Localization of synapsin I at the frog neuromuscular junction

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Nonsense-mediated mRNA decay and loss-of-function of the protein underlie the X-linked epilepsy associated with the W356× mutation in synapsin I

Synapsins are a family of neuronal phosphoproteins associated with the cytosolic surface of synaptic vesicles. Experimental evidence suggests a role for synapsins in synaptic vesicle clustering and recycling at the presynaptic terminal, as well as in neuronal development and synaptogenesis. Synapsin knock-out (Syn1(-/-) ) mice display an epileptic phenotype and mutations in the SYN1 gene have been identified in individuals affected by epilepsy and/or autism spectrum disorder. We investigated the impact of the c.1067G>A nonsense transition, the first mutation described in a family affected by X-linked syndromic epilepsy, on the expression and functional properties of the synapsin I protein. We found that the presence of a premature termination codon in the human SYN1 transcript renders it susceptible to nonsense-mediated mRNA decay (NMD). Given that the NMD efficiency is highly variable among individuals and cell types, we investigated also the effects of expression of the mutant protein and found that it is expressed at lower levels compared to wild-type synapsin I, forms perinuclear aggregates and is unable to reach presynaptic terminals in mature hippocampal neurons grown in culture. Taken together, these data indicate that in patients carrying the W356× mutation the function of synapsin I is markedly impaired, due to both the strongly decreased translation and the altered function of the NMD-escaped protein, and support the value of Syn1(-/-) mice as an experimental model mimicking the human pathology.

Pentylenetetrazol-induced epileptiform activity affects basal synaptic transmission and short-term plasticity in monosynaptic connections

Epileptic activity is generally induced in experimental models by local application of epileptogenic drugs, including pentylenetetrazol (PTZ), widely used on both vertebrate and invertebrate neurons. Despite the high prevalence of this neurological disorder and the extensive research on it, the cellular and molecular mechanisms underlying epileptogenesis still remain unclear. In this work, we examined PTZ-induced neuronal changes in Helix monosynaptic circuits formed in vitro, as a simpler experimental model to investigate the effects of epileptiform activity on both basal release and post-tetanic potentiation (PTP), a form of short-term plasticity. We observed a significant enhancement of basal synaptic strength, with kinetics resembling those of previously described use-dependent forms of plasticity, determined by changes in estimated quantal parameters, such as the readily releasable pool and the release probability. Moreover, these neurons exhibited a strong reduction in PTP expression and in its decay time constant, suggesting an impairment in the dynamic reorganization of synaptic vesicle pools following prolonged stimulation of synaptic transmission. In order to explain this imbalance, we determined whether epileptic activity is related to the phosphorylation level of synapsin, which is known to modulate synaptic plasticity. Using western blot and immunocytochemical staining we found a PTZ-dependent increase in synapsin phosphorylation at both PKA/CaMKI/IV and MAPK/Erk sites, both of which are important for modulating synaptic plasticity. Taken together, our findings suggest that prolonged epileptiform activity leads to an increase in the synapsin phosphorylation status, thereby contributing to an alteration of synaptic strength in both basal condition and tetanus-induced potentiation.

Normal levels of Rac1 are important for dendritic but not axonal development in hippocampal neurons

BACKGROUND INFORMATION

Rho family GTPases are required for cytoskeletal reorganization and are considered important for the maturation of neurons. Among these proteins, Rac1 is known to play a crucial role in the regulation of actin dynamics, and a number of studies indicate the involvement of this protein in different steps of vertebrate neuronal maturation. There are two distinct Rac proteins expressed in neurons, namely the ubiquitous Rac1 and the neuron-specific Rac3. The specific functions of each of these GTPases during early neuronal development are largely unknown.

RESULTS

The combination of the knockout of Rac3 with Rac1 down-regulation by siRNA (small interfering RNA) has been used to show that down-regulation of Rac1 affects dendritic development in mouse hippocampal neurons, without affecting axons. F-actin levels are strongly decreased in neuronal growth cones following down-regulation of Rac1, and time-lapse analysis indicated that the reduction of Rac1 levels decreases growth-cone dynamics.

CONCLUSIONS

These results show that normal levels of endogenous Rac1 activity are critical for early dendritic development, whereas dendritic outgrowth is not affected in hippocampal neurons from Rac3-null mice. On the other hand, early axonal development appears normal after Rac1 down-regulation. Our findings also suggest that the initial establishment of neuronal polarity is not affected by Rac1 down-regulation.

Essential role of Rac1 and Rac3 GTPases in neuronal development

Rac GTPases are members of the Rho family regulating the actin cytoskeleton and implicated in neuronal development. Ubiquitous Rac1 and neuron-specific Rac3 GTPases are coexpressed in the developing mammalian brain. We used Cre-mediated conditional deletion of Rac1 in neurons combined with knockout of neuron-specific Rac3 to study the role of these GTPases in neural development. We found that lack of both genes causes motor behavioral defects, epilepsy, and premature death of mice. Deletion of either GTPase does not produce evident phenotypes. Double-knockout mice show specific defects in the development of the hippocampus. Selective impairment of the dorsal hilus of double-knockout animals is associated with alteration in the formation of the hippocampal circuitry. Axonal pathways to and from the dorsal hilus are affected because of the deficit of hilar mossy cells. Moreover, analysis of Rac function in hippocampal cultures shows that spine formation is strongly hampered only in neurons lacking both Rac proteins. These findings show for the first time that both Rac1 and Rac3 are important for the development of the nervous system, wherein they play complementary roles during late stages of neuronal and brain development.

Regulation of synaptic transmission by presynaptic CaMKII and BK channels

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the BK channel are enriched at the presynaptic nerve terminal, where CaMKII associates with synaptic vesicles whereas the BK channel colocalizes with voltage-sensitive Ca(2+) channels in the plasma membrane. Mounting evidence suggests that these two proteins play important roles in controlling neurotransmitter release. Presynaptic BK channels primarily serve as a negative regulator of neurotransmitter release. In contrast, presynaptic CaMKII either enhances or inhibits neurotransmitter release and synaptic plasticity depending on experimental or physiological conditions and properties of specific synapses. The different functions of presynaptic CaMKII appear to be mediated by distinct downstream proteins, including the BK channel.

Profiling of experience-regulated proteins in the songbird auditory forebrain using quantitative proteomics

Auditory and perceptual processing of songs are required for a number of behaviors in songbirds such as vocal learning, territorial defense, mate selection and individual recognition. These neural processes are accompanied by increased expression of a few transcription factors, particularly in the caudomedial nidopallium (NCM), an auditory forebrain area believed to play a key role in auditory learning and song discrimination. However, these molecular changes are presumably part of a larger, yet uncharacterized, protein regulatory network. In order to gain further insight into this network, we performed two-dimensional differential in-gel expression (2D-DIGE) experiments, extensive protein quantification analyses, and tandem mass spectrometry in the NCM of adult songbirds hearing novel songs. A subset of proteins was selected for immunocytochemistry in NCM sections to confirm the 2D-DIGE findings and to provide additional quantitative and anatomical information. Using these methodologies, we found that stimulation of freely behaving birds with conspecific songs did not significantly impact the NCM proteome 5 min after stimulus onset. However, following 1 and 3 h of stimulation, a significant number of proteins were consistently regulated in NCM. These proteins spanned a range of functional categories that included metabolic enzymes, cytoskeletal molecules, and proteins involved in neurotransmitter secretion and calcium binding. Our findings suggest that auditory processing of vocal communication signals in freely behaving songbirds triggers a cascade of protein regulatory events that are dynamically regulated through activity-dependent changes in calcium levels.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

The synapsin domain E accelerates the exoendocytotic cycle of synaptic vesicles in cerebellar Purkinje cells

Synapsins are synaptic-vesicle-associated phosphoproteins implicated in the regulation of neurotransmitter release and excitability of neuronal networks. Mutation of synapsin genes in mouse and human causes epilepsy. To understand the role of the highly conserved synapsin domain E in the dynamics of release from mammalian inhibitory neurons, we generated mice that selectively overexpress the most conserved part of this domain in cerebellar Purkinje cells. At Purkinje-cell-nuclear-neuron synapses, transgenic mice were more resistant to depression induced by short or prolonged high-frequency stimulations. The increased synaptic performance was accompanied by accelerated release kinetics and shorter synaptic delay. Despite a marked decrease in the total number of synaptic vesicles, vesicles at the active zone were preserved or slightly increased. The data indicate that synapsin domain E increases synaptic efficiency by accelerating both the kinetics of exocytosis and the rate of synaptic vesicle cycling and decreasing depression at the inhibitory Purkinje-cell-nuclear-neuron synapse. These effects may increase the sensitivity of postsynaptic neurons to inhibition and thereby contribute to the inhibitory control of network activity.

Serotonin stimulates phosphorylation of Aplysia synapsin and alters its subcellular distribution in sensory neurons

Only a small fraction of neurotransmitter-containing synaptic vesicles (SVs), the readily releasable pool, is available for fast Ca(2+)-induced release at any synapse. Most SVs are sequestered at sites away from the plasma membrane and cannot be exocytosed directly. Recruitment of SVs to the releasable pool is thought to be an important component of short-term synaptic facilitation by serotonin (5-HT) at Aplysia sensorimotor synapses. Synapsins are associated with SVs and hypothesized to play a central role in the regulation of SV mobilization in nerve terminals. Aplysia synapsin was cloned to examine its role in synaptic plasticity at the well characterized sensorimotor neuron synapse of this animal. Acute 5-HT treatment of ganglia induced synapsin phosphorylation. Immunohistochemical analyses of cultured Aplysia neurons revealed that synapsin is distributed in distinct puncta in the neurites. These puncta are rapidly dispersed after treatment of the neurons with 5-HT. The dispersion of synapsin puncta by 5-HT was fully reversible after washout of the modulator. Both 5-HT-induced phosphorylation and dispersion of synapsin were mediated, at least in part, by cAMP-dependent protein kinase and mitogen-activated protein kinase. These experiments indicate that synapsin and its regulation by 5-HT may play an important role in the modulation of SV trafficking in short-term synaptic plasticity.

Opposing changes in phosphorylation of specific sites in synapsin I during Ca2+-dependent glutamate release in isolated nerve terminals

Synapsins are major neuronal phosphoproteins involved in regulation of neurotransmitter release. Synapsins are well established targets for multiple protein kinases within the nerve terminal, yet little is known about dephosphorylation processes involved in regulation of synapsin function. Here, we observed a reciprocal relationship in the phosphorylation-dephosphorylation of the established phosphorylation sites on synapsin I. We demonstrate that, in vitro, phosphorylation sites 1, 2, and 3 of synapsin I (P-site 1 phosphorylated by cAMP-dependent protein kinase; P-sites 2 and 3 phosphorylated by Ca(2+)-calmodulin-dependent protein kinase II) were excellent substrates for protein phosphatase 2A, whereas P-sites 4, 5, and 6 (phosphorylated by mitogen-activated protein kinase) were efficiently dephosphorylated only by Ca(2+)-calmodulin-dependent protein phosphatase 2B-calcineurin. In isolated nerve terminals, rapid changes in synapsin I phosphorylation were observed after Ca(2+) entry, namely, a Ca(2+)-dependent phosphorylation of P-sites 1, 2, and 3 and a Ca(2+)-dependent dephosphorylation of P-sites 4, 5, and 6. Inhibition of calcineurin activity by cyclosporin A resulted in a complete block of Ca(2+)-dependent dephosphorylation of P-sites 4, 5, and 6 and correlated with a prominent increase in ionomycin-evoked glutamate release. These two opposing, rapid, Ca(2+)-dependent processes may play a crucial role in the modulation of synaptic vesicle trafficking within the presynaptic terminal.

Synapsins as regulators of neurotransmitter release

One of the crucial issues in understanding neuronal transmission is to define the role(s) of the numerous proteins that are localized within presynaptic terminals and are thought to participate in the regulation of the synaptic vesicle life cycle. Synapsins are a multigene family of neuron-specific phosphoproteins and are the most abundant proteins on synaptic vesicles. Synapsins are able to interact in vitro with lipid and protein components of synaptic vesicles and with various cytoskeletal proteins, including actin. These and other studies have led to a model in which synapsins, by tethering synaptic vesicles to each other and to an actin-based cytoskeletal meshwork, maintain a reserve pool of vesicles in the vicinity of the active zone. Perturbation of synapsin function in a variety of preparations led to a selective disruption of this reserve pool and to an increase in synaptic depression, suggesting that the synapsin-dependent cluster of vesicles is required to sustain release of neurotransmitter in response to high levels of neuronal activity. In a recent study performed at the squid giant synapse, perturbation of synapsin function resulted in a selective disruption of the reserve pool of vesicles and in addition, led to an inhibition and slowing of the kinetics of neurotransmitter release, indicating a second role for synapsins downstream from vesicle docking. These data suggest that synapsins are involved in two distinct reactions which are crucial for exocytosis in presynaptic nerve terminals. This review describes our current understanding of the molecular mechanisms by which synapsins modulate synaptic transmission, while the increasingly well-documented role of the synapsins in synapse formation and stabilization lies beyond the scope of this review.

Presynaptic protein interactions in vivo: evidence from botulinum A, C, D and E action at frog neuromuscular junction

The present study examines the paralytic action of botulinum neurotoxins at their natural target, the neuromuscular junction. We asked whether syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and vesicle-associated membrane protein (VAMP/synaptobrevin), the proteins proteolysed by botulinum, are susceptible to cleavage in frog nerve terminals, and whether they form complexes in vivo. In control terminals, the three SNAREs were distributed in broad bands at 1 micrometer intervals, at sites consistent with presynaptic Ca2+ channels. Within 3 h, botulinum A, C, D and E (BoNT/A/C/D/E) blocked nerve-evoked muscle contractions but their effects on substrate immunoreactivity varied. The effect of BoNT/A on either C-terminus or N-terminus immunoreactivity of SNAP-25 was undetectable after 3-h incubation, although C-terminus immunoreactivity was reduced after 24 h; N-terminus immunoreactivity was not affected even after 36 h. BoNT/E reduced C-terminus immunoreactivity of SNAP-25 1.5 h after toxin application when transmitter release was blocked, but required 24 h to reduce N-terminus immunoreactivity. BoNT/C reduced syntaxin immunoreactivity after 24-h incubation but did not affect SNAP-25. BoNT/D reduced VAMP immunoreactivity at 3 h while it increased SNAP-25 C-terminal staining fourfold. BoNT/A and BoNT/C applied together for 24 h reduced syntaxin immunoreactivity and that of both C- and N-terminus of SNAP-25, indicating that retention of SNAP-25 N-terminus after cleavage by BoNT/A depended on intact syntaxin. Therefore, we infer that SNAP-25 interacts with VAMP and with syntaxin in vivo. Neurotoxin action abolished only 40-60% of SNAP-25, VAMP or syntaxin immunoreactivity suggesting that distinct pools of these proteins, not immediately involved in triggered exocytosis, are resistant to proteolysis.

Two sites of action for synapsin domain E in regulating neurotransmitter release

Synapsins, a family of synaptic vesicle proteins, have been shown to regulate neurotransmitter release; the mechanism(s) by which they act are not fully understood. Here we have studied the role of domain E of synapsins in neurotransmitter release at the squid giant synapse. Two squid synapsin isoforms were cloned and found to contain a carboxy (C)-terminal domain homologous to domain E of the vertebrate a-type synapsin isoforms. Presynaptic injection of a peptide fragment of domain E greatly reduced the number of synaptic vesicles in the periphery of the active zone, and increased the rate and extent of synaptic depression, suggesting that domain E is essential for synapsins to regulate a reserve pool of synaptic vesicles. Domain E peptide had no effect on the number of docked synaptic vesicles, yet reversibly inhibited and slowed the kinetics of neurotransmitter release, indicating a second role for synapsins that is more intimately associated with the release process itself. Thus, synapsin domain E is involved in at least two distinct reactions that are crucial for exocytosis in presynaptic terminals.

Anti-synapsin monoclonal antibodies: epitope mapping and inhibitory effects on phosphorylation and Grb2 binding

The synapsins are a family of major neuron-specific synaptic vesicle-associated phosphoproteins which play important roles in synaptic function. In an effort to identify molecular tools which can be used to perturb the activity of the synapsins in in vitro as well as in vivo experiments, we have localized the epitopes of a panel of monoclonal antibodies (mAbs) raised against synapsins I and II and have characterized their ability to interfere with the interactions of the synapsins with protein kinases, actin and Src homology-3 (SH3) domains. The epitopes of the six mAbs were found to be concentrated in the N-terminal region within domains A and B for the synapsin II-reactive mAbs 19.4, 19.11, 19.51 and 19.21, and in two C-terminal clusters in the proline-rich domains D for synapsin I (mAbs 10.22, 19.51, 19.11 and 19.8) and G for synapsin II (mAb 19.8). The synapsin II-specific mAbs 19.4 and 19.21, whose overlapping epitopes are adjacent to phosphorylation site 1, specifically inhibited synapsin II phosphorylation by endogenous or exogenous cAMP-dependent protein kinase. While all the anti-synapsin I mAbs were unable to affect the interactions of synapsin I both with Ca2+/calmodulin-dependent protein kinase II and with actin monomers and filaments, mAbs 19.8 and 19.51 were found to inhibit the binding of Grb2 SH3 domains to the proline-rich C-terminal region of synapsin I.

Sound damage and gentamicin treatment produce different patterns of damage to the efferent innervation of the chick cochlea

Both sound exposure and gentamicin treatment cause damage to sensory hair cells in the peripheral chick auditory organ, the basilar papilla. This induces a regeneration response which replaces hair cells and restores auditory function. Since functional recovery requires the re-establishment of connections between regenerated hair cells and the central nervous system, we have investigated the effects of sound damage and gentamicin treatment on the neuronal elements within the cochlea. Whole-mount preparations of basilar papillae were labeled with phalloidin to label the actin cytoskeleton and antibodies to neurofilaments, choline acetyltransferase, and synapsin to label neurons; and examined by confocal laser scanning microscopy. When chicks are treated with gentamicin or exposed to acoustic overstimulation, the transverse nerve fibers show no changes from normal cochleae assayed in parallel. Efferent nerve terminals, however, disappear from areas depleted of hair cells following acoustic trauma. In contrast, efferent nerve endings are still present in the areas of hair cell loss following gentamicin treatment, although their morphological appearance is greatly altered. These differences in the response of efferent nerve terminals to sound exposure versus gentamicin treatment may account, at least in part, for the discrepancies reported in the time of recovery of auditory function.

Synapsin I interacts with c-Src and stimulates its tyrosine kinase activity

Synapsin I is a synaptic vesicle-associated phosphoprotein that has been implicated in the formation of presynaptic specializations and in the regulation of neurotransmitter release. The nonreceptor tyrosine kinase c-Src is enriched on synaptic vesicles, where it accounts for most of the vesicle-associated tyrosine kinase activity. Using overlay, affinity chromatography, and coprecipitation assays, we have now shown that synapsin I is the major binding protein for the Src homology 3 (SH3) domain of c-Src in highly purified synaptic vesicle preparations. The interaction was mediated by the proline-rich domain D of synapsin I and was not significantly affected by stoichiometric phosphorylation of synapsin I at any of the known regulatory sites. The interaction of purified c-Src and synapsin I resulted in a severalfold stimulation of tyrosine kinase activity and was antagonized by the purified c-Src-SH3 domain. Depletion of synapsin I from purified synaptic vesicles resulted in a decrease of endogenous tyrosine kinase activity. Portions of the total cellular pools of synapsin I and Src were coprecipitated from detergent extracts of rat brain synaptosomal fractions using antibodies to either protein species. The interaction between synapsin I and c-Src, as well as the synapsin I-induced stimulation of tyrosine kinase activity, may be physiologically important in signal transduction and in the modulation of the function of axon terminals, both during synaptogenesis and at mature synapses.

Identification, expression, and crystallization of the protease-resistant conserved domain of synapsin I

A 35-37-kDa protease-resistant domain of synapsin Ia/ Ib, apparently produced by low levels of endogenous proteases in vapor diffusion droplets, slowly formed crystals diffracting X-rays to approximately 10 A resolution. The fragment mainly consisted of the highly conserved C domain common to the synapsin I/II family plus short N- and C-terminal flanking segments. Two constructs (SynA and SynB) of synthetic gene fragments coding for the C domain of synapsin with or without C-terminal flanking sequence were expressed in Escherichia coli as fusion proteins attached to the soluble protein glutathione-S-transferase. The fusion proteins were purified by affinity chromatography. Subsequent in situ cleavage with TEV protease resulted in the release of highly pure synapsin fragments, which were further purified by ion exchange chromatography. SynA and SynB formed crystals within three days, which diffracted to better than 3 A using a conventional X-ray source and to about 2 A using a synchrotron X-ray source. SynA crystals have the symmetry of the trigonal space groups P3(1)21 or P3(2)21 and the unit cell dimensions a = b = 77.4 A, c = 188.5 A, alpha = beta = 90 degrees, gamma = 120 degrees. SynB crystals have the symmetry of the orthorhombic space group C222(1) with the unit cell dimension a = 104.6 A, b = 113.3 A, and c = 273.8 A.

Expression of synapsin I correlates with maturation of the neuromuscular synapse

Synapsins are a family of neuron-specific phosphoproteins that are localized within the presynaptic terminals in adult brain. Previous work has demonstrated that introduction of exogenous synapsins I(a + b) or IIa into Xenopus spinal neurons promoted maturation of the neuromuscular synapse in a nerve-muscle co-culture system. We have now studied the expression of endogenous Xenopus synapsin I during synaptic maturation in vivo and in culture, using a polyclonal antibody raised against Xenopus synapsin I. Immunoprecipitation experiments indicated that synapsin I was not detectable during the early phase of synaptogenesis in vivo, and exhibited a marked increase during the period of synaptic maturation. In contrast, the expression of synaptophysin, another synaptic vesicle protein, was detected at the start of nervous system formation, and remained at a high level thereafter. Similar expression profiles for the two proteins were also observed in immunocytochemical studies of Xenopus spinal neurons in culture: intense staining of synaptophysin was found on the first day, while synapsin I was not detected until after three days in culture. The expression of synapsin I correlated very well with the appearance of a bell-shaped amplitude distribution of spontaneous synaptic currents, a physiological parameter which reflects functional maturation of the neuromuscular synapse. In one-day-old cultures grown in the absence of laminin, an extracellular matrix protein known to be present at the neuromuscular junction, the amplitude distribution of virtually all synapses was skewed towards smaller values. In contrast, when laminin was used as a culture substrate, many synapses exhibited a bell-shaped amplitude distribution. Laminin treatment also induced synapsin I expression in one-day-old cultures. These results suggest that the expression of endogenous synapsin I may regulate maturation at neuromuscular synapses.

Cd(2+)-and K(+)-evoked ACh release induce different synaptophysin and synaptobrevin immunolabelling at the frog neuromuscular junction

Synaptophysin and synaptobrevin, two integral proteins of synaptic vesicles, have been used as immunocytochemical markers of the synaptic vesicle membrane during Cd(2+)- or K(+)-induced ACH release at the frog neuromuscular junction. ACh release was stimulated in cutaneous pectoris nerve-muscle preparations by: (1) 1 mM Cd2+ in Ca(2+)-free medium for a period of 3 h, (2) 25 or 40 mM K+ in normal Ringer's solution. Synaptophysin and synaptobrevin were immunolabelled in single fibres teased from fixed muscles using rabbit antisera raised against synaptophysin and synaptobrevin revealed with fluorescein-conjugated IgG. The postsynaptic ACh receptors were simultaneously labelled with rhodaminated alpha-bungarotoxin. Unstimulated and K(+)-stimulated preparations showed synaptophysin and synaptobrevin immunolabelling only after membrane permeabilization with 0.1% Triton X-100. In preparations stimulated with Cd2+ in Ca(2+)-free medium, the immunofluorescence was also observed in non Triton X-100 treated muscle fibres. Confocal laser scanning microscopy analysis revealed that in unstimulated and K(+)-stimulated preparations, synaptophysin and synaptobrevin immunofluorescence appears as bands regularly spaced along the permeabilized nerve terminals and that their distribution corresponds to clusters of synaptic vesicles. After Cd2+ stimulation in Ca(2+)-free medium, labelling for both proteins is irregularly distributed, being more intense at the lateral margins of swollen nerve terminals, suggesting a translocation of synaptic vesicle proteins to the axolemma. At the electron microscopic level, Cd2+ stimulation in Ca(2+)-free medium produces nerve terminal swelling and synaptic vesicle depletion. The results show that when ACh release is stimulated under an impairment of synaptic vesicle recycling, which leads to synaptic vesicle depletion, synaptophysin and synaptobrevin translocation occurs. These findings are in favour of a permanent incorporation of synaptic vesicle membrane into the axolemma. In contrast, after K+ stimulation, the immunofluorescence and the normal synaptic vesicle population observed, suggest that a double process of synaptic vesicle exo-endocytosis rapidly occurs, without incorporation of synaptic vesicle components into the axolemma.

Synaptotagmin I is present mainly in autonomic and sensory neurons of the rat peripheral nervous system

The distribution of synaptotagmin I in the peripheral nervous system of the rat was investigated by immunofluorescence and confocal laser scanning microscopy. After crushing of the sciatic nerve, synaptotagmin I-like immunoreactivity accumulated proximally as well as distally to the crushes in thin and medium-sized axons. Double labelling studies revealed that synaptotagmin I co-localized with tyrosine hydroxylase, a marker of sympathetic adrenergic neurons, and with substance P, a marker for sensory neurons. No synaptotagmin I-like immunoreactivity was found in large axons, while accumulations of the synaptic vesicle proteins synaptophysin and synapsin I were found in all types of axons. Furthermore, no synaptotagmin I-like immunoreactivity was detected in motor endplates. In contrast, the protein was found in muscle spindles of young rats and in perivascular terminals, where it co-localized with synaptophysin and synapsin I. Lumbar sympathectomy resulted in a marked reduction of the amount and intensity of synaptotagmin I-like immunoreactivity in sciatic nerve. High magnification revealed that synaptotagmin I-like immunoreactivity was mainly distributed in a fine granular pattern, but large, brightly fluorescent granules which were not labelled by anti-synaptophysin or anti-synapsin I were occasionally observed. We conclude that synaptotagmin I is mainly expressed in adrenergic and sensory neurons and is absent from, or below detection levels, in motoneurons.

Visualization of synaptic structure and function with confocal microscopy: calcium fluctuations and oscillations

This article summarizes the basic principles of confocal microscopy and how they can be employed to visualize synaptic structure and function. Optical 'sectioning' of living cells allows the examination of a large number of biological processes at different subcellular localities. Different fluorescent markers enable the study of processes in the extracellular, intracellular and membrane domains of the nerve cell. The excellent spatial resolution of confocal microscopy permits to study the changes in intracellular calcium concentration in single synaptic boutons, without a substantial interference from supporting cells. Intracellular calcium concentration shows coordinated fluctuations in space and periodic oscillations. Periodic oscillations can serve as time keeping devices in nerve terminals. Oscillations were previously observed also in the process of transmitter release. We speculate therefore that these calcium oscillations may be of significance, if the quantal transmitter release is governed by a sequence of calcium dependent steps, which have a different affinity for calcium.

Synaptic vesicle phosphoproteins and regulation of synaptic function

Complex brain functions, such as learning and memory, are believed to involve changes in the efficiency of communication between nerve cells. Therefore, the elucidation of the molecular mechanisms that regulate synaptic transmission, the process of intercellular communication, is an essential step toward understanding nervous system function. Several proteins associated with synaptic vesicles, the organelles that store neurotransmitters, are targets for protein phosphorylation and dephosphorylation. One of these phosphoproteins, synapsin I, by means of changes in its state of phosphorylation, appears to control the fraction of synaptic vesicles available for release and thereby to regulate the efficiency of neurotransmitter release. This article describes current understanding of the mechanism by which synapsin I modulates communication between nerve cells and reviews the properties and putative functions of other phosphoproteins associated with synaptic vesicles.

Optical monitoring of transmitter release and synaptic vesicle recycling at the frog neuromuscular junction

1. Frog cutaneous pectoris motor nerve terminals were loaded with the fluorescent dye FM1-43, which produced a series of discrete spots along the length of terminals, each spot evidently marking a cluster of synaptic vesicles. Terminals were imaged for 2-10 min as they destained during repetitive nerve stimulation. Endplate potentials (EPPs) were recorded simultaneously from the muscle fibres innervated by these terminals; their summed amplitudes provided a measure of cumulative transmitter release. 2. Individual fluorescent spots in any one terminal varied in initial brightness but destained at similar fractional rates. 3. The rates of cumulative transmitter release and destaining increased with stimulus frequency in the range 2-30 Hz. At 40 Hz, however, both transmitter release and destaining were slower than at 30 Hz. 4. In twenty-six experiments, rates of dye loss and transmitter release were compared quantitatively. When the time course of summed EPPs was scaled to fit the time course of dye loss during the first 30-60 s of destaining, the two curves usually diverged at later times, the dye loss curve falling below the summed EPP curve. Thus, assuming that dye loss and transmitter release are proportional at early times, at later times the rate of dye loss decreases relative to the rate of transmitter release. 5. At stimulus frequencies from 2 to 30 Hz, the results could be fitted by a simple model in which vesicles lose their dye during exocytosis and, after a fixed recycle 'dead time', they re-enter the vesicle pool, mixing randomly with other vesicles. 6. Unlike stimulation at lower frequencies, at 40 Hz dye loss and summed EPP amplitude curves did not significantly diverge. Stimulation periods lasted up to about 2 min. Interpreted according to the model of vesicle recycling, this suggests that vesicle recycling is inhibited at 40 Hz. 7. The model led to predictions about the relative number, N, of vesicles (labelled and unlabelled) in the terminal at any time during stimulation. The calculated value of N decreased at times less than the recycle 'dead time', and then increased, reflecting the appearance of recycled vesicles in the vesicle pool. 8. From estimates of N and recorded EPP amplitudes, the fraction of vesicles released per shock, F, could be calculated during the entire stimulation period. At low stimulus frequencies (2-5 Hz), after an initial rapid fall, F decreased slowly and monotonically by about 50% in 6 min. At higher stimulus frequencies, a different process was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular movements of fluorescently labeled synaptic vesicles in frog motor nerve terminals during nerve stimulation

We stained synaptic vesicles in frog motor nerve terminals with FM1-43 and studied changes in the shape and position of vesicle clusters during nerve stimulation. Each stained vesicle cluster appeared as a fluorescent spot. During repetitive nerve stimulation the spots gradually dimmed, most without changing shape or position. Occasionally, however, a spot moved, appearing in some cases to stream toward and coalesce with a neighboring spot. This suggests the existence of translocation mechanisms that can actively move vesicles in a coordinated fashion between vesicle clusters. Within single clusters, we saw no signs of such directed vesicle movements. Fluorescent spots in terminals viewed from the side with a confocal microscope did not shrink toward the presynaptic membrane during nerve stimulation, but dimmed uniformly. This suggests that vesicles continuously mix within a cluster during destaining and provides no evidence of active vesicle translocators within single vesicle clusters for moving vesicles to the presynaptic membrane.

The morphological localization and biochemical characterization of a synapsin I-like antigen in the nervous system of Aplysia californica

Synapsins are a well-characterized class of phosphoproteins found at synapses in the mammalian nervous system. One member of this family, synapsin I, has been extensively studied and shown to associate in a phosphorylation-dependent manner with both small synaptic vesicles and cytoskeletal elements. Though the characteristics of synapsin I suggest an important function in synaptic transmission, its definitive role is still in question. In an effort to find a model system in which to test directly the function of synapsin I, we have looked in the nervous system of the marine mollusc Aplysia californica for synapsin I-like antigens (SILA). Light microscope immunocytochemical studies using polyclonal and monoclonal antibodies to bovine brain synapsin I demonstrate Aplysia SILA in neuronal somata, in the neuropil, and at some identified synapses. Though SILA were exclusively associated with neuronal structures in Aplysia, the pattern of staining suggested that they are not present at all synaptic terminals. This interpretation was corroborated by ultrastructural studies in which SILA were present at some synaptic terminals but absent, or in low abundance, in adjacent terminals. In axons, SILA were associated with vesicles of 120-150 nm diameter, as well as with filamentous structures. Biochemical studies identified small amounts of SILA of 40 and 50 kD molecular weight that are recognized by several antibodies to mammalian synapsin I, and are acid extractable, collagenase-sensitive phosphoproteins; these are criteria used to define synapsin I homologues in other species. Our studies indicate that SILA are present in neurons in Aplysia californica but suggested that they represent only a small percentage of the total protein within the nervous system.

Suppression of kinesin expression in cultured hippocampal neurons using antisense oligonucleotides

Kinesin, a microtubule-based force-generating molecule, is thought to translocate organelles along microtubules. To examine the function of kinesin in neurons, we sought to suppress kinesin heavy chain (KHC) expression in cultured hippocampal neurons using antisense oligonucleotides and study the phenotype of these KHC "null" cells. Two different antisense oligonucleotides complementary to the KHC sequence reduced the protein levels of the heavy chain by greater than 95% within 24 h after application and produced identical phenotypes. After inhibition of KHC expression for 24 or 48 h, neurons extended an array of neurites often with one neurite longer than the others; however, the length of all these neurites was significantly reduced. Inhibition of KHC expression also altered the distribution of GAP-43 and synapsin I, two proteins thought to be transported in association with membranous organelles. These proteins, which are normally localized at the tips of growing neurites, were confined to the cell body in antisense-treated cells. Treatment of the cells with the corresponding sense oligonucleotides affected neither the distribution of GAP-43 and synapsin I, nor the length of neurites. A full recovery of neurite length occurred after removal of the antisense oligonucleotides from the medium. These data indicate that KHC plays a role in the anterograde translocation of vesicles containing GAP-43 and synapsin I. A deficiency in vesicle delivery may also explain the inhibition of neurite outgrowth. Despite the inhibition of KHC and the failure of GAP-43 and synapsin I to move out of the cell body, hippocampal neurons can extend processes and acquire as asymmetric morphology.

Organization and physiology of the substantia nigra

A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of the substantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of the zona compacta neurons. This loop can enhance the transmission of information from the striatum through the pars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.

Optical analysis of synaptic vesicle recycling at the frog neuromuscular junction

The fluorescent dyes FM1-43 and RH414 label motor nerve terminals in an activity-dependent fashion that involves dye uptake by synaptic vesicles that are recycling. This allows optical monitoring of vesicle recycling in living nerve terminals to determine how recycled vesicles reenter the vesicle pool. The results suggest that recycled vesicles mix with the pool morphologically and functionally. One complete cycle of release of transmitter, recycling of a vesicle, and rerelease of transmitter appears to take about 1 minute.

Membrane routing during exocytosis and endocytosis in neuroendocrine neurones and endocrine cells: use of colloidal gold particles and immunocytochemical discrimination of membrane compartments

The hypothesis that the retrieval of membranes of neurohypophysial neurosecretory granules (NSG) and small electron-lucent microvesicles occurs by different routes was tested by incubating neurohypophysial neurosecretosomes with colloidal gold particles of various sizes. Neurosecretosomes derived from normal Long Evans rats and incubated in media of normal ionic composition endocytosed a few small (less than 25 nm) gold particles into 40-50 nm electron-lucent microvesicles. After depolarisation, more small gold particles were found in microvesicles, and small and large (greater than 25 nm) gold particles in vacuoles. Oxytocin-containing neurosecretosomes derived from Brattleboro rats, which contain 160 nm-diameter NSG, endocytosed gold particles in a pattern indistinguishable from that of neurosecretosomes from Long Evans rats. However, neurosecretosomes derived from defective vasopressin neurones of Brattleboro rats, which contain microvesicles, small vacuoles, and a few 100 nm dense-cored vesicles, but no 160 nm NSG, endocytosed only small colloidal gold particles. Early after depolarisation the gold particles were present only in microvesicles, but later some could be found in vacuoles and lysosome-like structures. Immunogold cytochemistry using a polyclonal antiserum raised against microvesicle-rich neurosecretosomes derived from Brattleboro rats labelled microvesicles in the posterior pituitary strongly, NSG weakly, and vacuoles to a variable extent. These data together indicate that, after exocytosis, the membranes of NSG are recaptured as large vacuoles. Microvesicles are exocytosed and endocytosed separately.

Cytoplasmic architecture of the axon terminal: filamentous strands specifically associated with synaptic vesicles

Cytoplasmic architecture of axon terminals in rat central nervous tissue was examined by quick-freeze deep-etch method to determine how synaptic vesicles and their associated cytoplasmic environment are organized in the terminal and to know how these structures participate in the mechanism for neurotransmitter release. The axoplasm is divisible into two domains: one occupied by mitochondria in the middle of the terminal, called the mitochondrial domain, the other situated in the periphery and exclusively filled with spherical synaptic vesicles, 50-60 nm in diameter, the synaptic vesicle domain. The most characteristic feature of the mitochondrial domain was the appearance of many microtubules connected with mitochondria by filamentous strands. Large vesicles, 80-100 nm in diameter, were preferentially associated with the mitochondrial domain, and linked with microtubules wherever they appeared. The cytoplasmic matrix of the synaptic vesicle domain showed a more fibrillar texture than that of the mitochondrial domain because of the distribution of filamentous strands associated with synaptic vesicles. These strands were significantly thicker and longer (mean 11.7 nm thick and 42.7 nm long) than those linking membrane-bound organelles to microtubules (mean 8.3 nm thick and 23.0 nm long), and connected vesicles to one another or to the plasma membrane, making a complicated network around the vesicles. Further, both strands were significantly different in dimension from actin filaments (mean 9.9 nm thick and 73.5 nm long) showing 5-nm axial periodicity. These strands, especially synaptic vesicle-associated ones including their network, were readily broken down in the most part by detergent treatment or chemical fixation, indicating that they are very delicate in nature. Granular materials, which are spherical and vary in size (6-20 nm in diameter), are also more conspicuous in the synaptic vesicle domain than in the mitochondrial domain. More fibrillar and granular cytoplasmic structure of the synaptic vesicle domain may be crucial for synaptic vesicles to perform an essential role in releasing the transmitter.

Synapsin I expression in the rat retina during postnatal development

The expression of the synapsin I gene was studied during postnatal development of the rat retina at the mRNA and protein levels. In situ hybridization histochemistry showed that synapsin I mRNA was expressed already in nerve cells in the ganglion cell layer of the neonatal retina, while it appeared in neurons of the inner nuclear layer from postnatal day 4 onward. Maximal expression of synapsin I mRNA was observed at P12 in ganglion cells and in neurons of the inner nuclear layer followed by moderate expression in the adult. At the protein level a shift of synapsin I appearance was observed from cytoplasmic to terminal localization during retinal development by immunohistochemistry. In early stages (P4 and P8), synapsin I was seen in neurons of the ganglion cell layer and in neurons of the developing inner nuclear layer as well as in the developing inner plexiform layer. In the developing outer plexiform layer synapsin I was localized only in horizontal cells and in their processes. Its early appearance at P4 indicated the early maturation of this cell type. A shift and strong increase of labelling to the plexiform layers at P12 indicated the localization of synapsin I in synaptic terminals. The inner plexiform layer exhibited a characteristic stratified pattern. Photoreceptor cells never exhibited synapsin I mRNA or synapsin I protein throughout development.

Synapsin I-like immunoreactivity in nerve fibers associated with lingual taste buds of the rat

Immunoreactivity to synapsin I, a neuronal phosphoprotein, was localized in free-floating tissue sections prepared from lingual tissue of rats. Many nerve fibers within the tissue exhibited clear immunoreactivity including motor endplates on striated muscle, autonomic fibers innervating blood vessels or glands, and sensory fibers innervating muscles or the lingual epithelium including taste buds. Numerous immunoreactive fibers occurred within each taste bud, with fewer, fine fibers being dispersed in the epithelium between taste buds. The majority of the intragemmal immunoreactive fibers extended throughout the taste buds most of the distance outward from the basal lamina toward the surface of the epithelium. Fine, perigemmal fibers reached nearly to the epithelial surface. Ultrastructural analysis of the immunoreactive sensory fibers revealed that synapsin I-immunoreactivity occurred diffusely throughout the cytoplasm, and heavily in association with microvesicles. The synaptic vesicles at the taste receptor cell-to-afferent fiber synapse were, however, not immunoreactive for synapsin I, although these vesicles fall into the size class shown to be immunoreactive in other systems. This absence of synapsin I may be a common property of vesicles in axonless short receptor cells.

Redistribution of synaptophysin and synapsin I during alpha-latrotoxin-induced release of neurotransmitter at the neuromuscular junction

The distribution of two synaptic vesicle-specific phosphoproteins, synaptophysin and synapsin I, during intense quantal secretion was studied by applying an immunogold labeling technique to ultrathin frozen sections. In nerve-muscle preparations treated for 1 h with a low dose of alpha-latrotoxin in the absence of extracellular Ca2+ (a condition under which nerve terminals are depleted of both quanta of neurotransmitter and synaptic vesicles), the immunolabeling for both proteins was distributed along the axolemma. These findings indicate that, in the presence of a block of endocytosis, exocytosis leads to the permanent incorporation of the synaptic vesicle membrane into the axolemma and suggest that, under this condition, at least some of the synapsin I molecules remain associated with the vesicle membrane after fusion. When the same dose of alpha-latrotoxin was applied in the presence of extracellular Ca2+, the immunoreactivity patterns resembled those obtained in resting preparations: immunogold particles were selectively associated with the membrane of synaptic vesicles, whereas the axolemma was virtually unlabeled. Under this condition an active recycling of both quanta of neurotransmitter and vesicles operates. These findings indicate that the retrieval of components of the synaptic vesicle membrane is an efficient process that does not involve extensive intermixing between molecular components of the vesicle and plasma membrane, and show that synaptic vesicles that are rapidly recycling still have the bulk of synapsin I associated with their membrane.

Substance P-like immunoreactivity at the frog neuromuscular junction

It has been recently shown that frog motoneurons and their nerve terminals contain calcitonin gene-related peptide-like immunoreactivity in large dense-core vesicles (Matteoli et al., Proc. natn. Acad. Sci. U.S.A. 85, 7366-7370, 1988). We report here by an immunofluorescence approach that the same neurons and nerve terminals also contain substance P-like immunoreactivity. The demonstration of substance P-like immunoreactivity in the frog motor nerve endings supports previous data suggesting a physiological role for this peptide in the modulation of cholinergic transmission.

Synaptophysin (p38) at the frog neuromuscular junction: its incorporation into the axolemma and recycling after intense quantal secretion

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

Differential effect of alpha-latrotoxin on exocytosis from small synaptic vesicles and from large dense-core vesicles containing calcitonin gene-related peptide at the frog neuromuscular junction

The regulatory peptide called calcitonin gene-related peptide (CGRP) was detected by immunofluorescence in frog motor neurons and motor nerve terminals. In motor nerve terminals, CGRP-like immunoreactivity was found to be segregated within large dense-core vesicles. To determine whether exocytosis from acetylcholine-containing small synaptic vesicles and from CGRP-containing large dense-core vesicles can be independently stimulated, nerve-muscle preparations were exposed to alpha-latrotoxin. This toxin induced complete depletion of acetylcholine-containing small synaptic vesicles but did not induce a parallel depletion of CGRP-like immunoreactivity and of large dense-core vesicles. These effects were independent of the presence of extracellular Ca2+ and occurred both at room temperature and at low temperature (1-3 degrees C). These findings suggest that exocytosis from the two vesicle populations is mediated by distinct biochemical mechanisms, which might be differentially regulated by physiological stimuli.