Calcium-dependent transmitter secretion from fibroblasts: modulation by synaptotagmin I

Activity-dependent BDNF release via endocytic pathways is regulated by synaptotagmin-6 and complexin

Brain-derived neurotrophic factor (BDNF) is known to modulate synapse development and plasticity, but the source of synaptic BDNF and molecular mechanisms regulating BDNF release remain unclear. Using exogenous BDNF tagged with quantum dots (BDNF-QDs), we found that endocytosed BDNF-QDs were preferentially localized to postsynaptic sites in the dendrite of cultured hippocampal neurons. Repetitive neuronal spiking induced the release of BDNF-QDs at these sites, and this process required activation of glutamate receptors. Down-regulating complexin 1/2 (Cpx1/2) expression eliminated activity-induced BDNF-QD secretion, although the overall activity-independent secretion was elevated. Among eight synaptotagmin (Syt) isoforms examined, down-regulation of only Syt6 impaired activity-induced BDNF-QD secretion. In contrast, activity-induced release of endogenously synthesized BDNF did not depend on Syt6. Thus, neuronal activity could trigger the release of endosomal BDNF from postsynaptic dendrites in a Cpx- and Syt6-dependent manner, and endosomes containing BDNF may serve as a source of BDNF for activity-dependent synaptic modulation.

Enhanced recruitment of endosomal Na+/H+ exchanger NHE6 into Dendritic spines of hippocampal pyramidal neurons during NMDA receptor-dependent long-term potentiation

Postsynaptic endosomal trafficking has emerged as a principal regulatory mechanism of structural and functional plasticity of glutamatergic synapses. Recycling endosomes perform activity-dependent transport of AMPA receptors (AMPARs) and lipids to the postsynaptic membrane, activities that are known to contribute to long-term synaptic potentiation and hypothesized to subserve learning and memory processes in the brain. Recently, genetic defects in a widely expressed vesicular pH-regulating transporter, the Na(+)/H(+) exchanger NHE6 isoform, have been implicated in neurodevelopmental disorders including severe X-linked mental retardation and autism. However, little information is available regarding the cellular properties of this transporter in the CNS. Here, we show by quantitative light microscopy that the protein abundance of NHE6 is developmentally regulated in area CA1 of the mouse hippocampus. Within pyramidal neurons, NHE6 was found to localize to discrete puncta throughout the soma and neurites, with noticeable accumulation at dendritic spines and presynaptic terminals. Dual immunolabeling of dendritic spines revealed that NHE6 partially colocalizes with typical markers of early and recycling endosomes as well as with the AMPAR subunit GluA1. Significantly, NHE6-containing vesicles exhibited enhanced translocation to dendritic spine heads during NMDA receptor (NMDAR)-dependent long-term potentiation. These data suggest that NHE6 may play a unique, previously unrecognized, role at glutamatergic synapses that are important for learning and memory.

The Na(+)/H (+) exchanger NHE5 is sorted to discrete intracellular vesicles in the central and peripheral nervous systems

The pH milieu of the central and peripheral nervous systems is an important determinant of neuronal excitability, function, and survival. In mammals, neural acid-base homeostasis is coordinately regulated by ion transporters belonging to the Na(+)/H(+) exchanger (NHE) and bicarbonate transporter gene families. However, the relative contributions of individual isoforms within the respective families are not fully understood. This report focuses on the NHE family, specifically the plasma membrane-type NHE5 which is preferentially transcribed in brain, but the distribution of the native protein has not been extensively characterized. To this end, we generated a rabbit polyclonal antibody that specifically recognizes NHE5. In both central (cortex, hippocampus) and peripheral (superior cervical ganglia, SCG) nervous tissue of mice, NHE5 immunostaining was punctate and highly concentrated in the somas and to lesser amounts in the dendrites of neurons. Very little signal was detected in axons. Similarly, in primary cultures of differentiated SCG neurons, NHE5 localized predominantly to vesicles in the somatodendritic compartment, though some immunostaining was also evident in punctate vesicles along the axons. NHE5 was also detected predominantly in intracellular vesicles of cultured SCG glial cells. Dual immunolabeling of SCG neurons showed that NHE5 did not colocalize with markers for early endosomes (EEA1) or synaptic vesicles (synaptophysin), but did partially colocalize with the transferrin receptor, a marker of recycling endosomes. Collectively, these data suggest that NHE5 partitions into a unique vesicular pool in neurons that shares some characteristics of recycling endosomes where it may serve as an important regulated store of functional transporters required to maintain cytoplasmic pH homeostasis.

Calcineurin homologous protein: a multifunctional Ca2+-binding protein family

The calcineurin homologous protein (CHP) belongs to an evolutionarily conserved Ca(2+)-binding protein subfamily. The CHP subfamily is composed of CHP1, CHP2, and CHP3, which in vertebrates share significant homology at the protein level with each other and between other Ca(2+)-binding proteins. The CHP structure consists of two globular domains containing from one to four EF-hand structural motifs (calcium-binding regions composed of two helixes, E and F, joined by a loop), the myristoylation, and nuclear export signals. These structural features are essential for the function of the three members of the CHP subfamily. Indeed, CHP1-CHP3 have multiple and diverse essential functions, ranging from the regulation of the plasma membrane Na(+)/H(+) exchanger protein function, to carrier vesicle trafficking and gene transcription. The diverse functions attributed to the CHP subfamily rendered an understanding of its action highly complex and often controversial. This review provides a comprehensive and organized examination of the properties and physiological roles of the CHP subfamily with a view to revealing a link between CHP diverse functions.

Calcium binding by synaptotagmin's C2A domain is an essential element of the electrostatic switch that triggers synchronous synaptic transmission

Synaptotagmin is the major calcium sensor for fast synaptic transmission that requires the synchronous fusion of synaptic vesicles. Synaptotagmin contains two calcium-binding domains: C2A and C2B. Mutation of a positively charged residue (R233Q in rat) showed that Ca2+-dependent interactions between the C2A domain and membranes play a role in the electrostatic switch that initiates fusion. Surprisingly, aspartate-to-asparagine mutations in C2A that inhibit Ca2+ binding support efficient synaptic transmission, suggesting that Ca2+ binding by C2A is not required for triggering synchronous fusion. Based on a structural analysis, we generated a novel mutation of a single Ca2+-binding residue in C2A (D229E in Drosophila) that inhibited Ca2+ binding but maintained the negative charge of the pocket. This C2A aspartate-to-glutamate mutation resulted in ∼80% decrease in synchronous transmitter release and a decrease in the apparent Ca2+ affinity of release. Previous aspartate-to-asparagine mutations in C2A partially mimicked Ca2+ binding by decreasing the negative charge of the pocket. We now show that the major function of Ca2+ binding to C2A is to neutralize the negative charge of the pocket, thereby unleashing the fusion-stimulating activity of synaptotagmin. Our results demonstrate that Ca2+ binding by C2A is a critical component of the electrostatic switch that triggers synchronous fusion. Thus, Ca2+ binding by C2B is necessary and sufficient to regulate the precise timing required for coupling vesicle fusion to Ca2+ influx, but Ca2+ binding by both C2 domains is required to flip the electrostatic switch that triggers efficient synchronous synaptic transmission.

Gβγ Interferes with Ca2+-Dependent Binding of Synaptotagmin to the SolubleN-Ethylmaleimide-Sensitive Factor Attachment Protein Receptor (SNARE) Complex

Presynaptic inhibitory G protein-coupled receptors (GPCRs) can decrease neurotransmission by inducing interaction of Gbetagamma with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. We have shown that this action of Gbetagamma requires the carboxyl terminus of the 25-kDa synaptosome-associated protein (SNAP25) and is downstream of the well known inhibition of Ca2+ entry through voltage-gated calcium channels. We propose a mechanism in which Gbetagamma and synaptotagmin compete for binding to the SNARE complex. Here, we characterized the Gbetagamma interaction sites on syntaxin1A and SNAP25 and demonstrated an overlap of the Gbetagamma- and synaptotagmin I -binding regions on each member of the SNARE complex. Synaptotagmin competes in a Ca2+-sensitive manner with binding of Gbetagamma to SNAP25, syntaxin1A, and the assembled SNARE complex. We predict, based on these findings, that at high intracellular Ca2+ concentrations, Ca2+-synaptotagmin I can displace Gbetagamma binding and the Gbetagamma-dependent inhibition of exocytosis can be blocked. We tested this hypothesis in giant synapses of the lamprey spinal cord, where 5-HT works via Gbetagamma to inhibit neurotransmission (Blackmer et al., 2001). We showed that increased presynaptic Ca2+ suppresses the 5-HT- and Gbetagamma-dependent inhibition of exocytosis. We suggest that this effect may be due to Ca2+-dependent competition between Gbetagamma and synaptotagmin I for SNARE binding. This type of dynamic regulation may represent a novel mechanism for modifying transmitter release in a graded manner based on the history of action potentials that increase intracellular Ca2+ concentrations and of inhibitory signals through G(i)-coupled GPCRs.

Afferent neurotransmission mediated by hemichannels in mammalian taste cells

In mammalian taste buds, ionotropic P2X receptors operate in gustatory nerve endings to mediate afferent inputs. Thus, ATP secretion represents a key aspect of taste transduction. Here, we characterized individual vallate taste cells electrophysiologically and assayed their secretion of ATP with a biosensor. Among electrophysiologically distinguishable taste cells, a population was found that released ATP in a manner that was Ca(2+) independent but voltage-dependent. Data from physiological and pharmacological experiments suggested that ATP was released from taste cells via specific channels, likely to be connexin or pannexin hemichannels. A small fraction of ATP-secreting taste cells responded to bitter compounds, indicating that they express taste receptors, their G-protein-coupled and downstream transduction elements. Single cell RT-PCR revealed that ATP-secreting taste cells expressed gustducin, TRPM5, PLCbeta2, multiple connexins and pannexin 1. Altogether, our data indicate that tastant-responsive taste cells release the neurotransmitter ATP via a non-exocytotic mechanism dependent upon the generation of an action potential.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

Nerve-evoked synchronous release and high K+ -induced quantal events are regulated separately by synaptotagmin I at Drosophila neuromuscular junctions

The distal Ca(2+)-binding domain of synaptotagmin I (Syt I), C2B, has two Ca(2+)-binding sites. To study their function in Drosophila, pairs of aspartates were mutated to asparagines and the mutated syt I was expressed in the syt I-null background (P[syt I(B-D1,2N)] and P[syt I(B-D3,4N)]). We examined the effects of these mutations on nerve-evoked synchronous synaptic transmission and high K(+)-induced quantal events at embryonic neuromuscular junctions. The P[syt I(B-D1,2N)] mutation virtually abolished synaptic transmission, whereas the P[syt I(B-D3,4N)] mutation strongly reduced but did not abolish it. The quantal content in P[syt I(B-D3,4N)] increased with the external Ca(2+) concentration, [Ca(2+)](e), with a slope of 1.86 in double-logarithmic plot, whereas that of control was 2.88. In high K(+) solutions the quantal event frequency in P[syt I(B-D3,4N)] increased progressively with [Ca(2+)](e) between 0 and 0.15 mM as in control. In contrast, in P[syt I(B-D1,2N)] the event frequency did not increase progressively between 0 and 0.15 mM and was significantly lower at 0.15 than at 0.05 mM [Ca(2+)](e). The P[syt I(B-D1,2N)] mutation inhibits high K(+)-induced quantal release in a narrow range of [Ca(2+)](e) (negative regulatory function). When Sr(2+) substituted for Ca(2+), nerve-evoked synchronous synaptic transmission was severely depressed and delayed asynchronous release was appreciably increased in control embryos. In high K(+) solutions with Sr(2+), the quantal event frequency was higher than that in Ca(2+) and increased progressively with [Sr(2+)](e) in control and in both mutants. Sr(2+) partially substitutes for Ca(2+) in synchronous release but does not support the negative regulatory function of Syt I.

Two-photon excitation imaging of exocytosis and endocytosis and determination of their spatial organization

Two-photon excitation imaging is the least invasive optical approach to study living tissues. We have established two-photon extracellular polar-tracer (TEP) imaging with which it is possible to visualize and quantify all exocytic events in the plane of focus within secretory tissues. This technology also enables estimate of the precise diameters of vesicles independently of the spatial resolution of the optical microscope, and determination of the fusion pore dynamics at nanometer resolution using TEP-imaging based quantification (TEPIQ). TEP imaging has been applied to representative secretory glands, e.g., exocrine pancreas, endocrine pancreas, adrenal medulla and a pheochromocytoma cell line (PC12), and has revealed unexpected diversity in the spatial organization of exocytosis and endocytosis crucial for the physiology and pathology of secretory tissues and neurons. TEP imaging and TEPIQ analysis are powerful tools for elucidating the molecular and cellular mechanisms of exocytosis and certain related diseases, such as diabetes mellitus, and the development of new therapeutic agents and diagnostic tools.

Differential properties of GTP- and Ca(2+)-stimulated exocytosis from large dense core vesicles

Many cells utilize a GTP-dependent pathway to trigger exocytosis in addition to Ca(2+)-triggered exocytosis. However, little is known about the mechanism by which GTP triggers exocytosis independent of Ca(2+). We used dual-color evanescent field microscopy to compare the motion and fusion of large dense core vesicles stimulated by either mastoparan (Mas) in Ca(2+)-free conditions or high K(+) in the presence of Ca(2+). We demonstrate that Mas is hardly effective in triggering the fusion of the predocked vesicles but predominantly mobilizes cytosolic vesicles. In contrast, Ca(2+)-dependent exocytosis is largely due to predocked vesicles. Fusion kinetics analysis and carbon-fiber amperometry reveal that Mas induces a brief 'kiss-and-run' fusion and releases only a small amount of the cargo, whereas Ca(2+) stimulates a more persistent opening of the fusion pore and larger release of the contents. Furthermore, we show that Mas-released vesicles require a much shorter time to reach fusion competence once they approach the plasma membrane. Our data suggest the involvement of different mechanisms not only in triggering and fusion but also in the docking and priming process for Ca(2+)- and GTP-dependent exocytosis.

Cell communication in taste buds

Taste bud cells communicate with sensory afferent fibers and may also exchange information with adjacent cells. Indeed, communication between taste cells via conventional and/or novel synaptic interactions may occur prior to signal output to primary afferent fibers. This review discusses synaptic processing in taste buds and summarizes results showing that it is now possible to measure real-time release of synaptic transmitters during taste stimulation using cellular biosensors. There is strong evidence that serotonin and ATP play a role in cell-to-cell signaling and sensory output in the gustatory end organs.

Modulation of Ca(2+)-dependent and Ca(2+)-independent miniature endplate potentials by phorbol ester and adenosine in frog

Phorbol esters and adenosine modulate transmitter release from frog motor nerves through actions at separate sites downstream of calcium entry. However, it is not known whether these agents have calcium-independent sites of action. We therefore characterised calcium independent miniature endplate potentials (mepps) generated in response to 4-aminoquinaldine (4-AQ(A)) and then compared the modulation of these mepps by phorbol esters and adenosine with that of normal calcium dependent mepps. Application of 30 microM 4-AQ(A) resulted in the appearance of a population of mepps with amplitudes greater than twice the total population mode (mepp(>2M)). In the presence of 4-AQ(A), K(+) depolarisation or hypertonicity increased the numbers of normal amplitude mepps (mepp(N)) but had no effect on the frequency of mepp(>2M) events, suggesting that mepp(>2M) are not dependent on calcium. Treatment with the botulinum toxin (Botx) fractions C, D, or E (which selectively cleave syntaxin, synaptobrevin and SNAP-25, respectively) produced equivalent reductions in both normal and 4-AQ(A) induced mepps, suggesting that both mepp populations have equal dependence on the intact SNARE proteins. Phorbol dibutyrate (PDBu, 100 nM) increased the frequencies of both populations of mepps recorded in the presence of 4-AQ(A). Adenosine (25 microM) selectively reduced the numbers of mepp(N) with no effect on the frequency of mepp(>2M) events. These results suggest that mepp(>2M) events released in response to 4-AQ(A) are dependent on intact forms of syntaxin, synaptobrevin and SNAP-25, but unlike mepp(N) are independent of a functional calcium sensor. The selective action of adenosine, to reduce the numbers of normal amplitude mepps without effecting the frequency of mepp(>2M) events, suggests that adenosine normally inhibits transmitter release through a mechanism that is dependent on the presence of a functional calcium sensor.

How to build a central synapse: clues from cell culture

Central neurons develop and maintain molecularly distinct synaptic specializations for excitatory and inhibitory transmitters, often only microns apart on their dendritic arbor. Progress towards understanding the molecular basis of synaptogenesis has come from several recent studies using a coculture system of non-neuronal cells expressing molecules that generate presynaptic or postsynaptic "hemi-synapses" on contacting neurons. Together with molecular properties of these protein families, such studies have yielded interesting clues to how glutamatergic and GABAergic synapses are assembled. Other clues come from heterochronic cultures, manipulations of activity in subsets of neurons in a network, and of course many in vivo studies. Taking into account these data, we consider here how basic parameters of synapses--competence, placement, composition, size and longevity--might be determined.

Mouse taste buds use serotonin as a neurotransmitter

Synapses between gustatory receptor cells and primary sensory afferent fibers transmit the output signal from taste buds to the CNS. Several transmitter candidates have been proposed for these synapses, including serotonin (5-HT), glutamate, acetylcholine, ATP, peptides, and others, but, to date, none has been unambiguously identified. We used Chinese hamster ovary cells stably expressing 5-HT2C receptors as biodetectors to monitor 5-HT release from taste buds. When taste buds were depolarized with KCl or stimulated with bitter, sweet, or sour (acid) tastants, serotonin was released. KCl- and acid-induced 5-HT release, but not release attributable to sweet or bitter stimulation, required Ca2+ influx. In contrast, 5-HT release evoked by sweet and bitter stimulation seemed to be triggered by intracellular Ca2+ release. These experiments strongly implicate serotonin as a taste bud neurotransmitter and reveal unexpected transmitter release mechanisms.

Synaptotagmin VII restricts fusion pore expansion during lysosomal exocytosis

Synaptotagmin is considered a calcium-dependent trigger for regulated exocytosis. We examined the role of synaptotagmin VII (Syt VII) in the calcium-dependent exocytosis of individual lysosomes in wild-type (WT) and Syt VII knockout (KO) mouse embryonic fibroblasts (MEFs) using total internal reflection fluorescence microscopy. In WT MEFs, most lysosomes only partially released their contents, their membrane proteins did not diffuse into the plasma membrane, and inner diameters of their fusion pores were smaller than 30 nm. In Syt VII KO MEFs, not only was lysosomal exocytosis triggered by calcium, but all of these restrictions on fusion were also removed. These observations indicate that Syt VII does not function as the calcium-dependent trigger for lysosomal exocytosis. Instead, it restricts the kinetics and extent of calcium-dependent lysosomal fusion.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Synaptotagmin I, a Ca2+ sensor for neurotransmitter release

The discovery that Ca(2+) triggers rapid neurotransmitter release has prompted the search for the Ca(2+) sensor. There is now general agreement that the vesicle-associated Ca(2+)-binding protein, synaptotagmin I, is required for the tight temporal coupling between Ca(2+) influx and synaptic vesicle fusion. However, the precise mechanism of Ca(2+)-sensing by synaptotagmin I is still under debate despite intensive investigation using genetic, biochemical and electrophysiological tools. Here, we discuss many of the genetic manipulations from the past few years that have shed light on the Ca(2+)-sensing function of synaptotagmin I. We also present our view as to how the Ca(2+) signal is translated rapidly into membrane fusion at fast chemical synapses.

Presynaptic quantal plasticity: Katz's original hypothesis revisited

Changes in the amplitudes of signals conveyed at synaptic contacts between neurons underlie many brain functions and pathologies. Here we review the possible determinants of the amplitude and plasticity of the elementary postsynaptic signal, the miniature. In the absence of a definite understanding of the molecular mechanism releasing transmitters, we investigated a possible alternative interpretation. Classically, both the quantal theory and the vesicle theory predict that the amount of transmitter producing a miniature is determined presynaptically prior to release and that rapid changes in miniature amplitude reflect essentially postsynaptic alterations. However, recent data indicates that short-term and long-lasting changes in miniature amplitude are in large part due to changes in the amount of transmitter in individual released packets that show no evidence of preformation. Current representations of transmitter release derive from basic properties of neuromuscular transmission and endocrine secretion. Reexamination of overlooked properties of these two systems indicate that the amplitude of miniatures may depend as much, if not more, on the Ca(2+) signals in the presynaptic terminal than on the number of postsynaptic receptors available or on vesicle's contents. Rapid recycling of transmitter and its possible adsorption at plasma and vesicle lumenal membrane surfaces suggest that exocytosis may reflect membrane traffic rather than actual transmitter release. This led us to reconsider the disregarded hypothesis introduced by Fatt and Katz (1952; J Physiol 117:109-128) that the excitability of the release site may account for the "quantal effect" in fast synaptic transmission. In this case, changes in excitability of release sites would contribute to the presynaptic quantal plasticity that is often recorded.

Synaptotagmin I functions as a calcium sensor to synchronize neurotransmitter release

To characterize Ca(2+)-mediated synaptic vesicle fusion, we analyzed Drosophila synaptotagmin I mutants deficient in specific interactions mediated by its two Ca(2+) binding C2 domains. In the absence of synaptotagmin I, synchronous release is abolished and a kinetically distinct delayed asynchronous release pathway is uncovered. Synapses containing only the C2A domain of synaptotagmin partially recover synchronous fusion, but have an abolished Ca(2+) cooperativity. Mutants that disrupt Ca(2+) sensing by the C2B domain have synchronous release with normal Ca(2+) cooperativity, but with reduced release probability. Our data suggest the Ca(2+) cooperativity of neurotransmitter release is likely mediated through synaptotagmin-SNARE interactions, while phospholipid binding and oligomerization trigger rapid fusion with increased release probability. These results indicate that synaptotagmin is the major Ca(2+) sensor for evoked release and functions to trigger synchronous fusion in response to Ca(2+), while suppressing asynchronous release.

Clathrin-mediated endocytosis and recycling of the neuron-specific Na+/H+ exchanger NHE5 isoform. Regulation by phosphatidylinositol 3'-kinase and the actin cytoskeleton

Mammalian Na+/H+ exchangers (NHEs) are a family of integral membrane proteins that play central roles in sodium, acid-base, and cell volume homeostasis. The recently cloned NHE5 isoform is expressed predominantly in brain, but its functional and cellular properties are poorly understood. To facilitate its characterization, an epitope-tagged construct of NHE5 was ectopically expressed in nonneuronal and neuronal cells. In NHE-deficient Chinese hamster ovary AP-1 cells, NHE5 localized at the plasmalemma, but a significant fraction accumulated intracellularly in vesicles that concentrated in a juxtanuclear region. Similarly, in nerve growth factor-differentiated neuroendocrine PC12 cells and primary hippocampal neurons, immunolabeling of NHE5 was detected in endomembrane vesicles in the perinuclear region of the cell body but also along the processes. More detailed characterization in AP-1 cells using organelle-specific markers showed that NHE5 co-localized with internalized transferrin, a marker of recycling endosomes. Transient transfection of a dominant negative mutant of dynamin-1, which inhibits clathrin-mediated endocytosis, blocked uptake of transferrin as well as internalization of NHE5. Likewise, wortmannin inhibition of phosphatidylinositol 3'-kinase, a lipid kinase implicated in endosomal traffic, induced coalescence of vesicles containing NHE5 and caused a pronounced inhibition of plasmalemmal Na+/H+ exchange. By contrast, disruption of the F-actin cytoskeleton with cytochalasin D increased cell surface NHE5 activity and abundance. These observations demonstrate that NHE5 is localized to the recycling endosomal pathway and is dynamically regulated by phosphatidylinositol 3'-kinase and by the state of F-actin assembly.

Neisseria gonorrhoeae porin P1.B induces endosome exocytosis and a redistribution of Lamp1 to the plasma membrane

The immunoglobulin A (IgA) protease secreted by pathogenic Neisseria spp. cleaves Lamp1, thereby altering lysosomes in a cell and promoting bacterial intracellular survival. We sought to determine how the IgA protease gains access to cellular Lamp1 in order to better understand the role of this cleavage event in bacterial infection. In a previous report, we demonstrated that the pilus-induced Ca(2+) transient triggers lysosome exocytosis in human epithelial cells. This, in turn, increases the level of Lamp1 at the plasma membrane, where it can be cleaved by IgA protease. Here, we show that porin also induces a Ca(2+) flux in epithelial cells. This transient is similar in nature to that observed in phagocytes exposed to porin. In contrast to the pilus-induced Ca(2+) transient, the porin-induced event does not trigger lysosome exocytosis. Instead, it stimulates exocytosis of early and late endosomes and increases Lamp1 on the cell surface. These results indicate that Neisseria pili and porin perturb Lamp1 trafficking in epithelial cells by triggering separate and distinct Ca(2+)-dependent exocytic events, bringing Lamp1 to the cell surface, where it can be cleaved by IgA protease.

Role of synaptotagmin in Ca2+-triggered exocytosis

The Ca(2+)-binding synaptic-vesicle protein synaptotagmin I has attracted considerable interest as a potential Ca(2+) sensor that regulates exocytosis from neurons and neuroendocrine cells. Recent studies have shed new light on the structure, biochemical/biophysical properties and function of synaptotagmin, and the emerging view is that it plays an important role in both exocytosis and endocytosis. At least a dozen additional isoforms exist, some of which are expressed outside of the nervous system, suggesting that synaptotagmins might regulate membrane traffic in a variety of cell types. Here we provide an overview of the members of this gene family, with particular emphasis on the question of whether and how synaptotagmin I functions during the final stages of membrane fusion: does it regulate the Ca(2+)-triggered opening and dilation of fusion pores?

Inhibition of Na+/Ca2+ exchange with KB-R7943 or bepridil diminished mineral deposition by osteoblasts

Osteoblasts form new bone by secreting a complex extracellular matrix that has the capacity to mineralize when adequate amounts of calcium and phosphate are supplied. The studies reported here show that long-term treatment of cultured, primary osteoblasts with Na+/Ca2+ exchanger (NCX) inhibitors, bepridil and KB-R7943, impacts in a dose-dependent manner the ability of the cells to form a calcified matrix. Treatment of confluent osteoblast cultures for 14 days with low levels of bepridil (3.0 microM) or KB-R7943 (1.0 microM and 0.1 microM) resulted in a significantly diminished capacity of these cells to mineralize bone matrix, without significantly altering cell morphology, viability, or cell differentiation. The data indicate that inhibition of NCX reduces mineral accumulation in the bone matrix by blocking the efflux of Ca2+ from the osteoblast into the bone fluid. In addition, immunocytochemistry of type I collagen (COLI) and bone sialoprotein (BSP) suggests that inhibition of NCX by 1.0 microM KB-R7943 also may impair the secretion of bone matrix proteins by the osteoblasts. This study is the first to show that NCX is an important regulator of the bone fluid microenvironment and that NCX appears critical to the mineralization process.

Dynamic signaling between astrocytes and neurons

Astrocytes, a sub-type of glia in the central nervous system, are dynamic signaling elements that integrate neuronal inputs, exhibit calcium excitability, and can modulate neighboring neurons. Neuronal activity can lead to neurotransmitter-evoked activation of astrocytic receptors, which mobilizes their internal calcium. Elevations in astrocytic calcium in turn trigger the release of chemical transmitters from astrocytes, which can cause sustained modulatory actions on neighboring neurons. Astrocytes, and perisynaptic Schwann cells, by virtue of their intimate association with synapses, are strategically positioned to regulate synaptic transmission. This capability, that has now been demonstrated in several studies, raises the untested possibility that astrocytes are an integral element of the circuitry for synaptic plasticity. Because the highest ratio of glia-to-neurons is found at the top of the phylogenetic tree in the human brain, these recent demonstrations of dynamic bi-directional signaling between astrocytes and neurons leave us with the question as to whether astrocytes are key regulatory elements of higher cortical functions.

Calmodulin Signals Capacitation and Triggers the Agonist-Induced Acrosome Reaction in Mouse Spermatozoa

Capacitated acrosome-intact spermatozoa interact with specific sugar residues on neoglycoproteins (ngps) or solubilized zona pellucida (ZP), the egg's extracellular glycocalyx, prior to the initiation of a signal transduction cascade that results in the fenestration and fusion of the sperm plasma membrane and the outer acrosomal membrane at multiple sites and exocytosis of acrosomal contents (i.e., induction of the acrosome reaction (AR)). The AR releases acrosomal contents at the site of sperm-zona binding and is thought to be a prerequisite event that allows spermatozoa to penetrate the ZP and fertilize the egg. Since Ca(2+)/calmodulin (CaM) plays a significant role in several cell signaling pathways and membrane fusion events, we have used a pharmacological approach to examine the role of CaM, a calcium-binding protein, in sperm capacitation and agonist-induced AR. Inclusion of CaM antagonists (calmodulin binding domain, calmidazolium, compound 48/80, ophiobolin A, W5, W7, and W13), either in in vitro capacitation medium or after sperm capacitation blocked the npg-/ZP-induced AR. Purified CaM largely reversed the AR blocking effects of antagonists during capacitation. Our results demonstrate that CaM plays an important role in priming (i.e., capacitation) of mouse spermatozoa as well as in the agonist-induced AR. These data allow us to propose that CaM regulates these events by modulating sperm membrane component(s).

ATP stimulates calcium-dependent glutamate release from cultured astrocytes

ATP caused a dose-dependent, receptor-mediated increase in the release of glutamate and aspartate from cultured astrocytes. Using calcium imaging in combination HPLC we found that the increase in intracellular calcium coincided with an increase in glutamate and aspartate release. Competitive antagonists of P(2) receptors blocked the response to ATP. The increase in intracellular calcium and release of glutamate evoked by ATP were not abolished in low Ca(2+)-EGTA saline, suggesting the involvement of intracellular calcium stores. Pre-treatment of glial cultures with an intracellular Ca(2+) chelator abolished the stimulatory effects of ATP. Thapsigargin (1 microM), an inhibitor of Ca(2+)-ATPase from the Ca(2+) pump of internal stores, significantly reduced the calcium transients and the release of aspartate and glutamate evoked by ATP. U73122 (10 microM, a phospholipase C inhibitor, attenuated the ATP-stimulatory effect on calcium transients and blocked ATP-evoked glutamate release in astrocytes. Replacement of extracellular sodium with choline failed to influence ATP-induced glutamate release. Furthermore, inhibition of the glutamate transporters p-chloromercuri-phenylsulfonic acid and Ltrans-pyrolidine-2,4-dicarboxylate failed to impair the ability of ATP to stimulate glutamate release from astrocytes. However, an anion transport inhibitor, furosemide, and a potent Cl(-) channel blocker, 5-nitro-2(3-phenylpropylamino)-benzoate, reduced ATP-induced glutamate release. These results suggest that ATP stimulates excitatory amino acid release from astrocytes via a calcium-dependent anion-transport sensitive mechanism.

Regulation of exocytosis by protein kinases and Ca(2+) in pancreatic duct epithelial cells

We asked if the mechanisms of exocytosis and its regulation in epithelial cells share features with those in excitable cells. Cultured dog pancreatic duct epithelial cells were loaded with an oxidizable neurotransmitter, dopamine or serotonin, and the subsequent release of these exogenous molecules during exocytosis was detected by carbon-fiber amperometry. Loaded cells displayed spontaneous exocytosis that may represent constitutive membrane transport. The quantal amperometric events induced by fusion of single vesicles had a rapid onset and decay, resembling those in adrenal chromaffin cells and serotonin-secreting leech neurons. Quantal events were frequently preceded by a "foot," assumed to be leak of transmitters through a transient fusion pore, suggesting that those cell types share a common fusion mechanism. As in neurons and endocrine cells, exocytosis in the epithelial cells could be evoked by elevating cytoplasmic Ca(2+) using ionomycin. Unlike in neurons, hyperosmotic solutions decreased exocytosis in the epithelial cells, and giant amperometric events composed of many concurrent quantal events were observed occasionally. Agents known to increase intracellular cAMP in the cells, such as forskolin, epinephrine, vasoactive intestinal peptide, or 8-Br-cAMP, increased the rate of exocytosis. The forskolin effect was inhibited by the Rp-isomer of cAMPS, a specific antagonist of protein kinase A, whereas the Sp-isomer, a specific agonist of PKA, evoked exocytosis. Thus, PKA is a downstream effector of cAMP. Finally, activation of protein kinase C by phorbol-12-myristate-13-acetate also increased exocytosis. The PMA effect was not mimicked by the inactive analogue, 4alpha-phorbol-12,13-didecanoate, and it was blocked by the PKC antagonist, bisindolylmaleimide I. Elevation of intracellular Ca(2+) was not needed for the actions of forskolin or PMA. In summary, exocytosis in epithelial cells can be stimulated directly by Ca(2+), PKA, or PKC, and is mediated by physical mechanisms similar to those in neurons and endocrine cells.

Regulated secretion of conventional lysosomes

Regulated secretion has been traditionally regarded as a specialized process present in only a few cell types. Similarly, the secretory lysosomes of hematopoietic cells have been viewed as 'modified' organelles that acquired the machinery for regulated exocytosis. However, there is evidence that conventional lysosomes can, in many cell types, respond to rises in the intracellular free Ca2+ concentration by fusing with the plasma membrane. These findings profoundly change the current view of lysosomes as a 'final' station of the endocytic pathway and suggest a previously unsuspected active role for this organelle.

Surface-accessible GABA supports tonic and quantal synaptic transmission

Exocytosis is commonly viewed as the only secretory process able to account for quantal forms of fast synaptic transmission. However, the demonstrated variability and composite properties of miniature postsynaptic signals are not easily explained by all-or-none exocytotic discharge of transmitter in solution from inside vesicles. Recent studies of endocrine secretion have shown that hormone release does not coincide with exocytosis due to its trapping in the core matrix of the granule. Thus, we tested whether the synaptic transmitter GABA could also be held in a matrix before being released. Using confocal microscopy and flow cytometry of embryonic rat hippocampal neurons, we found a GABA immunoreaction at the surface of live cell bodies and growth cones that coincided spatially and quantitatively with the binding of tetanus toxin fragment C (TTFC). TTFC binds predominantly at membrane sites containing the trisialoglycosphingolipid GT1b. Using flow cytometry, GT1b-containing liposomes preincubated in 100 nM GABA exhibited the same relationship between GABA and TTFC surface binding as found on neurons and growth cones. Embryonic neurons differentiated in culture expressed initially a tonic, and after 3-5 days, transient, postsynaptic signals mediated by GABA acting at GABA(A) receptor/Cl(-) channels. A stream of saline applied to the neuronal surface rapidly and reversibly suppressed both tonic and transient signals. A brief application of the GABAmimetic isoguvacine immediately transformed both tonic and transient GABAergic signals into tonic and transient isoguvacinergic signals. These results and those in the literature are consistent with an immediately releasable compartment of transmitter accessible from the presynaptic surface.

Spontaneous acetylcholine secretion from developing growth cones of Drosophila central neurons in culture: effects of cAMP-pathway mutations

We describe a novel bioassay system that uses Xenopus embryonic myocytes (myoballs) to detect the release of acetylcholine from Drosophila CNS neurons. When a voltage-clamped Xenopus myoball was manipulated into contact with cultured Drosophila "giant" neurons, spontaneous synaptic current-like events were registered. These events were observed within seconds after contact and were blocked by curare and alpha-bungarotoxin, but not by TTX and Cd(2+), suggesting that they are caused by the spontaneous quantal release of acetylcholine (ACh). The secretion occurred not only at the growth cone, but also along the neurite and at the soma, with significantly different release parameters among various regions. The amplitude of these currents displayed a skewed distribution. These features are distinct from synaptic transmission at more mature synapses or autapses formed in this culture system and are reminiscent of the transmitter release process during early development in other preparations. The usefulness of this coculture system in studying presynaptic secretion mechanisms is illustrated by a series of studies on the cAMP pathway mutations, dunce (dnc) and PKA-RI, which disrupt a cAMP-specific phosphodiesterase and the regulatory subunit of cAMP-dependent protein kinase A, respectively. We found that these mutations affected the ACh current kinetics, but not the quantal ACh packet, and that the release frequency was greatly enhanced by repetitive neuronal activity in dnc, but not wild-type, growth cones. These results suggest that the cAMP pathway plays an important role in the activity-dependent regulation of transmitter release not only in mature synapses as previously shown, but also in developing nerve terminals before synaptogenesis.

Reconstitution of mediatophore-supported quantal acetylcholine release

Synaptic transmission of a nerve impulse is an extremely rapid event relying on transfer of brief chemical impulses from one cell to another. This transmission is dependent upon Ca2+ and known to be quantal, which led to the widely accepted vesicular hypothesis of neurotransmitter release. However, at least in the case of rapid synaptic transmission the hypothesis has been found difficult to reconcile with a number of observations. In this article, we shall review data from experiments dealing with reconstitution of quantal and Ca2+-dependent acetylcholine release in: i) proteoliposomes, ii) Xenopus oocytes, and iii) release-deficient cell lines. In these three experimental models, release is dependent on the expression of the mediatophore, a protein isolated from the plasma membrane of cholinergic nerve terminals of the Torpedo electric organ. We shall discuss the role of mediatophore in quantal acetylcholine release, its possible involvement in morphological changes affecting presynaptic membrane during the release, and its interactions with others proteins of the cholinergic nerve terminal.

Acetylcholine synthesis and quantal release reconstituted by transfection of mediatophore and choline acetyltranferase cDNAs

Neuroblastoma N18TG-2 cells cannot synthesize or release acetylcholine (ACh), and do not express proteins involved in transmitter storage and vesicle fusion. We restored some of these functions by transfecting N18TG-2 cells with cDNAs of either rat choline acetyltransferase (ChAT), or Torpedo mediatophore 16-kDa subunit, or both. Cells transfected only with ChAT synthesized but did not release ACh. Cells transfected only with mediatophore expressed Ca2+-dependent ACh release provided they were previously filled with the transmitter. Cell lines produced after cotransfection of ChAT and mediatophore cDNAs released the ACh that was endogenously synthesized. Synaptic-like vesicles were found neither in native N18TG-2 cells nor in ChAT-mediatophore cotransfected clones, where all the ACh content was apparently cytosolic. Furthermore, restoration of release did not result from enhanced ACh accumulation in intracellular organelles consecutive to enhanced acidification by V-ATPase, as Torpedo 16 kDa transfection did not increase, but decreased the V-ATPase-driven proton transport. Using ACh-sensitive Xenopus myocytes for real-time recording of evoked release, we found that cotransfected cells released ACh in a quantal manner. We compared the quanta produced by ChAT-mediatophore cotransfected clones to those produced by clones transfected with mediatophore alone (artificially filled with ACh). The time characteristics and quantal size of currents generated in the myocyte were the same in both conditions. However, cotransfected cells released a larger proportion of their initial ACh store. Hence, expression of mediatophore at the plasma membrane seems to be necessary for quantal ACh release; the process works more efficiently when ChAT is operating as well, suggesting a functional coupling between ACh synthesis and release.

Long-range signaling within growing neurites mediated by neurotrophin-3

In addition to well established trophic functions, neurotrophins acutely affect neurotransmitter secretion from the presynaptic nerve terminal, influence synaptic development, and may serve as selective retrograde messengers that regulate synaptic efficacy. The crucial question related to the mechanisms of neurotrophin-mediated signaling is whether acute effects of neurotrophins are spatially restricted to the activated synapses. Here we have used a local perfusion technique for local delivery of neurotrophin-3 (NT-3) to various regions of developing Xenopus embryo neurons in culture. Within minutes after a focal exposure of a soma or a small ( approximately 30 micrometer in length) axonal segment to NT-3, we observed an increase in the spontaneous neurotransmitter secretion from the presynaptic nerve terminals located approximately 300-400 micrometer away from the site of NT-3 application. Secretory activity along the axonal shaft was not affected. Our findings suggest that the NT-3-mediated signal may rapidly travel through neuronal cytoplasm over unexpectedly long distances and modulate neurotransmitter release specifically at the presynaptic nerve terminals.

Distribution of neurotransmitter secretion in growing axons

Neurotransmitter secretion from the nerve terminal is mediated by the fusion of synaptic vesicles with the plasma membrane. It is generally believed that neurotransmitter release in mature synapses is localized to the presynaptic nerve terminals. To probe the topology of neurotransmitter secretion along developing axons in culture, we recorded membrane currents from myocytes manipulated into contact with axons. At the early stages of growth, exocytotic events were detected along the axon as well as at the growth cone. At the later stages of growth, neurotransmitter secretion adopted the form of a smooth proximodistal gradient, with the highest level of activity at the growth cone region. Our results reveal the existence of a previously undetected early stage of axonal growth and suggest developmental regulation in the pattern of neurotransmitter secretion along the growing nerve processes.

Neurotransmitter secretion along growing nerve processes: comparison with synaptic vesicle exocytosis

In mature neurons, synaptic vesicles continuously recycle within the presynaptic nerve terminal. In developing axons which are free of contact with a postsynaptic target, constitutive membrane recycling is not localized to the nerve terminal; instead, plasma membrane components undergo cycles of exoendocytosis throughout the whole axonal surface (Matteoli et al., 1992; Kraszewski et al., 1995). Moreover, in growing Xenopus spinal cord neurons in culture, acetylcholine (ACh) is spontaneously secreted in the quantal fashion along the axonal shaft (Evers et al., 1989; Antonov et al., 1998). Here we demonstrate that in Xenopus neurons ACh secretion is mediated by vesicles which recycle locally within the axon. Similar to neurotransmitter release at the presynaptic nerve terminal, ACh secretion along the axon could be elicited by the action potential or by hypertonic solutions. We found that the parameters of neurotransmitter secretion at the nerve terminal and at the middle axon were strikingly similar. These results lead us to conclude that, as in the case of the presynaptic nerve terminal, synaptic vesicles involved in neurotransmitter release along the axon contain a complement of proteins for vesicle docking and Ca2+-dependent fusion. Taken together, our results support the idea that, in developing axons, the rudimentary machinery for quantal neurotransmitter secretion is distributed throughout the whole axonal surface. Maturation of this machinery in the process of synaptic development would improve the fidelity of synaptic transmission during high-frequency stimulation of the presynaptic cell.

Comparative biology of Ca2+-dependent exocytosis: implications of kinetic diversity for secretory function

The application of caged-Ca2+ compounds to the study of Ca2+-dependent exocytosis has begun to reveal kinetic intermediates in this important process. The time course of exocytosis varies greatly among different cell and vesicle types, even in response to Ca2+ 'jumps' of identical amplitude. The kinetics of the binding of Ca2+ to the putative Ca2+ sensor for exocytosis also vary. Theoretical analysis reveals that the kinetic diversity of exocytotic and Ca2+-binding reactions has distinct roles in determining the probability of exocytosis occurring. It is proposed that both of these reactions are optimized for the secretory function of specific cell types and that the exocytotic reaction includes vesicle translocation in addition to the fusion of vesicles with the plasma membrane.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. Part I: trans-Golgi network-derived organelles undergo regulated exocytosis

Exocytosis is a widely observed cellular mechanism for delivering transmembrane proteins to the cell surface and releasing signaling molecules into the extracellular space. Calcium-evoked exocytosis, traditionally thought to be restricted to presynaptic specializations in neurons, has been described recently in many cells. Here, calcium-evoked dendritic exocytosis (CEDE) is visualized in living cultured hippocampal neurons. Organelles that undergo CEDE are in somata, dendrites, and perisynaptic regions, identified by using immunocytochemistry and correlative light and electron microscopy. CEDE is regulated developmentally: neurons <9 d in vitro do not show CEDE. In addition, CEDE is blocked by tetanus toxin, an inhibitor of regulated exocytosis, and nocodazole, an inhibitor of microtubule polymerization. Organelles that undergo CEDE often are found on the base of spines, putative sites of synaptic plasticity. CEDE therefore could be involved in structural and functional modification of spines and could play a role in synaptic plasticity, where it might involve changes in receptor/channel density, release of active compounds having effect on pre- and postsynaptic function, and/or growth of synaptic structures.

Constitutive secretion of exogenous neurotransmitter by nonneuronal cells: implications for neuronal secretion

Fibroblasts in cell culture were loaded with exogenous neurotransmitter acetylcholine (ACh). ACh secretion from loaded cells was detected by whole-cell patch clamp recordings from Xenopus myocytes manipulated into contact with ACh-loaded cells. Two different approaches were used for ACh loading. In the first approach, fibroblasts were incubated in the culture medium containing ACh. Recordings from myocytes revealed fast inward currents that resemble miniature endplate currents found at neuromuscular synapses. The currents observed in recordings from myocytes were due to exocytosis of ACh-containing vesicles. Although exogenous ACh penetrated through the plasma membrane of fibroblasts during incubation and was present in the cytoplasm at detectable levels, cytoplasmic ACh did not contribute to the quantal ACh secretion. In the second approach, exogenous ACh was loaded into the cytoplasm of fibroblasts by microinjection. Under these experimental conditions, fibroblasts also exhibited spontaneous quantal ACh secretion. Analysis of the exocytotic events in fibroblasts following two different protocols of ACh loading revealed that the vesicular compartments responsible for uptake of exogenous ACh are associated with the endocytic recycling pathway. Extrapolation of our results to neuronal cells suggest that in cholinergic neurons, in addition to genuine synaptic vesicles, ACh can be secreted by the vesicles participating in endosomal membrane recycling.

Acetylcholine release. Reconstitution of the elementary quantal mechanism

Choline acetyltransferase and vesicular acetylcholine transporter genes are the products of two adjacent genes defining a cholinergic locus. The release mechanism is expressed independently of this locus in some cell lines. A cholinergic neuron will therefore have to coordinate the expression of release with that of the cholinergic locus. Transfection of a plasmid encoding Torpedo mediatophore in cells that are unable to release this transmitter endows them with a Ca2(+)-dependent and quantal release mechanism. The synchronization of mediatophore activation results from a control of calcium microdomains by the synaptic vesicles. It is therefore dependent on the proteins that dock vesicles close to calcium channels.

Overexpression of synaptotagmin modulates short-term synaptic plasticity at developing neuromuscular junctions

The level of synaptotagmin I or II in developing spinal neurons was increased by injection of synaptotagmin messenger RNA into early blastomeres of Xenopus embryos. The effect of overexpression of synaptotagmin on synaptic function was assayed in Xenopus nerve-muscle cultures within two days after injection. At neuromuscular synapses made by synaptotagmin-overexpressing neurons, the frequency of miniature postsynaptic currents was markedly reduced, while their mean amplitude was unchanged, as compared to those of control neurons in the same culture. The amplitude of evoked postsynaptic currents elicited by low-frequency test stimuli was not affected by overexpression. However, synapses made by synaptotagmin-overexpressing neurons exhibited significantly higher paired-pulse facilitation and reduced tetanus-induced depression of the synaptic response, and there was also an increased number of synaptic vesicles at regions 100-300 nm from the plasmalemma at such synapses. These results show that synaptotagmins can exert an inhibitory action on the spontaneous exocytosis of synaptic vesicles. The effects on short-term plasticity suggest that synaptotagmin may facilitate vesicular supply for the evoked release during higher frequency transmission.

Presynaptic facilitation of synaptic transmission in the hippocampus

Biochemical and genetic characterization of proteins in presynaptic axon terminals have led to models of the biochemical pathways underlying synaptic vesicle docking, activation, and fusion. Several studies have attempted recently to assign a precise physiological role to these proteins. This review deals with some of these studies, concentrating on those performed with hippocampal synapses. It is shown that changes in the state of these presynaptic proteins, together with modifications in Ca2+ dynamics in axon terminals, functionally determine the level of basal synaptic transmission, and underlie pharmacologically induced and activity-dependent facilitation of transmitter release in the central nervous system.

GLUT4 and transferrin receptor are differentially sorted along the endocytic pathway in CHO cells

The trafficking of GLUT4, a facilitative glucose transporter, is examined in transfected CHO cells. In previous work, we expressed GLUT4 in neuroendocrine cells and fibroblasts and found that it was targeted to a population of small vesicles slightly larger than synaptic vesicles (Herman, G.A, F. Bonzelius, A.M. Cieutat, and R.B. Kelly. 1994. Proc. Natl. Acad. Sci. USA. 91: 12750-12754.). In this study, we demonstrate that at 37 degrees C, GLUT4-containing small vesicles (GSVs) are detected after cell surface radiolabeling of GLUT4 whereas uptake of radioiodinated human transferrin does not show appreciable accumulation within these small vesicles. Immunofluorescence microscopy experiments show that at 37 degrees C, cell surface-labeled GLUT4 as well as transferrin is internalized into peripheral and perinuclear structures. At 15 degrees C, endocytosis of GLUT4 continues to occur at a slowed rate, but whereas fluorescently labeled GLUT4 is seen to accumulate within large peripheral endosomes, no perinuclear structures are labeled, and no radiolabeled GSVs are detectable. Shifting cells to 37 degrees C after accumulating labeled GLUT4 at 15 degrees C results in the reappearance of GLUT4 in perinuclear structures and GSV reformation. Cytosol acidification or treatment with hypertonic media containing sucrose prevents the exit of GLUT4 from peripheral endosomes as well as GSV formation, suggesting that coat proteins may be involved in the endocytic trafficking of GLUT4. In contrast, at 15 degrees C, transferrin continues to traffic to perinuclear structures and overall labels structures similar in distribution to those observed at 37 degrees C. Furthermore, treatment with hypertonic media has no apparent effect on transferrin trafficking from peripheral endosomes. Double-labeling experiments after the internalization of both transferrin and surface-labeled GLUT4 show that GLUT4 accumulates within peripheral compartments that exclude the transferrin receptor (TfR) at both 15 degrees and 37 degrees C. Thus, GLUT4 is sorted differently from the transferrin receptor as evidenced by the targeting of each protein to distinct early endosomal compartments and by the formation of GSVs. These results suggest that the sorting of GLUT4 from TfR may occur primarily at the level of the plasma membrane into distinct endosomes and that the organization of the endocytic system in CHO cells more closely resembles that of neuroendocrine cells than previously appreciated.

Acetylcholine release and the cholinergic genomic locus

Choline acetyltransferase and vesicular acetylcholine-transporter genes are adjacent and coregulated. They define a cholinergic locus that can be turned on under the control of several factors, including the neurotrophins and the cytokines. Hirschprung's disease, or congenital megacolon, is characterized by agenesis of intramural cholinergic ganglia in the colorectal region. It results from mutations of the RET (GDNF-activated) and the endothelin-receptor genes, causing a disregulation in the cholinergic locus. Using cultured cells, it was shown that the cholinergic locus and the proteins involved in acetylcholine (ACh) release can be expressed separately ACh release could be demonstrated by means of biochemical and electrophysiological assays even in noncholinergic cells following preloading with the transmitter. Some noncholinergic or even nonneuronal cell types were found to be capable of releasing ACh quanta. In contrast, other cells were incompetent for ACh release. Among them, neuroblastoma N18TG-2 cells were rendered release-competent by transfection with the mediatophore gene. Mediatophore is an ACh-translocating protein that has been purified from plasma membranes of Torpedo nerve terminal; it confers a specificity for ACh to the release process. The mediatophores are activated by Ca2+; but with a slower time course, they can be desensitized by Ca2+. A strictly regulated calcium microdomain controls the synchronized release of ACh quanta at the active zone. In addition to ACh and ATP, synaptic vesicles have an ATP-dependent Ca2+ uptake system; they transiently accumulate Ca2+ after a brief period of stimulation. Those vesicles that are docked close to Ca2+ channels are therefore in the best position to control the profile and dynamics of the Ca2+ microdomains. Thus, vesicles and their whole set of associated proteins (SNAREs and others) are essential for the regulation of the release mechanism in which the mediatophore seems to play a key role.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Transferrin receptor functions as a signal-transduction molecule for its own recycling via increases in the internal Ca2+ concentration

Transferrin binding to its receptor modulates transferrin receptor (Tf-R) recycling rates in several cells [Klausner, R. D., Van Renswoude, J., Ashwell, G., Kempf, C., Schechter, A., Dean, A. & Bridges, K. R. (1983a) J. Biol. Chem. 258, 4715-4724; Gironès, N. & Davis, R. J. (1989) Biochem. J. 264, 35-46; Sainte-Marie, J., Vidal, M., Bette-Bobillo, P., Philippot, J. R. & Bienvenüe, A. (1991) Eur. J. Biochem. 201, 295-302]. To delineate the mechanism of this regulation, we hypothesized that the binding of the ligand to its receptor could lead to activation of several second-messenger pathways, which may redundantly stimulate recycling of the receptor. The effects of different regulators of Ca2+ flux or concentrations were investigated on the Tf-R-recycling pathway; these studies were carried out in two cell types. Perhexiline, a calcium antagonist, slowed receptor recycling in comparison with the control by more than 80% in L2C cells and by 60% in Jurkat cells (B and T lymphoblasts, respectively) but did not affect their internalization rate. Perhexiline thus trapped considerable amounts of Tf-R in the internal compartment. Ca2+ chelators, such as EGTA or 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid, and a Ca2+-channel inhibitor (Ni2+) decreased drastically the recycling rate of Tf-R. Tf-R recycling was shown to be slowed by a calmodulin antagonist. Conversely, artificial elevation of free internal Ca2+ in L2C cells, using lectin, accelerated the recycling rate. These results suggest that the intracellular Ca2+ concentration plays an important role in the outward flow of transferrin receptors. Consequently, we examined the role of transferrin in internal free Ca2+ regulation. The addition of transferrin or anti-(Tf-R) Ig specifically elicited a rise in [Ca2+], as demonstrated by inefficacy of apotransferrin or irrelevant antibodies. These results suggest that Ca2+ is a regulator of Tf-R recycling and that Tf-R seems to function as a signal-transduction molecule (perhaps in conjunction with other membrane proteins) rather than merely as an endocytic receptor.

Multiple exocytotic pathways in pancreatic beta cells

Ca2+-dependent exocytotic pathways in mouse pancreatic beta cells were investigated using both capacitance measurement and amperometric detection of vesicular contents. Serotonin was preloaded into large dense-core vesicles for the amperometry. Exocytosis was induced by rapid elevation of cytosolic Ca2+ concentrations using caged-Ca2+ compounds. Capacitance measurement revealed two major components of exocytosis, and only the slow component was accompanied by amperometric events reflecting quantal serotonin secretion. Moreover, the fast and slow exocytoses induced the two forms of endocytosis that were reported to follow the exocytoses of small-clear and large dense-core vesicles, respectively. Interestingly, we recorded two types of responses of quantal events: in the type-1 response, most quantal events occurred with a delay of 0.2 s and were rapidly exhausted with a time constant of 1.7 s, while, in the type-2 response, quantal events occurred with a delay of 2.5 s and were sustained. This suggests the existence of two pathways or modes of the exocytosis involving large dense-core vesicles. Thus, we have revealed three exocytotic pathways with divergent fusion kinetics in beta cells, which provide a new basis for the understanding of the physiology and pathology of beta cells.

The function of inositol high polyphosphate binding proteins

The inositol phosphate metabolism network has been found to be much more complex than previously thought, as more and more inositol phosphates and their metabolizing enzymes have been discovered. Some of the inositol phosphates have been shown to have biological activities, but little is known about their signal transduction mechanisms except for that of inositol 1,4,5-trisphosphate. The recent discovery, however, of a number of binding proteins for inositol high polyphosphate [inositol 1,3,4,5-tetrakisphosphate (IP4), inositol 1,3,4,5,6-pentakisphosphate, or inositol hexakisphosphate] enables us to speculate on the physiological function of these compounds. In this article we focus on two major issues: (1) the roles of inositol high polyphosphates in vesicular trafficking, especially exocytosis, and (2) pleckstrin homology domain-containing IP4 binding proteins involved in the Ras signaling pathway.

Lysosomes behave as Ca2+-regulated exocytic vesicles in fibroblasts and epithelial cells

Lysosomes are considered to be a terminal degradative compartment of the endocytic pathway, into which transport is mostly unidirectional. However, specialized secretory vesicles regulated by Ca2+, such as neutrophil azurophil granules, mast cell-specific granules, and cytotoxic lymphocyte lytic granules, share characteristics with lysosomes that may reflect a common biogenesis. In addition, the involvement of Ca2+ transients in the invasion mechanism of the parasite Trypanosoma cruzi, which occurs by fusion of lysosomes with the plasma membrane, suggested that lysosome exocytosis might be a generalized process present in most cell types. Here we demonstrate that elevation in the intracellular free Ca2+ concentration of normal rat kidney (NRK) fibroblasts induces fusion of lysosomes with the plasma membrane. This was verified by measuring the release of the lysosomal enzyme beta-hexosaminidase, the appearance on the plasma membrane of the lysosomal glycoprotein lgp120, the release of fluid-phase tracers previously loaded into lysosomes, and the release of the lysosomally processed form of cathepsin D. Exposure to the Ca2+ ionophore ionomycin or addition of Ca2+-containing buffers to streptolysin O-permeabilized cells induced exocytosis of approximately 10% of the total lysosomes of NRK cells. The process was also detected in other cell types such as epithelial cells and myoblasts. Lysosomal exocytosis was found to require micromolar levels of Ca2+ and to be temperature and ATP dependent, similar to Ca2+-regulated secretory mechanisms in specialized cells. These findings highlight a novel role for lysosomes in cellular membrane traffic and suggest that fusion of lysosomes with the plasma membrane may be an ubiquitous form of Ca2+-regulated exocytosis.