Synaptic vesicle phosphoproteins and regulation of synaptic function

Two proteins associated with secretory granule membranes identified in chicken regulated secretory cells

Lately, we have identified two polypeptides of 92–94 kDa (GRL1) and 45–60 kDa (GRL2), expressed in cytoplasmic granules of chicken granulocytes and thrombocytes. Here, we report that GRL1 and GRL2 are widely distributed in all exocrine and several endocrine cell types, but not in neurons of the central nervous system, during late stages of embryonic development, as well as in newly hatched and two-month-old chickens. Immunogold studies in ultrathin frozen sections of pancreatic acinar cells show that GRL1 and GRL2 are co-localized at the periphery of zymogen granules, in granules fused with apical acinar membranes and on apical membranes of acini, while the pregranular compartments of the secretory pathway are weakly or not labeled. Semiquantitative morphometric studies indicate that GRL1 and GRL2 are equally distributed in secretory granules. A variety of physical and metabolic studies reveal that GRL2, a highly N-glycosylated polypeptide, is an intrinsic membrane protein, while GRL1 is a peripheral membrane polypeptide released by Na2CO3 treatment of granulocyte membranes. In all hematopoietic, exocrine or endocrine cells examinated, GRL1 shows identical electrophoretic patterns, while GRL2 is identified as a diffuse band, at 40–65 kDa, in hematopoietic and pancreatic cells. Taken together, the morphological and biochemical studies indicate that GRL1 and GRL2 are components of the secretory granule membrane in chicken exocrine, endocrine and hemopoietic cell types.

Increase in synaptic vesicle proteins accompanies long-term potentiation in the dentate gyrus

Maintenance of long-term potentiation in synapses formed by the perforant path on to granule cells of the dentate gyrus is accompanied by a sustained increase in the extracellular concentration of glutamate, the presumed transmitter at this excitatory hippocampal pathway. Quantal analysis indicates that, at least in the first hour of induction, this reflects an increase in transmitter release rather than a decrease in glutamate uptake, while biochemical studies have suggested that the increase in release persists for several hours. Morphological studies have described early but persistent increases in the spine number and area. Increases in the number of segmented/perforated synapses persisting for at least 1 h after induction of long-term potentiation, have also been reported. These morphological changes suggest both presynaptic and postsynaptic modifications. Increases in synaptic vesicle number and distribution lasting for at least 1 h specifically indicate presynaptic changes. To explore further the role of the presynaptic terminal in long-term potentiation, we have investigated changes in three synaptic vesicle proteins, synapsin, synaptotagmin and synaptophysin, in control tissue and in tissue prepared from potentiated dentate gyrus 45 min and 3 h after induction of long-term potentiation. We found that there was an increase in the concentration of the three proteins 3 h after induction of long-term potentiation. No such increase was observed 45 min after induction or in tissue prepared from animals in which an intraventricular injection of the N-methyl-D-aspartate receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid, blocked induction of long-term potentiation.

Anchoring of protein kinase A is required for modulation of AMPA/kainate receptors on hippocampal neurons

Phosphorylation of molecules involved in synaptic transmission by multifunctional protein kinases modulates both pre- and post-synaptic events in the central nervous system. The positioning of kinases near their substrates may be an important part of the regulatory mechanism. The A-kinase-anchoring proteins (AKAPs; ref. 3) are known to bind the regulatory subunit of cyclic AMP-dependent protein kinase A with nanomolar affinity. Here we show that anchoring of protein kinase A by AKAPs is required for the modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate channels. Intracellular perfusion of cultured hippocampal neurons with peptides derived from the conserved kinase binding region of AKAPs prevented the protein kinase A-mediated regulation of AMPA/kainate currents as well as fast excitatory synaptic currents. This effect could be overcome by adding the purified catalytic subunit of protein kinase. A control peptide lacking kinase-binding activity had no effect. To our knowledge, these results provide the first evidence that anchoring of protein kinase A is crucial in the regulation of synaptic function.

Synapsin IIa accelerates functional development of neuromuscular synapses

We have investigated the possible involvement of the synaptic vesicle protein synapsin IIa in synapse development. Synapsin IIa was introduced into Xenopus embryonic spinal neurons by early blastomere injection, and nerve-muscle cultures were prepared. Synaptic currents were measured by comparing synapses in which the presynaptic neuron either contained [syn IIa (+)] or lacked (control) exogenous synapsin IIa. Syn IIa (+) synapses had a 3.6-fold increase in the frequency and a 2.1-fold increase in the amplitude of spontaneous synaptic currents, compared to controls, after 2 days in culture. Synapsin IIa also increased the amplitude of evoked synaptic currents by 2.3-fold in 2-day cultures. The evoked synaptic current amplitudes of syn IIa (+) synapses had a lower coefficient of variation indicating a more stable evoked response. These enhanced synaptic activities were independent of the presence or absence of the protein in the postsynaptic muscle cell. The findings indicate a role for synapsin IIa in synapse maturation.

Matrix-assisted laser desorption mass spectrometric peptide mapping of proteins separated by two-dimensional gel electrophoresis: determination of phosphorylation in synapsin I

A technique is described for the rapid, sensitive analysis of posttranslational modifications of proteins that have been separated by 2-dimensional electrophoresis and blotted onto a membrane with a cationic surface. The isolated protein spots visualized by reverse staining of the blotting membrane are excised, washed, and subjected to chemical (cyanogen bromide) and/or enzymatic (endoproteinase Lys-C) degradation directly on the membrane. The resulting mixture of peptide fragments is extracted from the membrane into a solution that is compatible with matrix-assisted laser desorption mass spectrometric analysis and analyzed without fractionation. Relatively accurate (+/- 1 Da) mass determination of these peptide fragments provides a facile and sensitive means for detecting the presence of modifications and for correlating such modifications with the differential mobility of different isoforms of a given protein during 2-dimensional electrophoresis. The technique is applied to the determination of sites of phosphorylation in synapsins Ia and Ib, neuronal phosphoproteins that are believed to function in the regulation of neurotransmitter release and are substrates for cAMP and Ca2+/calmodulin-dependent protein kinases, which appear to control their biological activity.

Tyrosine phosphorylation and activation of mitogen-activated protein kinase in the rat brain following transient cerebral ischemia

Activation of trophic factor receptors stimulates tyrosine phosphorylation on proteins and supports neuronal survival. We report that in the recovery phase following reversible cerebral ischemia, tyrosine phosphorylation increases in the membrane fraction of the resistant hippocampal CA3/dentate gyrus (DG) region, whereas in the sensitive CA1 region or striatum, tyrosine phosphorylation is less marked or decreases. In the cytosolic fractions, a 42-kDa protein, identified as mitogen-activated protein (MAP) kinase, is markedly phosphorylated and activated immediately following ischemia, in particular in CA3/DG, but not in striatum. In the CA1 region, phosphorylation of MAP kinase is less intense and decreases later during reperfusion, which could explain the delay of neuronal degeneration in this structure. The data suggest that in ischemia-resistant neurons the growth factor receptor-coupled signaling cascade is stimulated and, through its effects on DNA transcription and mRNA translation, supports neuronal survival.

Okadaic acid disrupts clusters of synaptic vesicles in frog motor nerve terminals

The fluorophore FM1-43 appears to stain membranes of recycled synaptic vesicles. We used FM1-43 to study mechanisms of synaptic vesicle clustering and mobilization in living frog motor nerve terminals. FM1-43 staining of these terminals produces a linear series of fluorescent spots, each spot marking the cluster of several hundred synaptic vesicles at an active zone. Most agents we tested did not affect staining, but the phosphatase inhibitor okadaic acid (OA) disrupted the fluorescent spots, causing dye to spread throughout the terminal. Consistent with this, electron microscopy showed that vesicle clusters were disrupted by OA treatment. However, dye did not spread passively to a uniform spatial distribution. Instead, time lapse movies showed clear evidence of active dye movements, as if synaptic vesicles were being swept along by an active translocation mechanism. Large dye accumulations sometimes occurred at sites of Schwann cell nuclei. These effects of OA were not significantly affected by pretreatment with colchicine or cytochalasin D. Electrophysiological recordings showed that OA treatment reduced the amount of acetylcholine released in response to nerve stimulation. The results suggest that an increased level of protein phosphorylation induced by OA treatment mobilizes synaptic vesicles and unmasks a powerful vesicle translocation mechanism, which may function normally to distribute synaptic vesicles between active zones.

Paralysis and early death in cysteine string protein mutants of Drosophila

Multimeric complexes of synaptic vesicle and terminal membrane proteins are important components of the neurotransmitter release mechanism. The csp gene of Drosophila encodes proteins homologous to synaptic vesicle proteins in Torpedo. Monoclonal antibodies demonstrate different distributions of isoforms at distinct subsets of terminals. Deletion of the csp gene in Drosophila causes a temperature-sensitive block of synaptic transmission, followed by paralysis and premature death.

Targeting learning

Novel transgenic approaches provide an exciting opportunity to assess the impact of the loss of specific genes in the biochemistry and electrophysiology of neurons involved in a learned behavior. Recent studies describing mice harboring mutations in five kinase genes expressed in the hippocampus found that two of these kinases, the alpha-Ca(2+)-calmodulin-dependent kinase II and the Fyn tyrosine kinase are necessary for the establishment of long-term potentiation. In addition to providing a new tool for the dissection of the molecular mechanisms of synaptic plasticity, these mutants will be important in determining how changes in synaptic strength affect not only learning and memory, but also a host of other processes thought to be associated with plasticity.

Cell membrane resealing by a vesicular mechanism similar to neurotransmitter release

After injury to the cell membrane, rapid resealing of the membrane occurs with little loss of intracellular contents. This process has been studied by measurement of the rate of dye loss after membrane puncture in both the sea urchin embryo and 3T3 fibroblasts. Resealing of disrupted cell membranes requires external calcium that can be antagonized by magnesium. Block of multifunctional calcium/calmodulin kinase, which regulates exocytotic vesicle availability at synapses, and of kinesin, which is required for outward-directed transport of vesicles, inhibited membrane resealing. Resealing was also inhibited by botulinum neurotoxins B and A, suggesting that the two synaptosomal-associated proteins synaptobrevin and SNAP-25 also participate in resealing. This pattern of inhibition indicates that the calcium-dependent mechanisms for cell membrane resealing may involve vesicle delivery, docking, and fusion, similar to the exocytosis of neurotransmitters.

Spatiotemporal pattern of quantal release of ATP and noradrenaline from sympathetic nerves: consequences for neuromuscular transmission

The recent explosive development in research concerning the fundamental mechanisms of synaptic transmission helps put the present paper in context. It is now evident that not all transmitter vesicles in a nerve terminal, not even all those docked at its active zones, are immediately available for release (36). We watch, fascinated, the unraveling of the amazingly complex cellular mechanisms and molecular machinery that determine whether or not a vesicle is "exocytosis-competent" (77,78,39,79). Studies on quantal release in different systems show that neurons are fundamentally similar in one respect: that transmitter release from individual active zones is monoquantal (2). But they also show that active zones in different neurons differ drastically in the probability of monoquantal release and in the number of quanta immediately available for release (3). This implies that one should not extrapolate directly from transmitter release in one set of presynaptic terminals (e.g., in neuromuscular endplate or squid giant synapse) to that in other nerve terminals, especially if they have a very different morphology. As shown here, one should not even extrapolate from transmitter release in sympathetic nerves in one tissue (e.g., rat tail artery) to that in other tissues or species (e.g., mouse vas deferens). It is noteworthy that most studies of quantal release are based on electrophysiological analysis and therefore deal with release of fast, ionotropic transmitters from small synaptic vesicles at the active zones, especially in neurons in which these events may be examined with high resolution (49,48,46,33,32). Such data are useful as general models of the release of both fast and slow transmitters from small synaptic vesicles at active zones in other systems, provided that these transmitters are released in parallel, as are apparently ATP and NA in sympathetic nerves. They tell us little or nothing, however, about the release of transmitters (e.g., neuropeptides) from the large vesicles, nor about the spatiotemporal pattern of monoquantal release from small synaptic vesicles in the many neurons that have boutons-en-passent terminals. They show that the time course of effector responses to fast, rapidly inactivated transmitters such as ACh or ATP is necessarily release related. But they do not even address the possibility that the effector responses to slow transmitters such as NA, co-released from the same terminals, may obey completely different rules and perhaps rather be clearance related (7).(ABSTRACT TRUNCATED AT 400 WORDS)

Persistent activation of protein kinase C during the development of long-term facilitation in Aplysia

We investigated activation of the two major neuronal protein kinase C (PKC) isoforms in Aplysia, Ca(2+)-activated Apl I and Ca(2+)-independent Apl II, during the induction and maintenance of behavioral sensitization of Aplysia defensive reflexes. Activation of PKC occurred during the training stimulus and persisted for at least 2 hr thereafter but was not maintained for 24 hr. The persistent activation required protein synthesis and was blocked by cyproheptidine, an agent that also blocked the initial activation of PKC. Persistent activation involved both an increase in membrane-associated Apl I and an increase in an autonomous kinase activity that may be related to a post-translational modification of Apl II. These results are consistent with the hypothesis that in addition to its role in producing the presynaptic facilitation of mechanosensory-motor neuron synapses that underlie short-term facilitation, PKC is needed for maintaining synaptic changes in an intermediate period that precedes the modifications accompanying consolidation of memory.

Molecular pathways for presynaptic calcium signaling

The results presented in this article describe two distinct, Ca-regulated molecular pathways in presynaptic terminals and implicate these two pathways in differentially mediating neurotransmitter secretion and PTP. Our current view of the Ca-dependent triggering of secretion and PTP is shown in Fig. 9. According to this scheme, differential activation of these two pathways is achieved by a combination of diffusion-based dilution of Ca that enters the terminal through voltage-gated Ca channels and by coupling these pathways to Ca receptors with different affinities for Ca ions. A simple way to achieve these conditions is to position these two receptors at different distances from the Ca channels, as shown in Fig. 2. Given that Ca ions are involved in activating many different presynaptic processes (Fig. 1), we propose that closer scrutiny of the molecular physiology of nerve terminals will reveal a wide variety of Ca-activated pathways responsible for producing these diverse processes.

"Strong" and "weak" synaptic differentiation in the crayfish opener muscle: structural correlates

The single excitor motoneuron to the limb opener muscle in the crayfish Procambarus clarkii provides multiterminal innervation to individual muscle fibers. At low impulse frequencies, these neuromuscular synapses generate a threefold larger junctional potential in fibers of the proximal region of the muscle compared to those in the central region. Focal extracellular recording from synapse-bearing "boutons" showed more quantal release at low frequencies in the proximal region. Structural correlates for the physiological differences were sought. Fluorescence microscopy of surface innervation stained with a vital fluorescent dye, 4-Di-2-Asp, showed that density of innervation was not greater in the proximal region and thus could not account for the overall differences in synaptic strength. Freeze fracture studies showed that the intramembrane organization of excitatory synapses and their active zones was qualitatively similar in proximal and central sites. Serial section electron microscopy of several innervation sites in proximal and central regions showed homogeneity in number and size of synapses. However, presynaptic dense bars (at release sites, or active zones) were longer and occurred at a higher density in proximal than in central synapses. The differences in number and length of presynaptic dense bars correlate positively with the differences in synaptic strength represented by junctional potential amplitudes and quantal contents of individual surface recording sites. Since many individual proximal synapses have multiple dense bars, co-operativity among these may serve to enhance transmitter output. It is concluded that occurrence of dense bars is a significant presynaptic correlate of synaptic strength in this neuron.

Intracellular membrane fusion

The NSF, SNAP, and SNAP receptors are key elements of the intracellular membrane fusion machinery. We use an affinity purification scheme, based on the function of SNAP receptor in assembling 20S fusion particles from NSF and SNAP proteins, to purify SNAP receptors from brain. Remarkably, each of the four SNAP receptors (or, SNAREs) thus delineated resides in synapses, with one receptor originating in the synaptic vesicle and another in the presynaptic plasma membrane that is targeted for fusion. This suggests a simple mechanism in which the general NSF/SNAP fusion machinery can assemble to bridge partner membranes in a complex containing elements of both vesicle and target membranes, and implies that similar fusion machines drive both constitutive fusion (ER-->Golgi-->surface and endocytosis) and regulated exocytosis. The vesicle (v-SNARE) and the target-associated t-SNAREs from the synapse are each members of compartmentally-specific families of membrane proteins found in yeast, animal cells, and neurons, thus raising the possibility that v-SNAREs and t-SNAREs encode specificity in membrane fusion processes that utilize a common mechanism.

Interactions of synapsin I with phospholipids: possible role in synaptic vesicle clustering and in the maintenance of bilayer structures

Synapsin I is a synaptic vesicle-specific phosphoprotein composed of a globular and hydrophobic head and of a proline-rich, elongated and basic tail. Synapsin I binds with high affinity to phospholipid and protein components of synaptic vesicles. The head region of the protein has a very high surface activity, strongly interacts with acidic phospholipids and penetrates the hydrophobic core of the vesicle membrane. In the present paper, we have investigated the possible functional effects of the interaction between synapsin I and vesicle phospholipids. Synapsin I enhances both the rate and the extent of Ca(2+)-dependent membrane fusion, although it has no detectable fusogenic activity per se. This effect, which appears to be independent of synapsin I phosphorylation and localized to the head region of the protein, is attributable to aggregation of adjacent vesicles. The facilitation of Ca(2+)-induced liposome fusion is maximal at 50-80% of vesicle saturation and then decreases steeply, whereas vesicle aggregation does not show this biphasic behavior. Association of synapsin I with phospholipid bilayers does not induce membrane destabilization. Rather, 31P-nuclear magnetic resonance spectroscopy demonstrated that synapsin I inhibits the transition of membrane phospholipids from the bilayer (L alpha) to the inverted hexagonal (HII) phase induced either by increases in temperature or by Ca2+. These properties might contribute to the remarkable selectivity of the fusion of synaptic vesicles with the presynaptic plasma membrane during exocytosis.

A Ca-dependent early step in the release of catecholamines from adrenal chromaffin cells

Intense stimuli, such as trains of depolarizing pulses or the caffeine-induced release of calcium from intracellular stores, readily depress the secretory response in neuroendocrine cells. Secretory responses are restored by rest periods of minutes in duration. This recovery was accelerated when the concentration of cytosolic calcium was moderately increased and probably resulted from calcium-dependent replenishment of a pool of release-ready granules. Continuously increased concentrations of calcium led the over-filling of such a pool. Subsequently, secretory responses to stronger calcium stimuli were augmented. Hormone-induced calcium transients with a plateau phase of increased concentration of calcium may enhance the secretory response in this way.

Cytoskeleton dynamics during neurotransmitter release

It has become apparent in recent years that the cytoskeleton and its associated proteins play a major role in secretion. This review summarizes recent findings on the cytoskeleton organization and the molecular topology of its regulatory proteins, as well as the dynamic changes that occur in this organelle during secretion from neurons and secretory cells. Although two apparently different ultrastructures and molecular organizations of the cytoskeleton seem to be involved in neuronal and secretory cell secretion, there are similarities between the two systems. In both neurons and secretory cells, Ca2+ plays a pivotal role in the control of cytoskeleton dynamics, especially in the changes in actin filament networks observed during secretion.

Neurotransmitter release regulated by nitric oxide in PC-12 cells and brain synaptosomes

BACKGROUND

Nitric oxide is a messenger molecule of the nervous system, which is produced by the enzyme nitric oxide synthase, which may regulate cyclic guanosine monophosphate levels and which has been implicated in the control of neurotransmitter release. PC-12 pheochromocytoma cells differentiate to form neuronal cells in culture when they are exposed to nerve growth factor. The levels of cyclic guanosine monophosphate in the cells and their ability to release acetylcholine in response to K(+)-depolarization are both maximal after eight days of treatment with nerve growth factor. We set out to assess a possible role for nitric oxide in the processes that occur in differentiating PC-12 cells.

RESULTS

Nitric oxide synthase is first evident in differentiating PC-12 cells eight days after beginning treatment with nerve growth factor, coinciding with the marked increase in K(+)-depolarization-induced release of acetylcholine. The release of both acetylcholine and dopamine in response to K(+)-depolarization is blocked by inhibitors of nitric oxide synthase and by hemoglobin, which binds nitric oxide. Providing l-arginine, a precursor required for nitric oxide synthesis, reverses the effects of the inhibitors. In synaptosomal preparations from the corpus striatum, inhibitors of nitric oxide synthase prevent the release of glutamate in response to the glutamate derivative N-methyl-d-aspartate but not in response to K(+)-depolarization.

CONCLUSION

Nitric oxide may mediate the release of acetylcholine and dopamine in response to K(+)-depolarization in PC-12 cells and the release of glutamate in response to N-methyl-d-aspartate in striatal synaptosomes. Nitric oxide synthase expression is induced after eight days of treating PC-12 cells with nerve growth factor, coinciding with a marked enhancement of the release of neurotransmitters in response to K(+)-depolarization.

Brain synapses contain inducible forms of the transcription factor NF-kappa B

We investigated the rat brain for the presence and activation state of the inducible transcription factor NF-kappa B. Two forms of NF-kappa B containing the transactivating p65 subunit were found in all brain regions investigated. The majority of NF-kappa B was in an inducible cytoplasmic form by virtue of its association with the inhibitory subunit I kappa B. Significant amounts of inducible NF-kappa B forms were present in synaptosomes, as suggested by electrophoretic mobility shift assay and Western blot analysis of subcellular brain fractions. A synaptic localization of NF-kappa B was further evident from immunostaining of inner and outer plexiform layers of the retina with an antibody directed against the p50 subunit of NF-kappa B. In cerebral cortex and striatum, NF-kappa B-specific antibodies showed a punctate immunostaining partially overlapping with that for the synaptic marker protein synaptophysin. NF-kappa B is thus the first transcription factor found in synapses of neurons. With its unusual subneuronal localization, the inducible transcription factor has the potential to function as retrograde messenger mediating stimulus-response coupling and long-term changes in gene expression following presynaptic stimulation.

Cone synapses of a flat diffuse cone bipolar cell in the primate retina

A Golgi-stained flat diffuse cone bipolar cell from a vervet monkey's retina (Cercopithecus aethiops), contacting six cones, was serially sectioned for electron microscopy (EM) to determine the types of synapses it made with the cone pedicles. All the synapses were basal (flat) contacts. Their distribution and ultrastructural type were similar at each pedicle. Approximately half the synapses were definable as triad-associated and the rest were elsewhere on the cone pedicle base. Their ultrastructure is the same regardless of those positions. About 25 synapses were made with each cone. Thus this type (DB2 of Boycott & Wässle, 1991) of flat diffuse cone bipolar cell is in contact with six cones through about 150 synapses. At the eccentricity studied each cone pedicle probably makes 90-100 basal synapses with between three and four DB2 bipolar cells. This is between two and three times the number that are made with all the types of invaginating bipolar cells. A brief review of cone photoreceptor synapses with bipolar cells shows that, for those so far examined in the primate retina, the dichotomy into two types of bipolar cell invaginating (ribbon-related), with axons ending in the b-layer of the inner plexiform layer (IPL) (hence presumptive On-bipolars) and flat (basal synapses), with axons ending in the a-layer of the inner plexiform layer (hence presumptive Off-bipolars) is the rule. But other vertebrate retinae, including that of the cat, also have bipolar cells which vary from this pattern.

The transglutaminase hypothesis for the action of tetanus toxin

Tetanus toxin potently and almost irreversibly inhibits the release of neurotransmitters from nerve terminals. The toxin binds to and activates transglutaminase, a Ca(2+)-dependent enzyme that can form stable crosslinks between substrate proteins. Transglutaminase is present in nerve terminals and recognizes synapsin I, an abundant synaptic vesicle phosphoprotein involved in neurotransmission, as an excellent substrate. The neuroparalytic action of tetanus toxin might be due, at least in part, to the stimulation of synaptic transglutaminase and the consequent crosslinking of synapsin I.

Rapid binding of synapsin I to F- and G-actin. A study using fluorescence resonance energy transfer

Synapsin I is a nerve terminal phosphoprotein which interacts with synaptic vesicles and actin in a phosphorylation-dependent manner. By using fluorescence resonance energy transfer between purified components labeled with fluorescent probes, we now show that the binding of synapsin I to actin is a rapid phenomenon. Binding of synapsin I to actin can also be demonstrated when synaptic vesicles are present in the medium and appears to be modulated by ionic strength and synapsin I phosphorylation.

alpha-Latrotoxin receptor. Implications in nerve terminal function

alpha-Latrotoxin is a potent stimulator of neurotransmitter release from nerve terminals. High affinity membrane alpha-latrotoxin receptor was purified in an active binding form. It is a membrane glycoprotein (M(r) 160,000-220,000) which may be complexed to a smaller polypeptide (M(r) 29,000). The structure of the receptor protein suggests that it may be a synapse-specific cell recognition molecule. Intracellularly, the alpha-latrotoxin receptor interacts with synaptotagmin, a calcium- and phospholipid-binding protein specifically localized in the synaptic vesicle membrane. This interaction may be important for targeting of synaptic vesicles to presynaptic release sites.

Reconstitution of transmitter secretion

Molecular mechanisms involved in the various stages of transmitter secretion have been studied by perturbing the composition of secretory cells using pharmacological and biochemical agents. An emerging approach is to reconstitute individual steps or the entire sequence of secretion mechanisms in non-secretory cells by loading the cell with presynaptic components or their mRNAs.

Calcium-independent potentiation of insulin release by cyclic AMP in single beta-cells

How does cyclic AMP potentiate insulin secretion from pancreatic islet beta-cells? This question is fundamental to understanding how hormones such as glucagon, which elevates cAMP, stimulate insulin secretion and so contribute to the normal secretory response of the islet. It is well established that a rise in the cytoplasmic Ca2+ concentration ([Ca2+]i) is essential for insulin secretion and therefore cAMP has been proposed to act by elevating [Ca2+]i. But studies on permeabilized beta-cells indicate that cAMP increases insulin release even when [Ca2+]i is held constant. We have used microfluorimetry and the patch-clamp technique to measure changes simultaneously in Ca2+ currents, [Ca2+]i and exocytosis in a single beta-cell in response to cAMP. We show here that cAMP, through activation of protein kinase A, increases Ca(2+)-influx through voltage-dependent L-type Ca2+ channels, thereby elevating [Ca2+]i and accelerating exocytosis. More importantly, cAMP also promotes insulin release by a direct interaction with the secretory machinery, which accounts for as much as 80% of its effect.

On the crawling of animal cells

Cells crawl in response to external stimuli by extending and remodeling peripheral elastic lamellae in the direction of locomotion. The remodeling requires vectorial assembly of actin subunits into linear polymers at the lamella's leading edge and the crosslinking of the filaments by bifunctional gelation proteins. The disassembly of the crosslinked filaments into short fragments or monomeric subunits away from the leading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic signals elicited by sensory cues escort actin through this cycle in which filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory receptors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.

SNAP receptors implicated in vesicle targeting and fusion

The N-ethylmaleimide-sensitive fusion protein (NSF) and the soluble NSF attachment proteins (SNAPs) appear to be essential components of the intracellular membrane fusion apparatus. An affinity purification procedure based on the natural binding of these proteins to their targets was used to isolate SNAP receptors (SNAREs) from bovine brain. Remarkably, the four principal proteins isolated were all proteins associated with the synapse, with one type located in the synaptic vesicle and another in the plasma membrane, suggesting a simple mechanism for vesicle docking. The existence of numerous SNARE-related proteins, each apparently specific for a single kind of vesicle or target membrane, indicates that NSF and SNAPs may be universal components of a vesicle fusion apparatus common to both constitutive and regulated fusion (including neurotransmitter release), in which the SNAREs may help to ensure vesicle-to-target specificity.