Synapsin I: an actin-bundling protein under phosphorylation control

Synaptic vesicle recycling in synapsin I knock-out mice

The synapsins are a family of four neuron-specific phosphoproteins that have been implicated in the regulation of neurotransmitter release. Nevertheless, knock-out mice lacking synapsin Ia and Ib, family members that are major substrates for cAMP and Ca2+/ Calmodulin (CaM)-dependent protein kinases, show limited phenotypic changes when analyzed electrophysiologically (Rosahl, T.W., D. Spillane, M. Missler, J. Herz, D.K. Selig, J.R. Wolff, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1995. Nature (Lond.). 375: 488-493; Rosahl, T.W., M. Geppert, D. Spillane, D., J. Herz, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1993. Cell. 75:661-670; Li, L., L.S. Chin, O. Shupliakov, L. Brodin, T.S. Sihra, O. Hvalby, V. Jensen, D. Zheng, J.O. McNamara, P. Greengard, and P. Andersen. 1995. Proc. Natl. Acad. Sci. USA. 92:9235-9239; see also Pieribone, V.A., O. Shupliakov, L. Brodin, S. Hilfiker-Rothenfluh, A.J. Czernik, and P. Greengard. 1995. Nature (Lond.). 375:493-497). Here, using the optical tracer FM 1-43, we characterize the details of synaptic vesicle recycling at individual synaptic boutons in hippocampal cell cultures derived from mice lacking synapsin I or wild-type equivalents. These studies show that both the number of vesicles exocytosed during brief action potential trains and the total recycling vesicle pool are significantly reduced in the synapsin I-deficient mice, while the kinetics of endocytosis and synaptic vesicle repriming appear normal.

Neurotrophins stimulate phosphorylation of synapsin I by MAP kinase and regulate synapsin I-actin interactions

The ability of neurotrophins to modulate the survival and differentiation of neuronal populations involves the Trk/MAP (mitogen-activated protein kinase) kinase signaling pathway. More recently, neurotrophins have also been shown to regulate synaptic transmission. The synapsins are a family of neuron-specific phosphoproteins that play a role in regulation of neurotransmitter release, in axonal elongation, and in formation and maintenance of synaptic contacts. We report here that synapsin I is a downstream effector for the neurotrophin/Trk/MAP kinase cascade. Using purified components, we show that MAP kinase stoichiometrically phosphorylated synapsin I at three sites (Ser-62, Ser-67, and Ser-549). Phosphorylation of these sites was detected in rat brain homogenates, in cultured cerebrocortical neurons, and in isolated presynaptic terminals. Brain-derived neurotrophic factor and nerve growth factor upregulated phosphorylation of synapsin I at MAP kinase-dependent sites in intact cerebrocortical neurons and PC12 cells, respectively, while KCl- induced depolarization of cultured neurons decreased the phosphorylation state at these sites. MAP kinase-dependent phosphorylation of synapsin I significantly reduced its ability to promote G-actin polymerization and to bundle actin filaments. The results suggest that MAP kinase-dependent phosphorylation of synapsin I may contribute to the modulation of synaptic plasticity by neurotrophins and by other signaling pathways that converge at the level of MAP kinase activation.

Synapsin I deficiency results in the structural change in the presynaptic terminals in the murine nervous system

Synapsin I is one of the major synaptic vesicle-associated proteins. Previous experiments implicated its crucial role in synaptogenesis and transmitter release. To better define the role of synapsin I in vivo, we used gene targeting to disrupt the murine synapsin I gene. Mutant mice lacking synapsin I appeared to develop normally and did not have gross anatomical abnormalities. However, when we examined the presynaptic structure of the hippocampal CA3 field in detail, we found that the sizes of mossy fiber giant terminals were significantly smaller, the number of synaptic vesicles became reduced, and the presynaptic structures altered, although the mossy fiber long-term potentiation remained intact. These results suggest significant contribution of synapsin I to the formation and maintenance of the presynaptic structure.

Long-term potentiation in mice lacking synapsins

Synapsin I and synapsin II are widely expressed synaptic vesicle phosphoproteins that have been proposed to play an important role in synaptic transmission and synaptic plasticity. To gain further insight into the functional significance of the phosphorylation sites on the synapsins, we have examined a number of synaptic processes thought to be mediated by protein kinases in knockout mice lacking both forms of synapsin (Rosahl et al., 1995). Long-term potentiation (LTP) at both the mossy fiber (MF)-CA3 pyramidal cell synapse and the Schaffer collateral-CA1 pyramidal cell synapse appears normal in hippocampal slices prepared from mice lacking synapsins. Moreover, the effects on synaptic transmission of forskolin at MF synapses and H-7 at synapses on CA1 cells are also normal in the mutant mice. These results indicate that the synapsins are not necessary for: (1) the induction or expression of two different forms of LTP in the hippocampus, (2) the enhancement in transmitter release elicited by activation of the cAMP-dependent protein kinase (PKA) and (3) the depression of synaptic transmission caused by H-7. Although disappointing, these results are important in that they exclude the most abundant family of synaptic phosphoproteins as an essential component of long-term synaptic plasticity.

The synaptic vesicle cycle: a cascade of protein-protein interactions

The synaptic vesicle cycle at the nerve terminal consists of vesicle exocytosis with neurotransmitter release, endocytosis of empty vesicles, and regeneration of fresh vesicles. Of all cellular transport pathways, the synaptic vesicle cycle is the fastest and the most tightly regulated. A convergence of results now allows formulation of molecular models for key steps of the cycle. These developments may form the basis for a mechanistic understanding of higher neural function.

Essential functions of synapsins I and II in synaptic vesicle regulation

Synaptic vesicles are coated by synapsins, phosphoproteins that account for 9% of the vesicle protein. To analyse the functions of these proteins, we have studied knockout mice lacking either synapsin I, synapsin II, or both. Mice lacking synapsins are viable and fertile with no gross anatomical abnormalities, but experience seizures with a frequency proportional to the number of mutant alleles. Synapsin-II and double knockouts, but not synapsin-I knockouts, exhibit decreased post-tetanic potentiation and severe synaptic depression upon repetitive stimulation. Intrinsic synaptic-vesicle membrane proteins, but not peripheral membrane proteins or other synaptic proteins, are slightly decreased in individual knockouts and more severely reduced in double knockouts, as is the number of synaptic vesicles. Thus synapsins are not required for neurite outgrowth, synaptogenesis or the basic mechanics of synaptic vesicle traffic, but are essential for accelerating this traffic during repetitive stimulation. The phenotype of the synapsin knockouts could be explained either by deficient recruitment of synaptic vesicles to the active zone, or by impaired maturation of vesicles at the active zone, both of which could lead to a secondary destabilization of synaptic vesicles.

Changes in mobility of synaptic vesicles with assembly and disassembly of actin network

In a presynaptic terminal, neurotransmitters are stored in synaptic vesicles and secreted into the synaptic cleft as a final step of cell signal transduction. At a static state, the vesicles are retained in the highly dense actin network. Prior to exocytosis, the dense actin network must disassemble or largely be organized. Actin networks are formed in vitro which retain synaptic vesicles prepared from rat cerebral cortex. Dynamic behaviors of synaptic vesicles are measured by the dynamic light scattering method. The D(app) values of synaptic vesicles confined in actin network became less than 1/4 those of free vesicles. The motions of synaptic vesicles are substantially restricted. This means that synaptic vesicles which are liberated from the actin network by detachment of synapsin 1 molecules are still trapped in the cage-like space of actin filaments. The actin network is disassembled by the actin severing protein, gelsolin, which is activated in the presence of microM level free Ca2+ ions. The D(app)(v) values of synaptic vesicles after severing the actin network return to those of free vesicles in the presence of short actin fragments. A molecular model for exocytosis in the synaptic terminal is constructed on the basis of these results.

Dephosphorylated synapsin I anchors synaptic vesicles to actin cytoskeleton: an analysis by videomicroscopy

Synapsin I is a synaptic vesicle-associated protein which inhibits neurotransmitter release, an effect which is abolished upon its phosphorylation by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Based on indirect evidence, it was suggested that this effect on neurotransmitter release may be achieved by the reversible anchoring of synaptic vesicles to the actin cytoskeleton of the nerve terminal. Using video-enhanced microscopy, we have now obtained experimental evidence in support of this model: the presence of dephosphorylated synapsin I is necessary for synaptic vesicles to bind actin; synapsin I is able to promote actin polymerization and bundling of actin filaments in the presence of synaptic vesicles; the ability to cross-link synaptic vesicles and actin is specific for synapsin I and is not shared by other basic proteins; the cross-linking between synaptic vesicles and actin is specific for the membrane of synaptic vesicles and does not reflect either a non-specific binding of membranes to the highly surface active synapsin I molecule or trapping of vesicles within the thick bundles of actin filaments; the formation of the ternary complex is virtually abolished when synapsin I is phosphorylated by CaM kinase II. The data indicate that synapsin I markedly affects synaptic vesicle traffic and cytoskeleton assembly in the nerve terminal and provide a molecular basis for the ability of synapsin I to regulate the availability of synaptic vesicles for exocytosis and thereby the efficiency of neurotransmitter release.

Accelerated structural maturation induced by synapsin I at developing neuromuscular synapses of Xenopus laevis

The role of synapsin I, a synaptic vesicle-associated phosphoprotein, in the maturation of nerve-muscle synapses was investigated in nerve-muscle co-cultures prepared from Xenopus embryos loaded with the protein by the early blastomere injection method. The stage of maturation of the synapses was analysed by electron microscopy as well as by whole-cell patch-clamp recording. The acceleration in the functional maturation of neuromuscular synapses induced by synapsin I was accompanied by a profound rearrangement in the ultrastructure of the nerve terminal. Nerve terminals formed by synapsin I-loaded neurons were characterized by a higher number of small synaptic vesicles organized in clusters and predominantly localized close to the nerve terminal plasma membrane, a smaller number of large dense-core vesicles and no significant change in the number of coated vesicles. Precocious development of active zone-like structures as well as deposition of basal lamina into the synaptic cleft were also observed at these synapses. These results support a role for synapsin I in the architectural changes which occur during synaptogenesis and lead to the maturation of quantal neurotransmitter release mechanisms.

Brain spectrin: of mice and men

This article reviews our current knowledge of the structure of alpha spectrins and beta spectrins in the brain, as well as their location and expression within neural tissue. We discuss the known protein interactions of brain spectrin isoforms, and then describe results that suggest an important role for spectrin (alpha SpII sigma 1/beta SpII sigma 1) in the Ca(2+)-regulated release of neurotransmitters. Evidence that supports a role for spectrin in the docking of synaptic vesicles to the presynaptic plasma membrane and as a Ca2+ sensor protein that unclamps the fusion machinery is described, along with the Casting the Line model, which summarizes the information. We finish with a discussion of the value of spectrin and ankyrin-deficient mouse models in deciphering spectrin function in neural tissue.

Synapsin I expression in spinal cord neurons during chick embryo development

The cellular distribution of synapsin I in chick spinal cord has been examined during embryo development and in cultured neurons from different developmental stages. Using immunocytochemical methods we have observed that synapsin I appears lightly detectable in spinal cord of embryonic day (E)5-E8 embryos when the motor neurons have already established functional contacts with muscle fibers, and increases at E9. Until E8 synapsin I immunoreactivity appeared mainly localized in the gray matter of spinal cord; immunostaining of white matter becomes clearly evident only at E9. These observations indicate that synapsin I expression and possibly its transport to the nerve terminals may be stimulated by sequential signals. The cellular distribution of synapsin I observed in vivo is maintained in E8 and E9 spinal cord neuron cell cultures. In fact, in E8 cultured neurons, synapsin I immunostaining is observed only in the cell body, while in E9 cultured neurons both cell body and fibers are stained. The addition of muscle extracts to E8 cultures induces synapsin I decoration of fibers similar to that observed in E9 cultured neurons. Indeed Western and Northern blot analysis and in situ hybridization demonstrate an increase of synapsin I and its mRNA in spinal cord neurons kept in the presence of muscle extracts. These data suggest that synapsin I expression, as previously reported for other neuronal markers, can be modulated by soluble factors present in target cells.

Fast and slow axonal transport-different methodological approaches give complementary information: contributions of the stop-flow/crush approach

This 'minireview' describes experiments in short term crush operated rat nerves, to study endogenous substances in anterograde and retrograde fast axonal transport. Immunofluorescence was used to recognize transported antigens, and cytofluorimetric scanning was employed to quantitate different antigens which had accumulated proximal and distal to the crushes. Vesicle membrane components p38 (synaptophysin) and SV2 accumulated on both sides of a crush. This was expected from a number of studies from different laboratories. Surface associated molecules, however, like synapsins and rab3a, have been studied by other groups with biochemical methods, and suggested to be transported with slow transport. The crush method, however, revealed that a considerable fraction of these two substances are transported with the fast transport system, and, thus, associated with fast transported organelles in the living neuron. Evidently, more than one technique is required to give a more complete picture of intraneuronal transport related events.

Protein phosphorylation and beta-cell function

The central role of reversible protein phosphorylation in regulation of beta-cell function is reviewed and the properties of the protein kinases so far defined in beta cells are summarised. The key effect of Ca2+ to initiate insulin secretion involves activation of a Ca2+/calmodulin-dependent protein kinase. Potentiation of secretion by agents activating protein kinase A or C appears to involve an increase in the sensitivity of the secretory system to intracellular Ca2+. The effects of MgATP on the binding of [3H]-glibenclamide to the beta-cell sulphonylurea receptor suggest that the properties of this receptor, which controls the activity of ATP-sensitive K-channels, are modulated by phosphorylation. The identity of the kinases and phosphatases responsible is not known but the presence in beta-cell membranes of various kinases not dependent on Ca2+ or cyclic AMP, and including tyrosine kinase, is documented, together with the presence of both Ca(2+)-dependent and Ca(2+)-independent protein phosphatases. Protein phosphorylation is also involved in regulation of beta-cell Ca2+ fluxes and evidence is presented that protein kinase C activation inhibits Ca2+ signalling by reducing influx of Ca2+ into the beta cell. The identity of the Ca2+/calmodulin-dependent protein kinase activity in beta cells is discussed. Comparison of its properties towards substrates and inhibitors with those of brain Ca2+/calmodulin-dependent protein kinase II suggests that the beta-cell enzyme may be similar or identical to the brain enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Remodeling of cytoskeletal architecture of nonneuronal cells induced by synapsin

The synapsins, a family of neuron-specific phosphoproteins, have been implicated in the functional and structural maturation of synapses. The cell biological basis for these effects is unknown. In vitro, the synapsins interact with cytoskeletal elements including actin. To examine, in vivo, the possible effect of the synapsins on cytoskeletal organization and cell morphology, we have transfected each of the four known members of the synapsin family into nonneuronal cells. We report here that synapsin expression in fibroblast cells gives rise to an alteration in cell morphology that is associated with formation of highly elongated processes. This morphological change is accompanied by a reorganization of filamentous actin (F-actin) characterized by disruption of existing stress fibers and formation of bundles of actin cables in the elongated processes. These results suggest that interactions of the synapsins with actin, and possibly with other cytoskeletal elements, may play a role in the morphological differentiation of neurons.

The formation of glutamatergic synapses in cultured central neurons: selective increase in miniature synaptic currents

The formation of synapses between cultured rat thalamic neurons was studied with electrophysiological and immunocytochemical methods. Thalamic neurons in culture form predominantly glutamatergic synapses. Already after 3 days in vitro glutamatergic miniature EPSCs occurred spontaneously and their frequency was strongly increased after K+ depolarization, while GABAergic mIPSCs were found after K+ depolarization at lower frequency. This demonstrates that both, excitatory glutamatergic and inhibitory GABAergic synapses were functional in close succession to initial neurite outgrowth. Synapses formed independent of spontaneous electrical activity, which was absent during the first week in culture. Spontaneous action potentials appeared during the second week and chronic action potential blockade by addition of tetrodotoxin reduced neuronal survival and the number of glutamatergic synapses per neuron. During in vitro differentiation the number of synapsin I immunoreactive presynaptic terminals and the frequency of spontaneous glutamatergic miniature EPSCs increased closely correlated, while the frequency of GABAergic mIPSCs after K+ depolarization did not increase. Thus, the continous formation of presynaptic terminals, including possible maturation of transmitter release, appeared to underlie the increase in mEPSC frequency. Analysis of miniature EPSC amplitudes at different stages in vitro revealed an increase in amplitudes, suggesting synaptic differentiation after initial establishment of functional transmission in glutamatergic synapses. This process was synapse specific as amplitudes of GABAergic mIPSCs were invariant.

Aberrant neurites and synaptic vesicle protein deficiency in synapsin II-depleted neurons

Synapsin I and synapsin II are neuron-specific phosphoproteins that have a role in the regulation of neurotransmitter release and in the formation of nerve terminals. After depletion of synapsin II by antisense oligonucleotides, rat hippocampal neurons in culture were unable to consolidate their minor processes and did not elongate axons. These aberrant morphological changes were accompanied by an abnormal distribution of intracellular filamentous actin (F-actin). In addition, synapsin II suppression resulted in a selective decrease in the amounts of several synaptic vesicle-associated proteins. These data suggest that synapsin II participates in cytoskeletal organization during the early stages of nerve cell development.

Characterization of GABAergic neurons in hippocampal cell cultures

The morphological characteristics of GABAergic neurons and the distribution of GABAergic synaptic terminals were examined in cultures of hippocampal neurons from 4-35 days in vitro. Neurons expressing GABA immunoreactivity represented about 6% of the total number of cultured neurons at all time points. Although the morphological characteristics of GABAergic cells suggested a heterogeneous population, GABAergic cells as a class were notably different from the non-GABAergic, presumably pyramidal cells. Most GABAergic cells had more fusiform or polygonal shaped somata, non-spiny and less tapering dendrites and appeared more phase-dense than nonGABAergic cells. Quantitative analysis revealed that GABAergic cells had fewer primary dendrites, more elongated dendritic arbors, and longer dendritic segments than non-GABAergic neurons-characteristics that are similar to GABAergic cells in situ. Double immunostaining revealed that GAD65-positive varicosities were also immunopositive for synapsin I, suggesting that GAD65-positive varicosities that contacted somata and dendrites represented presynaptic specializations. Confocal microscopy revealed the proportion of the synaptic specializations on the cell soma that were GAD65-positive was greater than on the dendrites, suggesting that somata and dendrites differ in their ability to induce the formation of presynaptic specializations by GABAergic axons. These data indicate that the GABAergic cells that develop in culture exhibit distinctive morphological characteristics and participate in different synaptic interactions that nonGABA cells. Thus many of the features that distinguish GABAergic neurons in culture are reminiscent of the characteristics that distinguish GABAergic neurons in situ.

Movement of axoplasmic organelles on actin filaments from skeletal muscle

It was recently shown that, in addition to the well-established microtubule-dependent mechanism, fast transport of organelles in squid giant axons also occurs in the presence of actin filaments [Kuznetsov et al., 1992, Nature 356:722-725]. The objectives of this study were to obtain direct evidence of axoplasmic organelle movement on actin filaments and to demonstrate that these organelles are able to move on skeletal muscle actin filaments. Organelles and actin filaments were visualized by video-enhanced contrast differential interference contrast (AVEC-DIC) microscopy and by video intensified fluorescence microscopy. Actin filaments, prepared by polymerization of monomeric actin purified from rabbit skeletal muscle, were stabilized with rhodamine-phalloidin and adsorbed to cover slips. When axoplasm was extruded on these cover slips in the buffer containing cytochalasin B that prevents the formation of endogenous axonal actin filaments, organelles were observed to move at the fast transport rate. Also, axoplasmic organelles were observed to move on bundles of actin filaments that were of sufficient thickness to be detected directly by AVEC-DIC microscopy. The range of average velocities of movement on the muscle actin filaments was not statistically different from that on axonal filaments. The level of motile activity (number of organelles moving/min/field) on the exogenous filaments was less than on endogenous filaments probably due to the entanglement of filaments on the cover slip surface. We also found that calmodulin (CaM) increased the level of motile activity of organelles on actin filaments. In addition, CaM stimulated the movement of elongated membranous organelles that appeared to be tubular elements of smooth endoplasmic reticulum or extensions of prelysosomes. These studies provide the first direct evidence that organelles from higher animal cells such as neurons move on biochemically defined actin filaments.

Stimulus-secretion coupling in pancreatic beta cells

Insulin secretion is triggered by a rise in the intracellular Ca2+ concentration that results from the activation of voltage-gated Ca2+ channels in the beta-cell plasma membrane. Multiple types of beta-cell Ca2+ channel have been identified in both electrophysiological and molecular biological studies, but it appears that the L-type Ca2+ channel plays a dominant role in regulating Ca2+ influx. Activity of this channel is potentiated by protein kinases A and C and is inhibited by GTP-binding proteins, which may mediate the effects of potentiators and inhibitors of insulin secretion on Ca2+ influx, respectively. The mechanisms by which elevation of intracellular Ca2+ leads to the release of insulin granules is not fully understood but appears to involve activation of Ca2+/calmodulin-dependent protein kinase. Phosphorylation by either protein kinase A or C, probably at different substrates, potentiates insulin secretion by acting at some late stage in the secretory process. There is also evidence that small GTP-binding proteins are involved in regulating exocytosis in beta cells. The identification and characterisation of the proteins involved in exocytosis in beta cells and clarification of the mechanism(s) of action of Ca2+ is clearly an important goal for the future.

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.

The 270 kDa splice variant of erythrocyte beta-spectrin (beta I sigma 2) segregates in vivo and in vitro to specific domains of cerebellar neurons

Spectrin isoforms arise from four distinct genes, three of which generate multiple alternative transcripts. With no biochemical restrictions on the assembly of alpha beta heterodimers, more than 25 distinct heterodimeric spectrin species may exist. Whether (and why) this subtle but substantial diversity is realized in any single cell is unknown. To address this question, sequence-specific antibodies to alternatively spliced regions of alpha- and beta-spectrin have been prepared. Reported here is the localization in rat cerebellar neurons at light and electron microscopic levels of an antibody against a unique sequence (beta I sigma 2-A = PGQHKDGQKSTGDERPT) from the 270 kDa transcript of the red cell beta-spectrin gene (spectrin beta I sigma 2). In this version, the 3′ sequence of erythroid beta-spectrin (beta I sigma 1) is replaced with an alternative sequence that shares substantial homology with the 3′ sequence of non-erythroid beta-spectrin (beta II sigma 1). The antibody to beta I sigma 2-A stains a single protein band at 270 kDa, determined by western blotting, in both rat cerebellum and in cultured cerebellar granule cells, and does not react with beta II sigma 1 spectrin (beta-fodrin). This antibody stains the dendritic spines of Purkinje cells in the molecular layer, and is concentrated at postsynaptic densities (PSDs) adjacent to synapsin I (which is confined to the presynaptic membrane). The soma of Purkinje cells do not stain. In the granular layer, cytoplasmic organelles and the postsynaptic densities of granular cells stain strongly. Astrocytes are also stained. In all cells, plasma membrane staining is confined to postsynaptic densities (PSD). The beta I sigma 2 isoform co-immunoprecipitates with non-erythroid alpha-spectrin (alpha II sigma), even though the distribution of alpha II sigma within neurons only partially overlaps that of beta I sigma 2. No hybrid beta I sigma 2 and beta II sigma 1 (beta-fodrin) spectrin complexes appear to exist. Spectrin beta I sigma 2 is also polarized in cultured rat cerebellar granule cells, where it is abundant in cell bodies but not neurites. The overall distribution of beta I sigma 2 is as a subset of the distribution of spectrins 240/235E previously detected with a generally reactive erythrocyte alpha beta-spectrin antibody. These findings establish the highly precise segregation of a beta-spectrin isoform to distinct cytoplasmic and membrane surface domains, indicate that it is complexed (partially) with non-erythroid alpha-spectrin, and demonstrate that cytoskeletal targeting mechanisms are preserved in cultured granular cells.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Comparison of immunolocalization patterns for the synaptic vesicle proteins p65 and synapsin I in macaque monkey retina

The distributions of the two synaptic vesicle proteins p65 [Matthew et al. (1981) J. Cell Biol., 91:257-269] and synapsin I [De Camilli et al. (1983) J. Cell Biol., 96:1337-1354] were compared in macaque monkey retina using pre-embedding immunocytochemistry for both light and electron microscopy. The monoclonal antibody AB-48 against p65 labeled ribbon-containing synaptic terminals of cone, rod, and bipolar cells as well as many conventional synapses of amacrine cells. In contrast, a polyclonal antiserum against synapsin I (SYN I) labeled many amacrine conventional synapses but no photoreceptor or bipolar ribbon synaptic terminals. Horizontal cell pre- and post-synaptic profiles in the outer plexiform layer were not labeled by either antibody. At the light microscopic level, the banding patterns in the inner plexiform layer also differed for the two antibodies, with four bands of AB-48 immunoreactivity in sublayers S1, S2, S4, and S5 but only three bands of SYN I immunoreactivity in S1, S3, and S5. SYN I also labeled varicose fibers in both the inner nuclear layer and the outer plexiform layer that are probably processes of dopaminergic and GABAergic interplexiform cells. Varicose fibers in the ganglion cell layer were labeled by both antibodies. These results provide the first electron microscopic immunocytochemical labeling for AB-48 and SYN I in intact retina and confirm that AB-48 labels both ribbon and conventional synaptic terminals, whereas SYN I labels only conventional synapses.

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.

Calcium-dependent serine phosphorylation of synaptophysin

The phosphorylation of synaptophysin, a major integral membrane protein of small synaptic vesicles, was found to be regulated in a Ca(2+)-dependent manner in rat cerebrocortical slices, synaptosome preparations, and highly purified synaptic vesicles isolated from rat forebrain. K(+)-induced depolarization of slices and synaptosomes prelabeled with 32P-orthophosphate produced a rapid, transient increase in serine phosphorylation of synaptophysin. In synaptosomes, the depolarization-dependent increase in synaptophysin phosphorylation required the presence of external Ca2+ in the incubation medium. The addition of Ca2+ plus calmodulin to purified synaptic vesicles resulted in a 4-fold increase in serine phosphorylation of synaptophysin, and this phosphorylation was antagonized by a peptide inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II(. Purified rat forebrain CaM kinase II phosphorylated both purified synaptophysin and endogenous, vesicle-associated synaptophysin, and the resulting 2-dimensional chymotryptic phosphopeptide maps were similar to those derived from synaptophysin phosphorylated in cerebrocortical slices. These data demonstrate that Ca(2+)-dependent phosphorylation of synaptophysin, mediated by CaM kinase II, occurs under physiological conditions.

Neuronotypic differentiation results in reduced levels and altered distribution of synaptophysin in PC12 cells

Several synaptic vesicle proteins including synaptophysin and p65/synaptotagmin are expressed by the pheochromocytoma cell line PC12. Stimulation of these cells with nerve growth factor for 7 days induces morphologic neuronotypic differentiation, but the levels of synaptophysin are markedly reduced. Stimulation with cyclic AMP analogs also produces neuronotypic differentiation of PC12 cells, and the degree of morphologic differentiation induced by these agents parallels their ability to effect reduction in synaptophysin levels. By contrast, levels of p65/synaptotagmin are increased following neuronotypic differentiation. The contrasting effects of neuronotypic differentiation on levels of synaptophysin and p65/synaptotagmin indicate potential differences in the regulation of these proteins in PC12 cells. Immunocytochemical labeling of undifferentiated PC12 cells reveals concentrations of synaptophysin in the perinuclear region. After neuronotypic differentiation, there is reduction in perinuclear labeling and concentration of label in swellings along PC12 cell processes. At the ultrastructural level, synaptophysin labeling is found on similar organelles in both undifferentiated and nerve growth factor-stimulated PC12 cells. Although the highest labeling densities were seen on small clear vesicles, specific labeling was also seen on dense core vesicles. The presence of synaptophysin on both small clear vesicles and dense core vesicles indicates potential functional similarities in these vesicle types. The changes in the levels and immunocytochemical distribution of synaptophysin after neuronotypic differentiation suggest possible functional heterogeneity among morphologically similar populations of small clear vesicles.

Synapsin I gene expression in the adult rat brain with comparative analysis of mRNA and protein in the hippocampus

Synapsin I is the best characterized member of a family of neuron-specific phosphoproteins thought to be involved in the regulation of neurotransmitter release. In this report, we present the first extensive in situ hybridization study detailing the regional and cellular distribution of synapsin I mRNA in the adult rat brain. Both the regional distribution and relative levels of synapsin I mRNA established by in situ hybridization were confirmed by RNA blot analysis. Our data demonstrate the widespread yet regionally variable expression of synapsin I mRNA throughout the adult rat brain. The greatest abundance of synapsin I mRNA was found in the pyramidal neurons of the CA3 and CA4 fields of the hippocampus, and in the mitral and internal granular cell layers of the olfactory bulb. Other areas abundant in synapsin I mRNA were the layer II neurons of the piriform cortex and layer II and V neurons of the entorhinal cortex, the granule cell neurons of the dentate gyrus, the pyramidal neurons of hippocampal fields CA1 and CA2, and the cells of the parasubiculum. In general, the pattern of expression of synapsin I mRNA paralleled those encoding other synaptic terminal-specific proteins, such as synaptophysin, VAMP-2, and SNAP-25, with noteworthy exceptions. To determine specifically how synapsin I mRNA levels are related to levels of synapsin I protein, we examined in detail the local distribution patterns of both synapsin I mRNA and protein in the rat hippocampus. These data revealed differential levels of expression of synapsin I mRNA and protein within defined synaptic circuits of the rat hippocampus.

Synapsin I, synapsin II, and synaptophysin: marker proteins of synaptic vesicles

The nerve terminal of neurons is filled with small synaptic vesicles, specialized secretory organelles involved in the storage and release of neurotransmitters. The synapsins are a family of four proteins that are the major peripheral proteins on the cytoplasmic face of synaptic vesicles. Synaptophysin is the major integral membrane protein of synaptic vesicles. The characterization of the synapsins and of synaptophysin during the last years has revealed exciting information about their structure, regulation and possible function. To understand the role of the synapsins and synaptophysin in the biology of a nerve cell means to elucidate the fundamental mechanism of brain function, the release of neurotransmitter.

Fluorescence approaches to the study of the actin-nucleating and bundling activities of synapsin I

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

Regulation and role of brain calcium/calmodulin-dependent protein kinase II

Ca2+/calmodulin-dependent protein kinase II (CaMKII) exhibits a broad substrate specificity and regulates diverse responses to physiological changes of intracellular Ca2+ concentrations. Five isozymic subunits of the highly abundant brain kinase are encoded by four distinct genes. Expression of each gene is tightly regulated in a cell-specific and developmental manner. CaMKII immunoreactivity is broadly distributed within neurons but is discretely associated with a number of subcellular structures. The unique regulatory properties of CaMKII have attracted a lot of attention. Ca2+/calmodulin-dependent autophosphorylation of a specific threonine residue (alpha-Thr286) within the autoinhibitory domain generates partially Ca(2+)-independent CaMKII activity. Phosphorylation of this threonine in CaMKII is modulated by changes in intracellular Ca2+ concentrations in a variety of cells, and may prolong physiological responses to transient increases in Ca2+. Additional residues within the calmodulin-binding domain are autophosphorylated in the presence of Ca2+ chelators and block activation by Ca2+/calmodulin. This Ca(2+)-independent autophosphorylation is very rapid following prior Ca2+/calmodulin-dependent autophosphorylation at alpha-Thr286 and generates constitutively active, Ca2+/calmodulin-insensitive CaMKII activity. Ca(2+)-independent autophosphorylation of CaMKII also occurs at a slower rate when alpha-Thr286 is not autophosphorylated and results in inactivation of CaMKII. Thus, Ca(2+)-independent autophosphorylation of CaMKII generates a form of the kinase that is refractory to activation by Ca2+/calmodulin. CaMKII phosphorylates a wide range of neuronal proteins in vitro, presumably reflecting its involvement in the regulation of diverse functions such as postsynaptic responses (e.g. long-term potentiation), neurotransmitter synthesis and exocytosis, cytoskeletal interactions and gene transcription. Recent evidence indicates that the levels of CaMKII are altered in pathological states such as Alzheimer's disease and also following ischemia.

Inhibition of phosphorylation of synapsin I and other synaptosomal proteins by beta-bungarotoxin, a phospholipase A2 neurotoxin

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX), which possess relatively low phospholipase A2 (PLA2) activity, act presynaptically to alter acetylcholine (ACh) release both in the periphery and in the CNS. In investigating the mechanism of this action, we found that beta-BuTX (5 and 15 nM) inhibited phosphorylation, in both resting and depolarized synaptosomes, of a wide range of proteins, including synapsin I. Naja naja atra PLA2, which has higher PLA2 activity, also inhibited phosphorylation but was less potent than beta-BuTX. At 1 nM, beta-BuTX and N. n. atra PLA2 inhibited phosphorylation of synapsin I only in depolarized synaptosomes. Synaptosomal ATP levels were not affected by 5 or 15 nM beta-BuTX or by 5 nM N. n. atra PLA2. Limited proteolysis, using Staphylococcus aureus V-8 protease, indicated that beta-BuTX inhibited phosphorylation of synapsin I in both the head and the tail regions. The inhibition of phosphorylation was not antagonized by nordihydroguaiaretic acid or indomethacin, suggesting that arachidonic acid derivatives do not mediate this inhibition. Furthermore, inhibition of phosphorylation by beta-BuTX and N. n. atra PLA2 was not altered in the presence of the phosphatase inhibitor okadaic acid, suggesting that stimulation of phosphatase activity is not responsible for this inhibition. Inhibition of protein phosphorylation by PLA2 neurotoxins and enzymes may be associated with an inhibition of ACh release.

Synapsin I partially dissociates from synaptic vesicles during exocytosis induced by electrical stimulation

The distribution of the synaptic vesicle-associated phosphoprotein synapsin I after electrical stimulation of the frog neuromuscular junction was investigated by immunogold labeling and compared with the distribution of the integral synaptic vesicle protein synaptophysin. In resting terminals both proteins were localized exclusively on synaptic vesicles. In stimulated terminals they appeared also in the axolemma and its infoldings, which however exhibited a lower synapsin I/synaptophysin ratio with respect to synaptic vesicles at rest. The value of this ratio was intermediate in synaptic vesicles of stimulated terminals, and an increased synapsin I labeling of the cytomatrix was observed. These results indicate that synapsin I undergoes partial dissociation from and reassociation with synaptic vesicles, following physiological stimulation, and are consistent with the proposed modulatory role of the protein in neurotransmitter release.

Identification of the ATP.Mg-dependent protein phosphatase activator (FA) as a synapsin I kinase that inhibits cross-linking of synapsin I with brain microtubules

The ATP.Mg-dependent protein phosphatase activating factor (FA) has been identified and purified to near homogeneity from brain. In this report, as evidenced on SDS-polyacrylamide gel electrophoresis followed by autoradiography, factor FA has further been identified as a cAMP and Ca(2+)-independent brain kinase that could phosphorylate synapsin I, a neuronal protein that coats synaptic vesicles, binds to cytoskeleton, and is believed to be involved in the modulation of neurotransmission. Kinetic study further indicated that factor FA could phosphorylate synapsin I with a low Km value of about 2 microM and with a molar ratio of 1 mol of phosphate per mole of protein. Peptide mapping analysis revealed that factor FA specifically phosphorylated the tail region of synapsin I but on a unique site distinct from those phosphorylated by Ca2+/calmodulin-dependent protein kinase II and cAMP-dependent protein kinase, the two well-established synapsin I kinases. Functional study further revealed that factor FA could phosphorylate this unique specific site on the tail region of synapsin I and thereby inhibit cross-linking of synapsin I with microtubules. The results further suggest the possible involvement of factor FA as a synapsin I kinase in the regulation of axonal transport process of synaptic vesicles via the promotion of vesicles motility during neurotransmission.

Neuronal compartments and axonal transport of synapsin I

Studies on the transport kinetics and the posttranslational modification of synapsin I in mouse retinal ganglion cells were performed to obtain an insight into the possible factors involved in forming the structural and functional differences between the axon and its terminals. Synapsin I, a neuronal phosphoprotein associated with small synaptic vesicles and cytoskeletal elements at the presynaptic terminals, is thought to be involved in modulating neurotransmitter release. The state of phosphorylation of synapsin I in vitro regulates its interaction with both synaptic vesicles and cytoskeletal components, including microtubules and microfilaments. Here we present the first evidence that in the mouse retinal ganglion cells most synapsin I is transported down the axon, together with the cytomatrix proteins, at the same rate as the slow component b of axonal transport, and is phosphorylated at both the head and tail regions. In addition, our data suggest that, after synapsin I has reached the nerve endings, the relative proportions of variously phosphorylated synapsin I molecules change, and that these changes lead to a decrease in the overall content of phosphorus. These results are consistent with the hypothesis that, in vivo, the phosphorylation of synapsin I along the axon prevents the formation of a dense network that could impair organelle movement. On the other hand, the dephosphorylation of synapsin I at the nerve endings may regulate the clustering of small synaptic vesicles and modulate neurotransmitter release by controlling the availability of small synaptic vesicles for exocytosis.

Organelles in fast axonal transport. What molecules do they carry in anterograde vs retrograde directions, as observed in mammalian systems?

The present minireview describes experiments carried out, in short-term crush-operated rat nerves, using immunofluorescence and cytofluorimetric scanning techniques to study endogenous substances in anterograde and retrograde fast axonal transport. Vesicle membrane components p38 (synaptophysin) and SV2 are accumulating on both sides of a crush, but a larger proportion of p38 (about 3/4) than of SV2 (about 1/2) is recycling toward the cell body, compared to the amount carried with anterograde transport. Matrix peptides, such as CGRP, ChRA, VIP, and DBH are recycling to a minor degree, although only 10-20% of surface-associated molecules, such as synapsins and kinesin, appear to recycle. The described methodological approach to study the composition of organelles in fast axonal transport, anterograde as compared to retrograde, is shown to be useful for investigating neurobiological processes. We make use of the "in vivo chromatography" process that the fast axonal transport system constitutes. Only substances that are in some way either stored in, or associated with, transported organelles can be clearly observed to accumulate relative to the crush region. Emphasis in this paper was given to the synapsins, because of diverging results published concerning the degree of affiliation with various neuronal organelles. Our previously published results have indicated that in the living axons the SYN I is affiliated with mainly anterogradely fast transported organelles. Therefore, some preliminary, previously unpublished results on the accumulations of the four different synapsins (SYN Ia, SYN Ib, SYN IIa, and SYN IIb), using antisera specific for each of the four members of the synapsin family, are described. It was found that SYN Ib clearly has a stronger affiliation to anterogradely transported organelles than SYN Ia, and that both SYN IIa and SYN IIb are bound to some degree to transported organelles.

Synapsin I regulates glutamate release from rat brain synaptosomes

Introduction of the dephosphorylated from of synapsin I into rat brain synaptosomes using freeze-thaw (transient) permeabilization significantly decreased the K(+)-induced release of glutamate. In contrast, introduction of synapsin I that had been phosphorylated by Ca2+/calmodulin-dependent protein kinase II was without effect on glutamate release. Addition of dephosphosynapsin I after freeze-thaw treatment also had no effect. Thus, the action of synapsin I was dependent on the phosphorylation state of synapsin I and on its entry into the synaptosomes. Our results implicate synapsin I as an important component in the regulation of neurotransmitter release in the mammalian nervous system.

Interaction of free and synaptic vesicle-bound synapsin I with F-actin

Synapsin I is a neuron-specific phosphoprotein that binds to small synaptic vesicles and F-actin in a phosphorylation-dependent fashion. We have found that dephosphorylated synapsin I induces a dose-dependent increase in the number of actin filaments, which at high ionic strength is abolished by synapsin I phosphorylation. The increase in filament number appears to be due to a nucleating effect of synapsin I and not to a barbed-end capping/severing activity. Synaptic vesicle-bound synapsin I was as effective as free synapsin I in increasing the number of filaments. These data support the view that synapsin I is involved in the regulation of the dynamics of the actin-based network during the exo-endocytotic cycle.

The synapsin I brain distribution in ischemia

We examined the distribution of synapsin I in the gerbil brain and investigated ischemic damage of presynaptic terminals immunohistochemically by using this protein as a marker protein of synaptic vesicles. The reaction for synapsin I in normal gerbil brain is exclusively localized in the neuropil, and other brain structures such as neuronal soma, dendrites, axon bundles, glia and endothelial cells exhibited little immunoreactivity. In a reproducible gerbil model of unilateral cerebral ischemia, ischemic loss of synapsin I immunoreactivity in the affected hemisphere was confined to the area exhibiting overt infarction, where the breakdown of this protein was also confirmed by the immunoblot analysis, and noted much later than that of microtubule-associated protein 2 immunoreactivity, which was demonstrated in neuronal soma and dendrites. In the non-affected hemisphere, selective damage of presynaptic terminals due to Wallerian degeneration and subsequently occurring resynaptogenesis at the molecular layer of the dentate gyrus were clearly demonstrated as a loss and recovery of immunoreaction for synapsin I, respectively. In a gerbil model of bilateral cerebral ischemia, immunoreaction for synapsin I was persistently preserved after seven days to two months recirculation following a brief period of global forebrain ischemia in the CA1 region of the hippocampus, where delayed neuronal death was consistently observed.(ABSTRACT TRUNCATED AT 250 WORDS)