Synapsin I is a spectrin-binding protein immunologically related to erythrocyte protein 4.1

The Role of Calmodulin vs. Synaptotagmin in Exocytosis

Exocytosis is a Ca²⁺-regulated process that requires the participation of Ca²⁺ sensors. In the 1980s, two classes of Ca²⁺-binding proteins were proposed as putative Ca²⁺ sensors: EF-hand protein calmodulin, and the C2 domain protein synaptotagmin. In the next few decades, numerous studies determined that in the final stage of membrane fusion triggered by a micromolar boost in the level of Ca²⁺, the low affinity Ca²⁺-binding protein synaptotagmin, especially synaptotagmin 1 and 2, acts as the primary Ca²⁺ sensor, whereas calmodulin is unlikely to be functional due to its high Ca²⁺ affinity. However, in the meantime emerging evidence has revealed that calmodulin is involved in the earlier exocytotic steps prior to fusion, such as vesicle trafficking, docking and priming by acting as a high affinity Ca²⁺ sensor activated at submicromolar level of Ca²⁺. Calmodulin directly interacts with multiple regulatory proteins involved in the regulation of exocytosis, including VAMP, myosin V, Munc13, synapsin, GAP43 and Rab3, and switches on key kinases, such as type II Ca²⁺/calmodulin-dependent protein kinase, to phosphorylate a series of exocytosis regulators, including syntaxin, synapsin, RIM and Ca²⁺ channels. Moreover, calmodulin interacts with synaptotagmin through either direct binding or indirect phosphorylation. In summary, calmodulin and synaptotagmin are Ca²⁺ sensors that play complementary roles throughout the process of exocytosis. In this review, we discuss the complementary roles that calmodulin and synaptotagmin play as Ca²⁺ sensors during exocytosis.

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.

Autoantibodies in the Extraintestinal Manifestations of Celiac Disease

Increased antibody reactivity towards self-antigens is often indicative of a disruption of homeostatic immune pathways in the body. In celiac disease, an autoimmune enteropathy triggered by the ingestion of gluten from wheat and related cereals in genetically predisposed individuals, autoantibody reactivity to transglutaminase 2 is reflective of the pathogenic role of the enzyme in driving the associated inflammatory immune response. Autoantibody reactivity to transglutaminase 2 closely corresponds with the gluten intake and clinical presentation in affected patients, serving as a highly useful biomarker in the diagnosis of celiac disease. In addition to gastrointestinal symptoms, celiac disease is associated with a number of extraintestinal manifestations, including those affecting skin, bones, and the nervous system. Investigations of these manifestations in celiac disease have identified a number of associated immune abnormalities, including B cell reactivity towards various autoantigens, such as transglutaminase 3, transglutaminase 6, synapsin I, gangliosides, and collagen. Clinical relevance, pathogenic potential, mechanism of development, and diagnostic and prognostic value of the various identified autoantibody reactivities continue to be subjects of investigation and will be reviewed here.

Synapsin regulation of vesicle organization and functional pools

Synaptic vesicles are organized in clusters, and synapsin maintains vesicle organization and abundance in nerve terminals. At the functional level, vesicles can be subdivided into three pools: the releasable pool, the recycling pool, and the reserve pool, and synapsin mediates transitions between these pools. Synapsin directs vesicles into the reserve pool, and synapsin II isoform has a primary role in this function. In addition, synapsin actively delivers vesicles to active zones. Finally, synapsin I isoform mediates coupling release events to action potentials at the latest stages of exocytosis. Thus, synapsin is involved in multiple stages of the vesicle cycle, including vesicle clustering, maintaining the reserve pool, vesicle delivery to active zones, and synchronizing release events. These processes are regulated via a dynamic synapsin phosphorylation/dephosphorylation cycle which involves multiple phosphorylation sites and several pathways. Different synapsin isoforms have unique and non-redundant roles in the multifaceted synapsin function.

The synapsins: multitask modulators of neuronal development

Neurons are examples of specialized cells that evolved the extraordinary ability to transmit electrochemical information in complex networks of interconnected cells. During their development, neurons undergo precisely regulated processes that define their lineage, positioning, morphogenesis and pattern of activity. The events leading to the establishment of functional neuronal networks follow a number of key steps, including asymmetric cell division from neuronal precursors, migration, establishment of polarity, neurite outgrowth and synaptogenesis. Synapsins are a family of abundant neuronal phosphoproteins that have been extensively studied for their role in the regulation of neurotransmission in presynaptic terminals. Beside their implication in the homeostasis of adult cells, synapsins influence the development of young neurons, interacting with cytoskeletal and vesicular components and regulating their dynamics. Although the exact molecular mechanisms determining synapsin function in neuronal development are still largely unknown, in this review we summarize the most important literature on the subject, providing a conceptual framework for the progress of present and future research.

Membrane trafficking events underlying axon repair, growth, and regeneration

Two central challenges for the field of neurobiology are to understand how axons grow and make proper synaptic connections under normal conditions and how they repair their membranes and mount regenerative responses after injury. At the most reductionist level, the first step toward addressing these challenges is to delineate the cellular and molecular processes by which an axon extends from its cell body. Underlying axon extension are questions of appropriate timing and mechanisms that establish or maintain the axon's polarity, initiate growth cone formation, and promote axon outgrowth and synapse formation. After injury, the problem is even more complicated because the neuron must also repair its damaged membrane, redistribute or manufacture what it needs in order to survive, and grow and form new synapses within a more mature, complex environment. While other reviews have focused extensively on the signaling events and cytoskeletal rearrangements that support axon outgrowth and regeneration, we focus this review instead on the underlying membrane trafficking events underlying these processes. Though the mechanisms are still under active investigation, the key roles played by membrane trafficking events during axon repair, growth, and regeneration have been elucidated through elegant comparative studies in both invertebrate and vertebrate organisms. Taken together, a model emerges indicating that the critical requirements for ensuring proper membrane sealing and axon extension include iterative bouts of SNARE mediated exocytosis, endocytosis, and functional links between vesicles and the actin cytoskeleton, similar to the mechanisms utilized during synaptic transmission. This article is part of a Special Issue entitled 'Neuronal Function'.

Mass spectrometric analysis of synapsins in Drosophila melanogaster and identification of novel phosphorylation sites

Synapsins are synaptic vesicle-associated phosphoproteins that play a major role in the fine regulation of neurotransmitter release. In Drosophila, synapsins are required for complex behavior including learning and memory. Synapsin isoforms were immunoprecipitated from homogenates of wild-type Drosophila heads using monoclonal antibody 3C11. Synapsin null mutants (Syn(97)) served as negative controls. The eluted proteins were separated by SDS-PAGE and visualized by silver staining. Gel pieces picked from five bands specific for wild type were analyzed by nano-LC-ESI-MS/MS following multienzyme digestion (trypsin, chymotrypsin, AspN, subtilisin, pepsin, and proteinase K). The protein was unambiguously identified with high sequence coverage (90.83%). A number of sequence conflicts were observed and the N-terminal amino acid was identified as methionine rather than leucine expected from the cDNA sequence. Several peptides from the larger isoform demonstrated that the in-frame UAG stop codon at position 582 which separates two large open reading frames is read through by tRNAs for lysine. Seven novel phosphorylation sites in Drosophila synapsin were identified at Thr-86, Ser-87, Ser-464, Thr-466, Ser-538, Ser-961, and Tyr-982 and verified by phosphatase treatment. No phosphorylation was observed at the conserved PKA/CaM kinase-I/IV site (RRFS, edited to RGFS) in domain A or a potential PKA site near domain E.

Gallium nitride induces neuronal differentiation markers in neural stem/precursor cells derived from rat cerebral cortex

In the present study, gallium nitride (GaN) was used as a substrate to culture neural stem/precursor cells (NSPCs), isolated from embryonic rat cerebral cortex, to examine the effect of GaN on the behavior of NSPCs in the presence of basic fibroblast growth factor (bFGF) in serum-free medium. Morphological studies showed that neurospheres maintained their initial shape and formed many long and thick processes with the fasciculate feature on GaN. Immunocytochemical characterization showed that GaN could induce the differentiation of NSPCs into neurons and astrocytes. Compared to poly-d-lysine (PDL), the most common substrate used for culturing neurons, there was considerable expression of synapsin I for differentiated neurons on GaN, suggesting GaN could induce the differentiation of NSPCs towards the mature differentiated neurons. Western blot analysis showed that the suppression of glycogen synthase kinase-3beta (GSK-3beta) activity was one of the effects of GaN-promoted NSPC differentiation into neurons. Finally, compared to PDL, GaN could significantly improve cell survival to reduce cell death after long-term culture. These results suggest that GaN potentially has a combination of electric characteristics suitable for developing neuron and/or NSPC chip systems.

A Golgi-associated protein 4.1B variant is required for assimilation of proteins in the membrane

The archetypal membrane skeleton is that of the erythrocyte, consisting predominantly of spectrin, actin, ankyrin R and protein 4.1R. The presence in the Golgi of a membrane skeleton with a similar structure has been inferred, based on the identification of Golgi-associated spectrin and ankyrin. It has long been assumed that a Golgi-specific protein 4.1 must also exist, but it has not previously been found. We demonstrate here that a hitherto unknown form of protein 4.1, a 200 kDa 4.1B, is associated with the Golgi of Madin-Darby canine kidney (MDCK) and human bronchial epithelial (HBE) cells. This 4.1B variant behaves like a Golgi marker after treatment with Brefeldin A and during mitosis. Depletion of the protein in HBE cells by siRNA resulted in disruption of the Golgi structure and failure of Na(+)/K(+)-ATPase, ZO-1 and ZO-2 to migrate to the membrane. Thus, this newly identified Golgi-specific protein 4.1 appears to have an essential role in maintaining the structure of the Golgi and in assembly of a subset of membrane proteins.

Cytoskeletal requirements in axonal transport of slow component-b

Slow component-b (SCb) translocates approximately 200 diverse proteins from the cell body to the axon and axon tip at average rates of approximately 2-8 mm/d. Several studies suggest that SCb proteins are cotransported as one or more macromolecular complexes, but the basis for this cotransport is unknown. The identification of actin and myosin in SCb led to the proposal that actin filaments function as a scaffold for the binding of other SCb proteins and that transport of these complexes is powered by myosin: the "microfilament-complex" model. Later, several SCb proteins were also found to bind F-actin, supporting the idea, but despite this, the model has never been directly tested. Here, we test this model by disrupting the cytoskeleton in a live-cell model system wherein we directly visualize transport of SCb cargoes. We focused on three SCb proteins that we previously showed were cotransported in our system: alpha-synuclein, synapsin-I, and glyceraldehyde-3-phosphate dehydrogenase. Disruption of actin filaments with latrunculin had no effect on the velocity or frequency of transport of these three proteins. Furthermore, cotransport of these three SCb proteins continued in actin-depleted axons. We conclude that actin filaments do not function as a scaffold to organize and transport these and possibly other SCb proteins. In contrast, depletion of microtubules led to a dramatic inhibition of vectorial transport of SCb cargoes. These findings do not support the microfilament-complex model, but instead indicate that the transport of protein complexes in SCb is powered by microtubule motors.

Synapsin maintains the reserve vesicle pool and spatial segregation of the recycling pool in Drosophila presynaptic boutons

We employed optical detection of the lipophylic dye FM1-43 and focal recordings of quantal release to investigate how synapsin affects vesicle cycling at the neuromuscular junction of synapsin knockout (Syn KO) Drosophila. Loading the dye employing high K+ stimulation, which presumably involves the recycling pool of vesicles in exo/endocytosis, stained the periphery of wild type (WT) boutons, while in Syn KO the dye was redistributed towards the center of the bouton. When endocytosis was promoted by cyclosporin A pretreatment, the dye uptake was significantly enhanced in WT boutons, and the entire boutons were stained, suggesting staining of the reserve vesicle pool. In Syn KO boutons, the same loading paradigm produced fainter staining and significantly faster destaining. When the axon was stimulated electrically, a distinct difference in dye loading patterns was observed in WT boutons at different stimulation frequencies: a low stimulation frequency (3 Hz) produced a ring-shaped staining pattern, while at a higher frequency (10 Hz) the dye was redistributed towards the center of the bouton and the fluorescence intensity was significantly increased. This difference in staining patterns was essentially disrupted in Syn KO boutons, although synapsin did not affect the rate of quantal release. Stimulation of the nerve in the presence of bafilomycin, the blocker of the transmitter uptake, produced significantly stronger depression in Syn KO boutons. These results, taken together, suggest that synapsin maintains the reserve pool of vesicles and segregation between the recycling and reserve pools, and that it mediates mobilization of the reserve pool during intense stimulation.

Immune cross-reactivity in celiac disease: anti-gliadin antibodies bind to neuronal synapsin I

Celiac disease is an immune-mediated disorder triggered by ingestion of wheat gliadin and related proteins in genetically susceptible individuals. In addition to the characteristic enteropathy, celiac disease is associated with various extraintestinal manifestations, including neurologic complications such as neuropathy, ataxia, seizures, and neurobehavioral changes. The cause of the neurologic manifestations is unknown, but autoimmunity resulting from molecular mimicry between gliadin and nervous system proteins has been proposed to play a role. In this study, we sought to investigate the immune reactivity of the anti-gliadin Ab response toward neural proteins. We characterized the binding of affinity-purified anti-gliadin Abs from immunized animals to brain proteins by one- and two-dimensional gel electrophoresis, immunoblotting, and peptide mass mapping. The major immunoreactive protein was identified as synapsin I. Anti-gliadin Abs from patients with celiac disease also bound to the protein. Such cross-reactivity may provide clues into the pathogenic mechanism of the neurologic deficits that are associated with gluten sensitivity.

Structural domains involved in the regulation of transmitter release by synapsins

Synapsins are a family of neuron-specific phosphoproteins that regulate neurotransmitter release by associating with synaptic vesicles. Synapsins consist of a series of conserved and variable structural domains of unknown function. We performed a systematic structure-function analysis of the various domains of synapsin by assessing the actions of synapsin fragments on neurotransmitter release, presynaptic ultrastructure, and the biochemical interactions of synapsin. Injecting a peptide derived from domain A into the squid giant presynaptic terminal inhibited neurotransmitter release in a phosphorylation-dependent manner. This peptide had no effect on vesicle pool size, synaptic depression, or transmitter release kinetics. In contrast, a peptide fragment from domain C reduced the number of synaptic vesicles in the periphery of the active zone and increased the rate and extent of synaptic depression. This peptide also slowed the kinetics of neurotransmitter release without affecting the number of docked vesicles. The domain C peptide, as well as another peptide from domain E that is known to have identical effects on vesicle pool size and release kinetics, both specifically interfered with the binding of synapsins to actin but not with the binding of synapsins to synaptic vesicles. This suggests that both peptides interfere with release by preventing interactions of synapsins with actin. Thus, interactions of domains C and E with the actin cytoskeleton may allow synapsins to perform two roles in regulating release, whereas domain A has an actin-independent function that regulates transmitter release in a phosphorylation-sensitive manner.

Cytoskeletal interactions of synapsin I in non-neuronal cells

Synapsin I is a neuronal phosphoprotein involved in the localization and stabilization of synaptic vesicles. Recently, synapsin I has been detected in several non-neuronal cell lines, but its function in these cells is unclear. To determine the localization of synapsin I in non-neuronal cells, it was transiently expressed in HeLa and NIH/3T3 cells as an enhanced green fluorescent protein fusion protein. Synapsin I-enhanced green fluorescent protein colocalized with F-actin in both cell lines, particularly with microspikes and membrane ruffles. It did not colocalize with microtubules or vimentin and it did not cause major alterations in cytoskeletal organization. Synapsin Ia-enhanced green fluorescent protein colocalized with microtubule bundles in taxol-treated HeLa cells and with F-actin spots at the plasma membrane in cells treated with cytochalasin B. It did not noticeably affect F-actin reassembly following drug removal. Synapsin Ia-enhanced green fluorescent protein remained colocalized with F-actin in cells treated with nocodazole, and it did not affect reassembly of microtubules following drug removal. These results demonstrate that synapsin I interacts with F-actin in non-neuronal cells and suggest that synapsin I may have a role in regions where actin is highly dynamic.

Synapsin I is expressed in epithelial cells: localization to a unique trans-Golgi compartment

Synapsin I is abundant in neural tissues. Its phosphorylation is thought to regulate synaptic vesicle exocytosis in the pre-synaptic terminal by mediating vesicle tethering to the cytoskeleton. Using anti-synapsin antibodies, we detected an 85 kDa protein in liver cells and identified it as synapsin I. Like brain synapsin I, non-neuronal synapsin I is phosphorylated in vitro by protein kinase A and yields identical 32P-peptide maps after limited proteolysis. We also detected synapsin I mRNA in liver by northern blot analysis. These results indicate that the expression of synapsin I is more widespread than previously thought. Immunofluorescence analysis of several non-neuronal cell lines localizes synapsin I to a vesicular compartment adjacent to trans-elements of the Golgi complex, which is also labeled with antibodies against myosin II; no sub-plasma membrane synapsin I is evident. We conclude that synapsin I is present in epithelial cells and is associated with a trans-Golgi network-derived compartment; this localization suggests that it plays a role in modulating post-TGN trafficking pathways.

The domain of brain beta-spectrin responsible for synaptic vesicle association is essential for synaptic transmission

We have examined the interaction between synapsin I, the major phosphoprotein on the membrane of small synaptic vesicles, and brain spectrin. Using recombinant peptides we have localized the synapsin I attachment site upon the beta-spectrin isoform betaSpIISigmaI to a region of 25 amino acids, residues 211 through 235. This segment is adjacent to the actin binding domain and is within the region of the betaSpIISigmaI that we previously predicted as a candidate synapsin I binding domain based upon sequence homology. We used differential centrifugation techniques to quantitatively assess the interaction of spectrin with synaptic vesicles. Using this assay, high affinity saturable binding of recombinant betaSpIISigmaI proteins was observed with synaptic vesicles. Binding was only observed when the 25 amino acid synapsin I binding site was included on the recombinant peptides. Further, we demonstrate that antibodies directed against 15 amino acids of the synapsin I binding domain specifically blocked synaptic transmission in cultured hippocampal neurons. Thus, the synapsin I attachment site on betaSpIISigmaI spectrin comprises a approximately 25 amino acid segment of the molecule and interaction of these two proteins is an essential step for the process of neurotransmission.

Protein 4.1N binding to nuclear mitotic apparatus protein in PC12 cells mediates the antiproliferative actions of nerve growth factor

Protein 4.1N is a neuronal selective isoform of the erythrocyte membrane cytoskeleton protein 4.1R. In the present study, we demonstrate an interaction between 4.1N and nuclear mitotic apparatus protein (NuMA), a nuclear protein required for mitosis. The binding involves the C-terminal domain of 4.1N. In PC12 cells treatment with nerve growth factor (NGF) elicits translocation of 4. 1N to the nucleus and promotes its association with NuMA. Specific targeting of 4.1N to the nucleus arrests PC12 cells at the G1 phase and produces an aberrant nuclear morphology. Inhibition of 4.1N nuclear translocation prevents the NGF-mediated arrest of cell division, which can be reversed by overexpression of 4.1N. Thus, nuclear 4.1N appears to mediate the antiproliferative actions of NGF by antagonizing the role of NuMA in mitosis.

Spectrin (betaSpIIsigma1) is an essential component of synaptic transmission

The cellular mechanism that underlies the regulated release of synaptic vesicles during neurotransmission is not fully known. Our previous data has shown that brain spectrin (alphaSpIIsigma1/betaSpIIsigma1)2 is localized in axons and nerve terminals and we have shown that the beta subunit (betaSpIIsigma1) contains a synapsin-binding domain capable of interacting with synapsin and small synaptic vesicles in vitro and in vivo. These findings suggested a role for brain beta-spectrin in synaptic neurotransmission. To examine this possibility further, peptide-specific antibodies directed against epitopes within the synapsin-binding domain of brain beta-spectrin, or against flanking regions, were injected into the presynaptic neuron of synaptically paired rat hippocampal neurons in culture. Here, we show that the antibodies directed against the synapsin-binding domain specifically blocked synaptic neurotransmission.

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.

Synapsin III, a novel synapsin with an unusual regulation by Ca2+

Synapsins I and II are synaptic vesicle proteins essential for normal Ca2+ regulation of neurotransmitter release. Synapsins are composed of combinations of common and variable sequences, with the central C-domain as the largest conserved domain. The C-domain is structurally homologous to ATPases, suggesting that synapsins function as ATP-dependent phosphotransfer enzymes. We have now identified an unanticipated third synapsin gene that is also expressed in brain. The product of this gene, synapsin IIIa, shares with synapsins Ia and IIa three conserved domains that are connected by variable sequences: the phosphorylated A-domain at the amino terminus, the large ATP-binding C-domain in the center, and the E-domain at the carboxyl terminus. Like other synapsins, synapsin IIIa binds ATP with high affinity and ADP with a lower affinity, consistent with a cycle of ATP binding and hydrolysis. ATP binding to the different synapsins is directly regulated by Ca2+ in a dramatically different fashion: Ca2+ activates ATP binding to synapsin I, has no effect on synapsin II, and inhibits synapsin III. Thus vertebrates express three distinct synapsins that utilize ATP but are specialized for different modes of direct Ca2+ regulation in synaptic function.

Synapsin I is structurally similar to ATP-utilizing enzymes

Synapsins are abundant synaptic vesicle proteins with an essential regulatory function in the nerve terminal. We determined the crystal structure of a fragment (synC) consisting of residues 110-420 of bovine synapsin I; synC coincides with the large middle domain (C-domain), the most conserved domain of synapsins. SynC molecules are folded into compact domains and form closely associated dimers. SynC monomers are strikingly similar in structure to a family of ATP-utilizing enzymes, which includes glutathione synthetase and D-alanine:D-alanine ligase. SynC binds ATP in a Ca2+-dependent manner. The crystal structure of synC in complex with ATPgammaS and Ca2+ explains the preference of synC for Ca2+ over Mg2+. Our results suggest that synapsins may also be ATP-utilizing enzymes.

Characterization of the interaction between synapsin I and calspectin (brain spectrin or fodrin)

We characterized the properties of the interaction between synapsin I and calspectin using purified proteins. The binding assay in the native state using antibodies specific to the tail region of synapsin I revealed that the binding is a high affinity with Kd of 9 nM, which is almost comparable to that of synapsin I to synaptic vesicles and to F-actin. We demonstrated that the head-middle region of synapsin I binds the NH2-terminal domain of beta subunit of calspectin, which also contains an actin binding site. Furthermore, the interaction was significantly inhibited by phosphorylation of synapsin I by cAMP-dependent protein kinase or by Ca2+, calmodulin-dependent protein kinase II. These properties of the interaction between synapsin I and calspectin may help understanding of its modulatory roles in neurotransmitter release.

Characterization of the 78 kDa mast cell protein phosphorylated by the antiallergic drug cromolyn and homology to moesin

Mast cells (MC) can be stimulated to secrete by cross-linking immunoglobulin E bound to specific surface receptors, as well as in response to polycationic molecules such as substance P and compound 48/80. The antiallergic drug disodium cromoglycate (cromolyn) inhibited MC secretion and rapidly incorporated phosphate into a 78 kDa protein, speculated to be its mode of action. This protein was purified by single and two-dimensional gel electrophoresis, and was shown to be phosphorylated primarily on serine residues by protein kinase C. Partial amino acid sequencing of two generated fragments was identical to that of portions of mouse moesin, a member of the band 4.1 superfamily of proteins, with no definitive function known to date. Polyclonal antibodies raised against the rat basophil leukemia cell moesin cDNA expressed in Escherichia coli immunoprecipitated the 78 kDa phosphoprotein quantitatively, and immunocytochemistry localized it to the plasma membrane. Reversible phosphorylation of this 78 kDa phosphoprotein could affect its possible cytoskeletal binding through which it may regulate stimulus-secretion coupling in MC.

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.

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.

Brain alpha erythroid spectrin: identification, compartmentalization, and beta spectrin associations

Using isoform and subunit specific antibodies we have determined the presence, localization, and beta spectrin associations of alpha erythroid spectrin, alpha SpI sigma*, as well as alpha non-erythroid spectrin, alpha SpII sigma 1, in mouse brain. Peptide specific antibodies against unique sequences within the beta SpII sigma 1, non-erythroid beta spectrin isoform, and within beta SpI sigma 1, erythrocyte beta spectrin isoform were used to compare the immunolocalization of beta spectrin subunit isoforms with that of alpha spectrin subunit isoforms and to immunoprecipitate spectrin tetramers in order to identify the subunit components by immunoblot analysis. The specificity and sensitivity of antibodies for isoform specific alpha and beta subunits was determined by immunodot and immunoblot methods. Immunohistochemical analyses indicated that beta SpI sigma 2 is located in neuronal somata and dendrites in mouse cerebellum. beta SpII sigma 1 is located in the medullary layer, chiefly composed of axonal tracts. Parallel immunohistochemical analysis with antibodies for the alpha and beta spectrin isoforms revealed that antibodies specific for the alpha subunit of erythrocyte spectrin (alpha SpI sigma 1) localized antigen to the somata and dendrites of cerebellar granule cell neurons, a pattern similar to that for the localization of the erythroid beta subunit (beta SpI sigma 2). In contrast antibodies specific for the non-erythroid alpha subunit (alpha SpII sigma 1) localized antigen to axons in the cerebellum corresponding to the pattern for the non-erythroid beta subunit (beta SpII sigma 1). The distinct localization of antigens by antisera which recognize either the alpha subunit of red blood cell spectrin or the alpha subunit of non-erythroid brain spectrin, together with the correspondence of their localization with appropriate beta subunits, clearly indicate that brain contains at least two species of spectrin each with distinct alpha and beta subunits. Immunoprecipitation experiments of cerebellar extracts using beta spectrin peptide specific antibodies followed by immunoblotting analysis confirmed the association of an erythroid alpha subunit isoform with a beta erythroid subunit isoform, as well as the association of non-erythroid alpha and beta subunits. In addition the immunoblot analysis of the immunoprecipitated material suggested there are minor populations of various hybrid tetramers in brain consisting of mixed erythroid and non-erythroid subunits. In summary these data collectively demonstrate that in mouse brain there are at least two alpha spectrin subunits, one erythroid alpha SpI sigma* and one non-erythroid alpha SpII sigma 1; these associate with an erythroid beta SpI sigma 1, and a non-erythroid beta SpII sigma 1 in the cerebellum of mouse.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for the association of protein 4.1 immunoreactive forms with neurofibrillary tangles in Alzheimer's disease brains

The formation of neurofibrillary tangles (NFTs) and paired-helical filaments (PHFs) in Alzheimer's disease (AD) reflects a major disorganization of the cytoskeleton. The role of the neuronal membrane skeleton in the development of these abnormalities has not previously been investigated. In this study, we used 9 antibodies raised against the erythrocyte membrane skeleton protein 4.1 (P4.1) for immunocytochemical and immunoblot analyses to investigate whether or not the brain homologues of this protein were constituents of NFTs or PHFs. Our results show that 7 of the 9 monospecific antibodies against the human and pig erythrocyte P4.1 stained NFTs in the prefrontal cortex and hippocampus of AD brains. The P4.1 antibodies used here did not cross-react with tau protein isolated from AD brain, and preabsorption of these antibodies with tau protein did not cause loss of NFT staining. In age-matched control brains, these P4.1 antibodies stained neuronal cell bodies or nuclei. Six of the antibodies also stained isolated NFTs but the SDS-insoluble NFTs were immunostained only by two of the P4.1 antibodies. By using inositol hexaphosphate affinity chromatography and immunoblot analysis, we identified a 68-kDa protein as the most likely brain analogue of P4.1. When SDS-extracted proteins from the isolated NFTs were immunoblotted, a 50-kDa band was immunostained. The 68-kDa and 50-kDa proteins were not stained by tau protein and neurofilament subunit NF-H antibodies, that strongly stained NFTs. We conclude that brain protein 4.1 isoform(s) are constituents of NFTs in AD.

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.

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.

Solid-phase binding analysis of N-CAM interactions with brain fodrin

The large cytoplasmic domain form of the neural cell adhesion molecule N-CAM has been reported to interact specifically with fodrin, a submembranous cytoskeletal protein. We tested the abilities of fodrins from bovine brain and embryonic chicken brain to bind to N-CAM that had been isolated from differentiated or undifferentiated mouse N2A neuroblastoma cells or from the brains of embryonic day 11 or day 14 chickens. Labeled fodrin samples bound with immobilized fodrin at a minimum soluble fodrin concentration of 2.5 x 10(-8) M, but the labeled fodrin did not bind to the immobilized N-CAM when incubated at 20-fold higher fodrin concentrations.

Cytochemical polarity in lateral geniculate interneurons

To determine the cytochemical composition of presynaptic dendrites, we have examined the distribution of synapsin 1, calcium and calmodulin-dependent protein kinase II (CaM-II), microtubule-associated protein 2 (MAP-2) and spectrin in cat lateral geniculate (LGN) class III cells by immune-EM. Special attention was paid to the dendrites of these interneurons because they are both pre- and postsynaptic. The dendritic proteins MAP-2 and RBC spectrin were not observed in interneuron dendrites but these proteins were localized in relay cell dendrites. The synaptic vesicle-associated protein synapsin 1 was present in all synaptic vesicle containing profiles, including dendritic terminals. CaM-II, the major postsynaptic density protein, was found in all dendrites. Thus, the LGN interneuron dendritic compartment displays both axonal and dendritic cytochemical properties. The results suggest the possibility of unique molecular interactions in interneuron dendritic terminals.

Molecular pathways for presynaptic calcium signaling

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

Axonal membrane-skeletal protein A60: association with a brain spectrin-binding activity and entry into cerebellar axons at a stage after the initiation of axonal growth

A60 is a 60-kDa component of the axonal cortical cytoskeleton in CNS neurones. It appears to be neurone specific and is tightly bound to brain membranes. In this study the cytoskeletal activities and developmental expression of A60 in rat cerebellum have been examined using the monoclonal antibody DR1. A60 in a partially purified soluble extract of brain membranes interacts selectively with brain but not erythrocyte spectrin. Because erythrocyte spectrin is more closely related to the dendritic form of spectrin than the axonal form, this raises the possibility that A60 localises in axons by interaction with the axonal form of spectrin only. A60 is not found in rat cerebellum before the day of birth. However, during postnatal development of the cerebellum (days 1-13) DR1 reactivity appears progressively. On postnatal day 1, a small population of cells in the mantle layer (presumptive Purkinje cells) is DR1 positive. There is no DR1 reactivity found in Purkinje cell axons during their initial phase of growth. By postnatal day 7, Purkinje cell bodies, initial dendritic segments, and the cerebellar white matter are all positive. This pattern of labelling is strengthened up until postnatal day 13. By contrast, in adult rat cerebellum, the location of A60 has changed so that it is most concentrated in axons, and dendritic staining is lost. These data indicate that A60 is a spectrin-binding component of the adult axonal membrane skeleton, the presence of which is only required in axons after the initial phase of growth.

Cytoskeleton dynamics during neurotransmitter release

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

Localization of spectrin isoforms in the adult mouse heart

The distribution of two isoforms of spectrin in the adult mouse heart was investigated by Western blotting and immunocytochemistry by use of monospecific antibodies to erythrocyte spectrin and nonerythroid brain spectrin (240/235). Western blotting revealed proteins analogous to both isoforms of alpha-spectrin in adult heart. Light-microscopic immunocytochemistry indicated that erythroid spectrin was distributed throughout the myocardium, with immunofluorescence localized to plasma membranes, Z-lines, and intercalated discs. Antibodies to brain spectrin (240/235) exhibited staining throughout the heart, with a generally diffuse distribution except for the prominent immunoreactivity associated with the intercalated discs. Nonerythroid spectrin immunofluorescence was detected in the endothelial cells of the endocardium and the mesothelial cell lining of the epicardium. Erythrocyte spectrin was not detected in the endocardium or the epicardium. The identification and localization of spectrin isoforms in the mammalian heart suggest the importance of spectrin proteins in the structural integrity and proper function of cardiac cells and tissues. This is the first demonstration of two different alpha-spectrin subunits in the mammalian heart.

The complete amino acid sequence for brain beta spectrin (beta fodrin): relationship to globin sequences

The amino acid sequence of mouse brain beta spectrin (beta fodrin), deduced from the nucleotide sequence of complementary DNA clones, reveals that this non-erythroid beta spectrin comprises 2363 residues, with a molecular weight of 274,449 Da. Brain beta spectrin contains three structural domains and we suggest the position of several functional domains including f-actin, synapsin I, ankyrin and spectrin self association sites. Analysis of deduced amino acid sequences indicated striking homology and similar structural characteristics of brain beta spectrin repeats beta 11 and beta 12 to globins. In vitro analysis has demonstrated that heme is capable of specific attachment to brain spectrin, suggesting possible new functions in electron transfer, oxygen binding, nitric oxide binding or heme scavenging.

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.

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.

Developmental expression of brain beta-spectrin isoform messenger RNAs

We have investigated the expression of brain beta SpIIa and beta SpIb (previously referred to as the beta-subunits of brain spectrin (240/235) and brain spectrin (240/235E), respectively) during mouse brain development. The 9 kb transcript which encodes beta SpIIa is present in fetal mouse brain tissue and increases to a maximal level in a 30-day-old mouse. There is a coordinate accumulation of the 7.8 kb alpha SpIIa mRNA (with beta SpIIa) during mouse brain development. The coordinate expression of alpha SpIIa and beta SpIIa at the mRNA and protein level allows formation of (alpha SpIIa/beta SpIIa)2 tetramers (brain spectrin(240/235)) early in premitotic neuronal development; and avoids turnover of unassembled alpha and beta-subunits. An 11 kb transcript which encodes beta SpIb is not produced in embryonic tissue, and is first seen in a 6-day-old mouse. The protein translation products beta SpIIa and beta SpIb have previously been demonstrated by our laboratory to first appear in fetal mouse brain tissue and at postnatal day 6-8, respectively [J. Neurosci., 7 (1987) 864-874]. The expression of beta SpIb mRNA on postnatal day 6-8, and the appearance of brain spectrin(240/235E) in postmitotic and postmigratory neurons of the cerebellum at this same time; suggests that brain spectrin(240/235E) is involved in differentiated functions of the neuron (formation of cell-cell contacts, formation of dendritic processes and postsynaptic contacts). Thus, the data from the present study demonstrates that the expression of these two neuronal beta-spectrin isoforms is regulated at the level of mRNA expression.

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.