neurons and/or effector cells by the release of neurotransmitter across a small intervening space (synaptic cleft). Such neurotransmitter is contained within synaptic vesicles that fuse with the plasma membrane when it depolarizes, to release their contents. The vesicles are then rapidly retrieved and refurbished to sustain further synaptic activity. Keywords: neurotransmitter release; synapse physiology; clathrin; dynamin; endocytosis; exocytosis; neurotransmitter; SNARE

Response from Fesce, Meldolesi and ValtortaWe appreciate that our colleagues, in commenting on our Opinion article 1xSynaptic vesicles: is kissing a matter of competence?. Valtorta, F. et al. Trends Cell Biol. 2001; 11: 324–328Abstract | Full Text | Full Text PDF | PubMed | Scopus (88)See all References1, agree on the validity of the ‘kiss-and-run’ hypothesis. Their comment provides a nice complement to our article, but we would like to respond to two points.First, we feel the possibility raised by Burgoyne et al. that kiss-and-run exocytosis does not produce full emptying of secretory granules in chromaffin cells is still controversial. On the one hand, differences in the sizes and time-courses of release events (measured by amperometry) were indeed proposed as a means to discern kiss-and-run from full fusion; on the other hand, however, Ales et al.2xHigh calcium concentrations shift the mode of exocytosis to the kiss-and-run mechanism. Ales, E. et al. Nat. Cell Biol. 1999; 1: 40–44Crossref | PubMedSee all References2 performed focal amperometry, by positioning a carbon fibre electrode within a patch pipette in a cell-attached configuration and claimed that similarly sized (total charge) events were produced, no matter whether the fused granule was incorporated into the plasma membrane or underwent kiss-and-run recycling.Second, while Burgoyne et al. discuss important evidence about the possible molecular mechanisms of recycling competence, in our paper, we deliberately quoted only a limited selection of references about this for two main reasons: (i) to emphasize our incomplete knowledge of the process, which is still far from being comprehensively understood; and (ii) to focus the attention of the reader on several possibilities: namely the possible differences among vesicles in their molecular make-up, localization with respect to specific synaptic structures (e.g. active zones, Ca2+ channels) and in the dynamic assembly of their supramolecular complexes.In our commentary, we put forward the hypothesis that a process of vesicle maturation is involved in determining the competence not only for rapid evoked release but, similarly, for rapid recycling through kiss-and-run. In this view, several steps might be indispensable, but none of them is the ‘switch’ to kiss-and-run. This comprehensive view could well be disproved by future experimental evidence. Currently, however, we feel it is a promising hypothesis, especially if one considers that, in neurosecretory cells, the (at least) two recycling pathways appear always to be operating in parallel. Furthermore, the relative contributions of either pathway depend not only on the protein composition of the cell and/or activation of signal-transduction pathways and related biochemical changes – but also on the mode of stimulation. The latter aspect is particularly relevant at synapses. All this suggests that, at any one moment, there are vesicles differentially responsive to distinct stimuli within nerve terminals; by the same token, these vesicles might be differentially competent for kiss-and-run endocytosis.

Within pancreatic islet cells, rhythmic changes in the cytosolic Ca2+ concentration have been reported to occur in response to stimulatory glucose concentrations and to be synchronous with pulsatile release of insulin. We explored the possible mechanisms responsible for Ca2+ signal propagation within islet cells, with particular regard to gap junction communication, the pathway widely credited with being responsible for coordination of the secretory activity. Using fura-2 imaging, we found that multiple mechanisms control Ca2+ signaling in pancreatic islet cells. Gap junction blockade by 18 alpha-glycyrrhetinic acid greatly restricted the propagation of Ca2+ waves induced by mechanical stimulation of cells but affected neither Ca2+ signals nor insulin secretion elicited by glucose elevation. The source of Ca2+ elevation was also different under the two experimental conditions, the first being sustained by release from inner stores and the second by nifedipine-sensitive Ca2+ influx. Furthermore, glucose-induced Ca2+ waves were able to propagate across cell-free clefts, indicating that diffusible factors can control Ca2+ signal coordination. Our results provide evidence that multiple mechanisms of Ca2+ signaling operate in beta-cells and that gap junctions are not required for intercellular Ca2+ wave propagation or insulin secretion in response to glucose.

Synapsin I is a neuron-specific phosphoprotein which binds to small synaptic vesicles and actin in a phosphorylation-dependent fashion. We have analyzed the ability of synapsin I to interact with actin monomers and filaments using purified proteins derivatized with fluorescent probes. Synapsin I accelerates the initial rate of actin polymerization and increases the final steady-state levels of polymerized actin. The fraction of total actin polymerized by synapsin I strongly depends on the synapsin I-actin ratio. We have visualized the actin-bundling activity of synapsin I using a non-perturbing method, video-enhanced microscopy of fluoresceinated synapsin I and actin filaments. Our findings suggest that synapsin I exerts a control on the physical characteristics of the cytoskeletal network of the nerve terminal and are consistent with the proposed role of synapsin I in mediating the interaction of synaptic vesicles with actin.

Synapsin I is a synaptic vesicle-specific phosphoprotein which is able to bind and bundle actin filaments in a phosphorylation-dependent fashion. In the present paper we have analyzed the effects of synapsin I on the kinetics of actin polymerization and their modulation by site-specific phosphorylation of synapsin I. We found that dephosphorylated synapsin I accelerates the initial rate of actin polymerization and decreases the rate of filament elongation. The effect was observed at both low and high ionic strength, was specific for synapsin I, and was still present when polymerization was triggered by F-actin seeds. Dephosphorylated synapsin I was also able to induce actin polymerization and bundle formation in the absence of KCl and MgCl2. The effects of synapsin I were strongly decreased after its phosphorylation by Ca2+/calmodulin-dependent protein kinase II. These observations suggest that synapsin I has a phosphorylation-dependent nucleating effect on actin polymerization. The data are compatible with the view that changes in the phosphorylation state of synapsin I play a functional role in regulating the interactions between the nerve terminal cytoskeleton and synaptic vesicles in various stages of the exoendocytotic cycle.

The general features of the kinetics of actin polymerization are investigated by mathematical models, with the aim of identifying the kinetically relevant parameters in the process and detecting and interpreting the alterations occurring in actin polymerization under various experimental conditions. Polymerization curves, obtained by following the increase in fluorescence of actin derivatized with N-(1-pyrenyl) iodoacetamide, are fitted using analytical equations derived from biochemical models of the actin polymerization process. Particular attention is given to the evaluation of the effects of the neuronal phosphoprotein synapsin I. The models obtained under various ionic conditions reveal that synapsin I interacts with actin in a very complex fashion, sharing some of the properties of classical nucleating proteins but displaying also actions not described previously for other actin-binding proteins. Synapsin I appears to bind G-actin with a very high stoichiometry (1:2-4), and the complex behaves as an F-actin nucleus, producing actin filaments under conditions where spontaneous polymerization is negligible. These actions of synapsin I are markedly affected by site-specific phosphorylation of the protein. An original transformation of the fluorescence data, which estimates the disappearance rate of actin monomer toward the critical concentration, is presented and shown to be of general usefulness for the study of actin-binding proteins.

Kiss-and-run fusion, a quick way of coupling exocytotic vesicle fusion to vesicle recycling by endocytosis, can allow full discharge of secretory-vesicle contents, such as neurotransmitters, and is more common than classical endocytosis at high calcium concentrations.

Quantal acetylcholine release was studied in sciatic nerve-extensor digitorum longus muscle preparations from normal rats and in reinnervated muscles at different times following peroneus nerve crush. The efficiency of nerve-evoked transmission was regained almost completely at a time (20 days after crush) when the secretory response to increased extracellular K+ was very weak. This suggests that K+-induced secretion might be activated through mechanisms at least partially different from those involved in the response to nerve stimulation.

The intramembrane particles on the presynaptic membrane and on the membrane of synaptic vesicles were studied at freeze-fractured neuromuscular junctions of the frog. The particles on the P face of the presynaptic membrane belong to two major classes: small particles with diameters less than 9 nm and large particles with diameters between 9 and 13 nm. In addition, there were a few extralarge particles with diameters greater than 13 nm. Indirect stimulation of the muscle, or the application of black widow spider venom, decreased the concentration of small particles on the presynaptic membrane but did not change the concentration of large particles. Three similar classes of particles were found on the P face of the membrane of the synaptic vesicles. The concentrations of large and extralarge particles on the vesicle membrane were comparable to the concentrations of these particles on the presynaptic membrane, whereas the concentration of small particles on the vesicle membrane was less than than the concentration of small particles on the presynaptic membrane. These results are compatible with the idea that synaptic vesicles fuse with the presynaptic membrane when quanta of transmitter are released. However, neither the large nor the extralarge particles on the P face of the presynaptic membrane can be used to trace the movement of vesicle membrane that has been incorporated into the axolemma.

Recycling of synaptophysin (p38), a synaptic vesicle integral membrane protein, was studied by the use of antisera raised against the protein purified from frog brain. When frog cutaneous pectoris muscles were fixed at rest, a bright, specific immunofluorescent signal was observed in nerve-terminal regions only if their plasma membranes had been previously permeabilized. When muscles were fixed after they had been treated for 1 h with a low dose of alpha-latrotoxin in Ca2+-free medium, an equally intense fluorescence could be observed without previous permeabilization. Under this condition, alpha-latrotoxin depletes nerve terminals of their quantal store of acetylcholine and of synaptic vesicles. These results indicate that fusion of synaptic vesicles leads to the exposure of intravesicular antigenic determinants of synaptophysin on the outer surface of the axolemma, and provide direct support for the vesicle hypothesis of neurotransmitter release. After 1 h treatment with the same dose of alpha-latrotoxin in the presence of 1.8 mM extracellular Ca2+, immunofluorescent images were obtained only after permeabilization with detergents. Under this condition, the vesicle population was maintained by an active process of recycling and more than two times the initial store of quanta were secreted. Thus, despite the active turnover of synaptic vesicles and of quanta of neurotransmitter, no extensive intermixing occurs between components of the vesicle and presynaptic plasma membrane.

We applied the quick-freezing technique to investigate the precise temporal coincidence between the onset of quantal secretion and the appearance of fusions of synaptic vesicles with the prejunctional membrane. Frog cutaneous pectoris nerve-muscle preparations were soaked in modified Ringer's solution with 1 mM 4-aminopyridine, 10 mM Ca2+, and 10(-4) M d-Tubocurarine and quick-frozen 1-10 ms after a single supramaximal shock. The frozen muscles were then either freeze-fractured or cryosubstituted in acetone with 13% OsO4 and processed for thin section electron microscopy. Temporal resolution of less than 1 ms can be achieved using a quick-freeze device that increases the rate of freezing of the muscle after it strikes the chilled copper block (15 degrees K) and that minimizes the precooling of the muscle during its descent toward the block. We minimized variations in transmission time by examining thin sections taken only from the medial edge of the muscle, which was at a fixed distance from the point of stimulation of the nerve. The ultrastructure of the cryosubstituted preparations was well preserved to a depth of 5 - 10 micron, and within this narrow band vesicles were found fused with the axolemma after a minimum delay of 2.5 ms after stimulation of the nerve. Since the total transmission time to this edge of the muscle was approximately 3 ms, these results indicate that the vesicles fuse with the axolemma precisely at the same time the quanta are released. Freeze-fracture does not seem to be an adequate experimental technique for this work because in the well-preserved band of the muscle the fracture plane crosses, but does not cleave, the inner hydrophobic domain of the plasmalemma. Fracture faces may form in deeper regions of the muscle where tissue preservation is unsatisfactory and freezing is delayed.

Ouabain (0.1 and 0.05 mM) was applied to frog cutaneous pectoris nerve-muscle preparations bathed in modified Ringer's solution containing either 1.8 mM Ca2+ (and 4 mM Mg2+) or no added Ca2+ (4 mM Mg2+ and 1 mM EGTA). During the intense quantal release of acetylcholine (ACh) induced by ouabain, the parameters of the miniature endplate potentials (mepps) were deduced from the variance, skew, and power spectra of the endplate recordings by applying a recently described modification of classical fluctuation analysis. Often the high frequency of mepps is not stationary; therefore, the signal was high-pass filtered (time constant of the resistance-capacitance filter of 2 ms) to remove the errors introduced by nonstationarity. When ouabain was applied in the presence of Ca2+, mepp frequency started to rise exponentially after a lag of 1.5-2 h, reached an average peak frequency of 1,300/s in approximately 30 min, and then suddenly subsided to low level (10/s). In Ca2+-free solution, after a shorter lag (1-1.5 h), mepp frequency rose to peak rate of 700/s in approximately 20 min and then gradually subsided. In spite of the different time course of secretion in the two experimental conditions, the cumulative quantal release was not significantly different (7.4 +/- 1.3 X 10(5) in Ca2+-containing and 8.8 +/- 2.7 X 10(5) in Ca2+-free solutions). 60 min after the peak secretion, the muscles were fixed for observation in the electron microscope. Morphometric analysis on micrographs of neuromuscular junctions revealed in both cases a profound depletion of synaptic vesicles and deep infoldings of presynaptic membrane. This rapid depletion and the lack of uptake of horseradish peroxidase suggest that ouabain impairs the recycling process that tends to conserve the vesicle population during intense secretion of neurotransmitter. The good correlation observed between the reduction in the store of synaptic vesicles and the total number of quanta of ACh secreted in the absence of a vigorous membrane recycling strongly supports the view that the secretion of a quantum of ACh requires the fusion of a synaptic vesicle with the axolemma.

The time course of exocytosis of quanta of acetylcholine induced by 20 mM K + was studied at the frog neuromuscular junction. Images of vesicle fusion on freeze-fracture replicas were mostly localized at the active zones in resting preparations fixed in 20 mM K + . Fusions appreared also outside the active zones in preparations fixed after 1 min exposure to 20 mM K + and were evenly distributed over the presynaptic membrane after 5 min in 20 mM K + (even though secretion was prevented by withdrawing Ca 2+ until 30 s before fixation). The mean densities of vesicle fusions were comparable in all conditions, as were the total number of quanta released during the fixation period. This indicates that fusions outside active zones represent ectopic exocytosis, slowly activated by potassium. Partial inactivation of K + -induced quantal release (time and concentration-dependent) was observed electrophysiologically; this may be related to the observed decrease in density of vesicle fusions along the active zones, with time. Consistently, after 5 min in 15 mM K + fusion density at the active zones remained high. It is concluded that active zone-associated and ectopic fusions are two exocytotic processes activated with differential time courses and concentration-dependence by K + .