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

Exposure to bisphenol A induces neurotoxicity associated with synaptic and cytoskeletal dysfunction in neuro-2a cells

Bisphenol A (BPA) has been reported to injure the developing and adult brain. However, the underlying mechanism still remains elusive. This study used neuro-2a cells as a cellular model to investigate the neurotoxic effects of BPA. Microtubule-associated protein 2 (MAP2) and tau protein maintain microtubule normal function and promote the normal development of the nervous system. Synaptophysin (SYP) and drebrin (Dbn) proteins are involved in regulating synaptic plasticity. Cells were exposed to the minimum essential medium (MEM), 0.01% (v/v) DMSO, and 150 μM BPA for 12, 24, or 36 h. Morphological analysis revealed that the cells in the BPA-treated groups shrank and collapsed compared with those in the control groups. CCK-8 and lactate dehydrogenase assay (LDH) assays showed that the mortality of neuro-2a cells increased as the BPA treatment time was prolonged. Ultrastructural analysis further revealed that cells demonstrated nucleolar swelling, dissolution of nuclear and mitochondrial membranes, and partial mitochondrial condensation following exposure to BPA. BPA also decreased the relative protein expression levels of MAP2, tau, and Dbn. Interestingly, the relative protein expression levels of SYP increased. These results indicated that BPA inhibited the proliferation and disrupted cytoskeleton and synaptic integrity of neuro-2a cells.

Hub genes for early diagnosis and therapy of adamantinomatous craniopharyngioma

BACKGROUND

Adamantinomatous craniopharyngioma (ACP) is a subtype of craniopharyngioma, a neoplastic disease with a benign pathological phenotype but a poor prognosis in the sellar region. The disease has been considered the most common congenital tumor in the skull. Therefore, this article aims to identify hub genes that might serve as genetic markers of diagnosis, treatment, and prognosis of ACP.

METHODS

The procedure of this research includes the acquisition of public data, identification and functional annotation of differentially expressed genes (DEGs), construction and analysis of protein-protein interaction network, and the mining and analysis of hub genes by Spearman-rho test, multivariable linear regression, and receiver operator characteristic curve analysis. Quantitative real-time polymerase chain reaction was used to detect the level of mRNA of relative genes.

RESULTS

Among 2 datasets, a total of 703 DEGs were identified, mainly enriched in chemical synaptic transmission, cell adhesion, odontogenesis of the dentin-containing tooth, cell junction, extracellular region, extracellular space, structural molecule activity, and structural constituent of cytoskeleton. The protein-protein interaction network was composed of 4379 edges and 589 nodes. Its significant module had 10 hub genes, and SYN1, SYP, and GRIA2 were significantly down-regulated with ACP.

CONCLUSION

In a word, we find out the DEGs between ACP patients and standard samples, which are likely to play an essential role in the development of ACP. At the same time, these DEGs are of great value in tumors' diagnosis and targeted therapy and could even be mined as biological molecular targets for diagnosing and treating ACP patients.

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.

Resveratrol reverses hippocampal synaptic markers injury and SIRT1 inhibition against developmental Pb exposure

Lead (Pb) exposure damages synaptic structural plasticity that results in cognitive impairment. Resveratrol, a natural polyphenolic compound, is one of the most potent agonists of silencing information regulator 1 (SIRT1) discovered to date. However, the effects of SIRT1 on synaptic functional plasticity in early life Pb exposure are not well studied. Herein, the purpose of this study is to investigate the expression of synaptic markers and SIRT1 in rats exposed to Pb and to evaluate the regulatory effect of resveratrol during this process. The Pb exposed male SD pups were treated with resveratrol (50 mg/kg/d) or EDTA (150 mg/kg/d) followed by hippocampal and blood sampling for analysis at postnatal day 21 (PND21). In the Morrris water maze test, resveratrol treatement protected the rats against Pb-induced impairment of learning and memory (P < 0.05). Resveratrol also enhanced the expression of brain-derived neurotrophic factor (BDNF, P < 0.001 vs 0.2% Pb group), and reversed the effects of Pb exposure on SIRT1(P < 0.001 vs 0.2% Pb group). The DG, CA1 and CA3 regions of the hippocampus showed a considerable increase in the expression of pre- and postsynaptic proteins (P < 0.001 vs 0.2% Pb group). In conclusion, our study demonstrated that resveratrol, through the activation of SIRT1, played a protective role against Pb-induced defects in synaptic plasticity, and suggested a new potential adjuvant treatment for Pb poisoning.

Effects of febrile seizures in mesial temporal lobe epilepsy with hippocampal sclerosis on gene expression using bioinformatical analysis

Background

To investigate the effect of long-term febrile convulsions on gene expression in mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) and explore the molecular mechanism of MTLE-HS.

Methods

Microarray data of MTLE-HS were obtained from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) between MTLE-HS with and without febrile seizure history were screened by the GEO2R software. Pathway enrichment and gene ontology of the DEGs were analyzed using the DAVID online database and FunRich software. Protein–protein interaction (PPI) networks among DEGs were constructed using the STRING database and analyzed by Cytoscape.

Results

A total of 515 DEGs were identified in MTLE-HS samples with a febrile seizure history compared to MTLE-HS samples without febrile seizure, including 25 down-regulated and 490 up-regulated genes. These DEGs were expressed mostly in plasma membrane and synaptic vesicles. The major molecular functions of those genes were voltage-gated ion channel activity, extracellular ligand-gated ion channel activity and calcium ion binding. The DEGs were mainly involved in biological pathways of cell communication signal transduction and transport. Five genes (SNAP25, SLC32A1, SYN1, GRIN1,andGRIA1) were significantly expressed in the MTLE-HS with prolonged febrile seizures.

Conclusion

The pathogenesis of MTLE-HS involves multiple genes, and prolonged febrile seizures could cause differential expression of genes. Thus, investigations of those genes may provide a new perspective into the mechanism of MTLE-HS.

Growth and excitability at synapsin II deficient hippocampal neurons

Synapsins are neuronal phosphoproteins that fine-tune synaptic transmission and suppress seizure activity. Synapsin II (SynII) deletion produces epileptic seizures and overexcitability in neuronal networks. Early studies in primary neuronal cultures have shown that SynII deletion results in a delay in synapse formation. More recent studies at hippocampal slices have revealed increased spontaneous activity in SynII knockout (SynII(-)) mice. To reconcile these observations, we systematically re-examined synaptic transmission, synapse formation, and neurite growth in primary hippocampal neuronal cultures. We find that spontaneous glutamatergic synaptic activity was suppressed in SynII(-) neurons during the initial developmental epoch (7 days in vitro, DIV) but was enhanced at later times (12 and18 DIV). The density of synapses, transmission between connected pairs of neurons, and the number of docked synaptic vesicles were not affected by SynII deletion. However, we found that neurite outgrowth in SynII(-) neurons was suppressed during the initial developmental epoch (7 DIV) but enhanced at subsequent developmental stages (12 and18 DIV). This finding can account for the observed effect of SynII deletion on synaptic activity. To test whether the observed phenotype resulted directly from the deletion of SynII we expressed SynII in SynII(-) cultures using an adeno-associated virus (AAV). Expression of SynII at 2 DIV rescued the SynII deletion-dependent alterations in both synaptic activity and neuronal growth. To test whether the increased neurite outgrowth in SynII(-) observed at DIV 12 and18 represents an overcompensation for the initial developmental delay or results directly from SynII deletion we performed "late expression" experiments, transfecting SynII(-) cultures with AAV at 7 DIV. The late SynII expression suppressed neurite outgrowth at 12 and 18 DIV to the levels observed in control neurons, suggesting that these phenotypes directly depend on SynII. These results reveal a novel developmentally regulated role for SynII function in the control of neurite growth.

Synapsins regulate brain-derived neurotrophic factor-mediated synaptic potentiation and axon elongation by acting on membrane rafts

In neurons, intracellular membrane rafts are essential for specific actions of brain-derived neurotrophic factor (BDNF), which include the regulation of axon outgrowth, growth cone turning and synaptic transmission. Virtually, all the actions of BDNF are mediated by binding to its receptor, TrkB. The association of TrkB with the tyrosine kinase, Fyn, is critical for its localization to intracellular membrane rafts. Here, we show that synapsins, a family of highly amphipathic neuronal phosphoproteins, regulate membrane raft lipid composition and consequently, the ability of BDNF to regulate axon/neurite development and potentiate synaptic transmission. In the brains of mice lacking all synapsins, the expression of both BDNF and TrkB were increased, suggesting that BDNF/TrkB-mediated signaling is impaired. Consistent with this finding, synapsin-depleted neurons exhibit altered raft lipid composition, deficient targeting of Fyn to rafts, attenuated TrkB activation, and abrogation of BDNF-stimulated axon outgrowth and synaptic potentiation. Conversely, overexpression of synapsins in neuroblastoma cells results in corresponding reciprocal changes in raft lipid composition, increased localization of Fyn to rafts and promotion of BDNF-stimulated neurite formation. In the presence of synapsins, the ratio of cholesterol to estimated total phospholipids converged to 1, suggesting that synapsins act by regulating the ratio of lipids in intracellular membranes, thereby promoting lipid raft formation. These studies reveal a mechanistic link between BDNF and synapsins, impacting early development and synaptic transmission.

Antidepressant Potential of Chlorogenic Acid-Enriched Extract from Eucommia ulmoides Oliver Bark with Neuron Protection and Promotion of Serotonin Release through Enhancing Synapsin I Expression

Eucommia ulmoides Oliver (E. ulmoides) is a traditional Chinese medicine with many beneficial effects, used as a tonic medicine in China and other countries. Chlorogenic acid (CGA) is an important compound in E. ulmoides with neuroprotective, cognition improvement and other pharmacological effects. However, it is unknown whether chlorogenic acid-enriched Eucommia ulmoides Oliver bark has antidepressant potential through neuron protection, serotonin release promotion and penetration of blood-cerebrospinal fluid barrier. In the present study, we demonstrated that CGA could stimulate axon and dendrite growth and promote serotonin release through enhancing synapsin I expression in the cells of fetal rat raphe neurons in vitro. More importantly, CGA-enriched extract of E. ulmoides (EUWE) at 200 and 400 mg/kg/day orally administered for 7 days showed antidepressant-like effects in the tail suspension test of KM mice. Furthermore, we also found CGA could be detected in the the cerebrospinal fluid of the rats orally treated with EUWE and reach the level of pharmacological effect for neuroprotection by UHPLC-ESI-MS/MS. The findings indicate CGA is able to cross the blood-cerebrospinal fluid barrier to exhibit its neuron protection and promotion of serotonin release through enhancing synapsin I expression. This is the first report of the effect of CGA on promoting 5-HT release through enhancing synapsin I expression and CGA-enriched EUWE has antidepressant-like effect in vivo. EUWE may be developed as the natural drugs for the treatment of depression.

Synapsins Are Downstream Players of the BDNF-Mediated Axonal Growth

Synapsins (Syns) are synaptic vesicle-associated phosphoproteins involved in neuronal development and neurotransmitter release. While Syns are implicated in the regulation of brain-derived neurotrophic factor (BDNF)-induced neurotransmitter release, their role in the BDNF developmental effects has not been fully elucidated. By using primary cortical neurons from Syn I knockout (KO) and Syn I/II/III KO mice, we studied the effects of BDNF and nerve growth factor (NGF) on axonal growth. While NGF had similar effects in all genotypes, BDNF induced significant differences in Syn KO axonal outgrowth compared to wild type (WT), an effect that was rescued by the re-expression of Syn I. Moreover, the significant increase of axonal branching induced by BDNF in WT neurons was not detectable in Syn KO neurons. The expression analysis of BDNF receptors in Syn KO neurons revealed a significant decrease of the full length TrkB receptor and an increase in the levels of the truncated TrkB.t1 isoform and p75^NTR associated with a marked reduction of the BDNF-induced MAPK/Erk activation. By using the Trk inhibitor K252a, we demonstrated that these differences in BDNF effects were dependent on a TrkB/p75^NTR imbalance. The data indicate that Syn I plays a pivotal role in the BDNF signal transduction during axonal growth.

Synapsin regulates activity-dependent outgrowth of synaptic boutons at the Drosophila neuromuscular junction

Patterned depolarization of Drosophila motor neurons can rapidly induce the outgrowth of new synaptic boutons at the larval neuromuscular junction (NMJ), providing a model system to investigate mechanisms underlying acute structural plasticity. Correlative light and electron microscopy analysis revealed that new boutons typically form near the edge of postsynaptic reticulums of presynaptic boutons. Unlike mature boutons, new varicosities have synaptic vesicles which are distributed uniformly throughout the bouton and undeveloped postsynaptic specializations. To characterize the presynaptic mechanisms mediating new synaptic growth induced by patterned activity, we investigated the formation of new boutons in NMJs lacking synapsin [Syn(-)], a synaptic protein important for vesicle clustering, neurodevelopment, and plasticity. We found that budding of new boutons at Syn(-) NMJs was significantly diminished, and that new boutons in Syn(-) preparations were smaller and had reduced synaptic vesicle density. Since synapsin is a target of protein kinase A (PKA), we assayed whether activity-dependent synaptic growth is regulated via a cAMP/PKA/synapsin pathway. We pretreated preparations with forskolin to raise cAMP levels and found this manipulation significantly enhanced activity-dependent synaptic growth in control but not Syn(-) preparations. To examine the trafficking of synapsin during synaptic growth, we generated transgenic animals expressing fluorescently tagged synapsin. Fluorescence recovery after photobleaching analysis revealed that patterned depolarization promoted synapsin movement between boutons. During new synaptic bouton formation, synapsin redistributed upon stimulation toward the sites of varicosity outgrowth. These findings support a model whereby synapsin accumulates at sites of synaptic growth and facilitates budding of new boutons via a cAMP/PKA-dependent pathway.

Porcine synapsin 1: SYN1 gene analysis and functional characterization of the promoter

Synapsin 1 (SYN1) is a phosphoprotein involved in nerve signal transmission. The porcine SYN1 promoter orthologue was cloned and characterized to provide a means of expressing a transgene specifically in neurons. The nucleotide sequence of the promoter displayed a high degree of conservation of elements responsible for neuron-specific expression. Expression analysis of SYN1 demonstrated presence of transcript during embryonic development. Analysis of GFP expression in transgenic zebrafish embryos suggests that the pig SYN1 promoter directs expression in neuronal cells. Thus, the SYN1 promoter is a good candidate for use in the generation of pig models of human neurodegenerative disorders.

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The porcine synapsin1 (SYN1) promoter was cloned and characterized.

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SYN1 mRNA expression is detected in brain during embryo development.

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The SYN1 gene is mapped to pig chromosome X.

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Porcine SYN1 directs GFP expression in neuronal cells of transgenic zebrafish.

Synaptic functions of invertebrate varicosities: what molecular mechanisms lie beneath

In mammalian brain, the cellular and molecular events occurring in both synapse formation and plasticity are difficult to study due to the large number of factors involved in these processes and because the contribution of each component is not well defined. Invertebrates, such as Drosophila, Aplysia, Helix, Lymnaea, and Helisoma, have proven to be useful models for studying synaptic assembly and elementary forms of learning. Simple nervous system, cellular accessibility, and genetic simplicity are some examples of the invertebrate advantages that allowed to improve our knowledge about evolutionary neuronal conserved mechanisms. In this paper, we present an overview of progresses that elucidates cellular and molecular mechanisms underlying synaptogenesis and synapse plasticity in invertebrate varicosities and their validation in vertebrates. In particular, the role of invertebrate synapsin in the formation of presynaptic terminals and the cell-to-cell interactions that induce specific structural and functional changes in their respective targets will be analyzed.

Epilepsy as a neurodevelopmental disorder

Epilepsy is characterized by spontaneous recurrent seizures and comprises a diverse group of syndromes with different etiologies. Epileptogenesis refers to the process whereby the brain becomes epileptic and can be related to several factors, such as acquired structural brain lesions, inborn brain malformations, alterations in neuronal signaling, and defects in maturation and plasticity of neuronal networks. In this review, we will focus on alterations of brain development that lead to an hyperexcitability phenotype in adulthood, providing examples from both animal and human studies. Malformations of cortical development (including focal cortical dysplasia, lissencephaly, heterotopia, and polymicrogyria) are frequently epileptogenic and result from defects in cell proliferation in the germinal zone and/or impaired neuronal migration and differentiation. Delayed or reduced arrival of inhibitory interneurons into the cortical plate is another possible cause of epileptogenesis. GABAergic neurons are generated during early development in the ganglionic eminences, and failure to pursue migration toward the cortex alters the excitatory/inhibitory balance resulting in aberrant network hyperexcitability. More subtle defects in the developmental assembly of excitatory and inhibitory synapses are also involved in epilepsy. For example, mutations in the presynaptic proteins synapsins and SNAP-25 cause derangements of synaptic transmission and plasticity which underlie appearance of an epileptic phenotype. Finally, there is evidence that defects in synapse elimination and remodeling during early "critical periods" can trigger hyperexcitability later in life. Further clarification of the developmental pathways to epilepsy has important implications for disease prevention and therapy.

Effects of phosphorylation and neuronal activity on the control of synapse formation by synapsin I

Synapsins are synaptic vesicle (SV)-associated proteins that regulate synaptic transmission and neuronal differentiation. At early stages, Syn I and II phosphorylation at Ser9 by cAMP-dependent protein kinase (PKA) and Ca2+/calmodulin-dependent protein kinase I/IV modulates axon elongation and SV-precursor dynamics. We evaluated the requirement of Syn I for synapse formation by siRNA-mediated knockdown as well as by overexpression of either its wild-type (WT) form or its phosphorylation mutants. Syn1 knockdown at 14 days in vitro caused a decrease in the number of synapses, accompanied by a reduction of SV recycling. Although overexpression of WT Syn I was ineffective, overexpression of its phosphorylation mutants resulted in a complex temporal regulation of synapse density. At early stages of synaptogenesis, phosphomimetic Syn I S9E significantly increased the number of synapses. Conversely, dephosphomimetic Syn I S9A decreased synapse number at more advanced stages. Overexpression of either WT Syn I or its phosphomimetic S9E mutant rescued the decrease in synapse number caused by chronic treatment with tetrodotoxin at early stages, suggesting that Syn I participates in an alternative PKA-dependent mechanism that can compensate for the impairment of the activity-dependent synaptogenic pathway. Altogether these results indicate that Syn I is an important regulator of synapse formation, which adjusts synapse number in response to extracellular signals.

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'.

The role of synapsins in neuronal development

The synapsins, the first identified synaptic vesicle-specific proteins, are phosphorylated on multiple sites by a number of protein kinases and are involved in neurite outgrowth and synapse formation as well as in synaptic transmission. In mammals, the synapsin family consists of at least 10 isoforms encoded by 3 distinct genes and composed by a mosaic of conserved and variable domains. The synapsins are highly conserved evolutionarily, and orthologues have been found in invertebrates and lower vertebrates. Within nerve terminals, synapsins are implicated in multiple interactions with presynaptic proteins and the actin cytoskeleton. Via these interactions, synapsins control several mechanisms important for neuronal homeostasis. In this review, we describe the main functional features of the synapsins, in relation to the complex role played by these phosphoproteins in neuronal development.

The synapsin gene family in basal chordates: evolutionary perspectives in metazoans

Background

Synapsins are neuronal phosphoproteins involved in several functions correlated with both neurotransmitter release and synaptogenesis. The comprehension of the basal role of the synapsin family is hampered in vertebrates by the existence of multiple synapsin genes. Therefore, studying homologous genes in basal chordates, devoid of genome duplication, could help to achieve a better understanding of the complex functions of these proteins.

Results

In this study we report the cloning and characterization of the Ciona intestinalis and amphioxus Branchiostoma floridae synapsin transcripts and the definition of their gene structure using available C. intestinalis and B. floridae genomic sequences. We demonstrate the occurrence, in both model organisms, of a single member of the synapsin gene family. Full-length synapsin genes were identified in the recently sequenced genomes of phylogenetically diverse metazoans. Comparative genome analysis reveals extensive conservation of the SYN locus in several metazoans. Moreover, developmental expression studies underline that synapsin is a neuronal-specific marker in basal chordates and is expressed in several cell types of PNS and in many, if not all, CNS neurons.

Conclusion

Our study demonstrates that synapsin genes are metazoan genes present in a single copy per genome, except for vertebrates. Moreover, we hypothesize that, during the evolution of synapsin proteins, new domains are added at different stages probably to cope up with the increased complexity in the nervous system organization. Finally, we demonstrate that protochordate synapsin is restricted to the post-mitotic phase of CNS development and thereby is a good marker of postmitotic neurons.

Early involvement of synapsin III in neural progenitor cell development in the adult hippocampus

Synapsin III is a synaptic vesicle-associated protein that is expressed in cells of the subgranular layer of the hippocampal dentate gyrus, a brain region known to sustain substantial levels of neurogenesis into adulthood. Here we tested the hypothesis that synapsin III plays a role in adult neurogenesis with synapsin III knockout and wild-type mice. Immunocytochemistry of the adult hippocampal dentate gyrus revealed that synapsin III colocalizes with markers of neural progenitor cell development (nestin, PSA-NCAM, NeuN, and Tuj1) but did not colocalize with markers of mitosis (Ki67 and PCNA). Because neurogenesis consists of a number of stages, the proliferation, survival, and differentiation of neural progenitor cells were systematically quantitated in the hippocampal dentate gyrus of adult synapsin III knockout and wild-type mice. We found a 30% decrease in proliferation and a 55% increase in survival of neural progenitor cells in synapsin III knockout mice. We also observed a 6% increase in the number of neural progenitor cells that differentiated into neurons. No difference in the volume of the dentate gyrus was observed between synapsin III knockout and wild-type mice. Collectively, our results demonstrate a novel role for synapsin III in regulating the proliferation of neural progenitor cells in the adult hippocampal dentate gyrus. These findings suggest a distinct function for this synaptic vesicle protein, in addition to its role in neurotransmission.

The conserved protein kinase-A target motif in synapsin of Drosophila is effectively modified by pre-mRNA editing

Background

Synapsins are abundant synaptic vesicle associated phosphoproteins that are involved in the fine regulation of neurotransmitter release. The Drosophila member of this protein family contains three conserved domains (A, C, and E) and is expressed in most or all synaptic terminals. Similar to mouse mutants, synapsin knock-out flies show no obvious structural defects but are disturbed in complex behaviour, notably learning and memory.

Results

We demonstrate that the N-terminal phosphorylation consensus motif RRxS that is conserved in all synapsins investigated so far, is modified in Drosophila by pre-mRNA editing. In mammals this motif represents the target site P1 of protein kinase A (PKA) and calcium/calmodulin dependent protein kinase I/IV. The result of this editing, by which RRFS is modified to RGFS, can be observed in cDNAs of larvae and adults and in both isolated heads and bodies. It is also seen in several newly collected wild-type strains and thus does not represent an adaptation to laboratory culture conditions. A likely editing site complementary sequence is found in a downstream intron indicating that the synapsin pre-mRNA can form a double-stranded RNA structure that is required for editing by the adenosine deaminase acting on RNA (ADAR) enzyme. A deletion in the Drosophila Adar gene generated by transposon remobilization prevents this modification, proving that the ADAR enzyme is responsible for the pre-mRNA editing described here. We also provide evidence for a likely function of synapsin editing in Drosophila. The N-terminal synapsin undeca-peptide containing the genomic motif (RRFS) represents an excellent substrate for in-vitro phosphorylation by bovine PKA while the edited peptide (RGFS) is not significantly phosphorylated. Thus pre-mRNA editing by ADAR could modulate the function of ubiquitously expressed synapsin in a cell-specific manner during development and adulthood.

Conclusion

Similar to several other neuronal proteins of Drosophila, synapsin is modified by ADAR-mediated recoding at the pre-mRNA level. This editing likely reduces or abolishes synapsin phosphorylation by PKA. Since synapsin in Drosophila is required for various forms of behavioural plasticity, it will be fascinating to investigate the effect of this recoding on learning and memory.

Protein sorting in the synaptic vesicle life cycle

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

Deletion of synapsins I and II genes alters the size of vesicular pools and rabphilin phosphorylation

Previous studies established that genetic deletion of synapsins, synaptic vesicle-associated phosphoproteins that regulate neurotransmitter release, decreases the number of synaptic vesicles in nerve terminals. To investigate whether these changes affect the release properties of the remaining synaptic vesicles, we used a radioactive labeling technique to measure release independently of the total number of synaptic vesicles. 3H-glutamate and 14C-gamma-amino-butyric-acid (GABA) release from isolated nerve terminals prepared from the neocortex of synapsins I and II double knock-out mice (DKO) was assayed and compared to wild-type preparations. Hyperosmotic shock-evoked 3H-glutamate was reduced by 20+/-3% from DKO nerve terminals and potassium depolarization-evoked glutamate release was also decreased by 28+/-2%. Surprisingly, sucrose or potassium depolarization-evoked release of 14C-GABA was increased by 32+/-4% and 29+/-5%, respectively. The basal efflux of both 3H-glutamate and 14C-GABA increased by 17+/-2% and 12+/-2% from DKO nerve terminals. As lack of synapsins I and II, major phosphoproteins of synaptic vesicles, may lead to deregulation of phosphorylation events, we compared phosphorylation state of another synaptic vesicle protein, rabphilin. In DKO nerve terminals, membrane-associated rabphilin level was reduced by approximately 0.28-fold, its phosphorylation at 234serine was increased by approximately 1.61-fold whereas cytosolic rabphilin levels showed both more dramatic reduction in abundance, approximately 16.5-fold, and increase in phosphorylation, approximately 4.8-fold. Collectively, these data suggest that deletion of major synapsin isoforms leads to (1) deregulation of basal neurotransmission causing "leaky" basal release, (2) changes in either the size or mobilization of releasable or reserve pools, and (3) a decrease in rabphilin abundance accompanied by an increase in basal phosphorylation of the remaining rabphilin.

Phosphorylation of synapsin I by cAMP-dependent protein kinase controls synaptic vesicle dynamics in developing neurons

In developing neurons, synaptic vesicles (SVs) undergo cycles of exo-endocytosis along isolated axons. However, it is currently unknown whether SV exocytosis is regulated before synaptogenesis. Here, we show that cAMP-dependent pathways affect SV distribution and recycling in the axonal growth cone and that these effects are mediated by the SV-associated phosphoprotein synapsin I. The presence of synapsin I on SVs is necessary for the correct localization of the vesicles in the central portion of the growth cone. Phosphorylation of synapsin I by cAMP-dependent protein kinase (protein kinase A) causes the dissociation of the protein from the SV membrane, allowing diffusion of the vesicles to the periphery of the growth cone and enhancing their rate of recycling. These results provide new clues as to the bases of the well known activity of synapsin I in synapse maturation and indicate that molecular mechanisms similar to those operating at mature nerve terminals are active in developing neurons to regulate the SV life cycle before synaptogenesis.

Protein fucosylation regulates synapsin Ia/Ib expression and neuronal morphology in primary hippocampal neurons

Although fucose-alpha(1-2)-galactose [Fucalpha(1-2)Gal] carbohydrates have been implicated in cognitive processes such as long-term memory, the molecular mechanisms by which these sugars influence neuronal communication are not well understood. Here, we present molecular insights into the functions of Fucalpha(1-2)Gal sugars, demonstrating that they play a role in the regulation of synaptic proteins and neuronal morphology. We show that synapsins Ia and Ib, synapse-specific proteins involved in neurotransmitter release and synaptogenesis, are the major Fucalpha(1-2)Gal glycoproteins in mature cultured neurons and the adult rat hippocampus. Fucosylation has profound effects on the expression and turnover of synapsin in cells and protects synapsin from degradation by the calcium-activated protease calpain. Our studies suggest that defucosylation of synapsin has critical consequences for neuronal growth and morphology, leading to stunted neurite outgrowth and delayed synapse formation. We also demonstrate that Fucalpha(1-2)Gal carbohydrates are not limited to synapsin but are found on additional glycoproteins involved in modulating neuronal architecture. Together, our studies identify important roles for Fucalpha(1-2)Gal sugars in the regulation of neuronal proteins and morphological changes that may underlie synaptic plasticity.

Signaling at the vertebrate synapse: new roles for embryonic morphogens?

The formation of synapses is critical for functional neuronal connectivity. The coordinated assembly at both sides of the synapse is fundamental for the proper apposition of the neurotransmitter release machinery on the presynaptic neuron and the clustering of neurotransmitter receptors and ion channels on the receptive postsynaptic cell. This process requires bidirectional communication between the presynaptic neuron and its postsynaptic target, another neuron, or muscle fiber. Extracellular signals such as WNT, TGF-beta, and FGF factors are emerging as key target-derived signals required for the initial stages of synaptic assembly. Studies in invertebrates are also providing new insights into the function of these signals in synaptic growth and homeostasis. During early embryonic patterning, WNT, TGF-beta, and FGF factors function as typical morphogens in a concentration-dependent manner to regulate cell fate decisions. This mode of action raises the provocative idea that these same morphogens might also provide a coordinate system for axons to establish the distance to their targets during axon guidance and synapse formation.

Nitric oxide synthase inhibition during synaptic maturation decreases synapsin I immunoreactivity in rat brain

During the development of the brain, nitric oxide and synapsins, the latter being phosphoproteins associated to presynaptic membrane vesicles, are abundant in presynaptic terminals and play important and similar roles in neurotransmitter release, morphogenesis, synaptogenesis, and synaptic plasticity. These mechanisms are fundamental for neuronal development and plasticity and constitute important factors for the formation of neuroanatomical structures. Neural nitric oxide synthase (nNOS), synapsin I, and nNOS adapter protein (CAPON) constitute a ternary complex necessary for specific NO and synapsin functions at a presynaptic level. It is not known whether NO absence may affect the presence and/or activity of synapsins during brain development. To understand the role of NO in synaptogenesis, we studied the effects of NOS inhibition on synapsin I expression at a postnatal stage. Rat pups were treated with a competitive NOS antagonist, N-nitro-L-arginine methyl ester, from postnatal days 3 to 23. Control pups received exclusively an equivalent volume of saline solution. Histochemical and immunochemical techniques for NADPH-d and synapsin I, respectively, were carried out. NOS inhibition elicited a significant reduction in synapsin I immunoreactive density and NADPH-d activity in the brain in the analyzed areas-prefrontal cortex, hippocampus, and dorsal thalamus. These data show that the alterations originated by NO and synapsin deficiencies produce a diminution in synaptic density. Thus, functions that depend on the formation of synaptic connections such as learning and memory could be affected by NO deficiency.

Synapsin Is a Novel Rab3 Effector Protein on Small Synaptic Vesicles

Synapsins, a family of neuron-specific phosphoproteins, have been demonstrated to regulate the availability of synaptic vesicles for exocytosis by binding to both synaptic vesicles and the actin cytoskeleton in a phosphorylation-dependent manner. Although the above-mentioned observations strongly support a pre-docking role of the synapsins in the assembly and maintenance of a reserve pool of synaptic vesicles, recent results suggest that the synapsins may also be involved in some later step of exocytosis. In order to investigate additional interactions of the synapsins with nerve terminal proteins, we have employed phage display library technology to select peptide sequences binding with high affinity to synapsin I. Antibodies raised against the peptide YQYIETSMQ (syn21) specifically recognized Rab3A, a synaptic vesicle-specific small G protein implicated in multiple steps of exocytosis. The interaction between synapsin I and Rab3A was confirmed by photoaffinity labeling experiments on purified synaptic vesicles and by the formation of a chemically cross-linked complex between synapsin I and Rab3A in intact nerve terminals. Synapsin I could be effectively co-precipitated from synaptosomal extracts by immobilized recombinant Rab3A in a GTP-dependent fashion. In vitro binding assays using purified proteins confirmed the binding preference of synapsin I for Rab3A-GTP and revealed that the COOH-terminal regions of synapsin I and the Rab3A effector domain are required for the interaction with Rab3A to occur. The data indicate that synapsin I is a novel Rab3 interactor on synaptic vesicles and suggest that the synapsin-Rab3 interaction may participate in the regulation of synaptic vesicle trafficking within the nerve terminals.

Phosphorylation by cAMP-dependent protein kinase is essential for synapsin-induced enhancement of neurotransmitter release in invertebrate neurons

Synapsins are synaptic vesicle-associated phosphoproteins involved in the regulation of neurotransmitter release and synapse formation; they are substrates for multiple protein kinases that phosphorylate them on distinct sites. We have previously found that injection of synapsin into Helix snail neurons cultured under low-release conditions increases the efficiency of neurotransmitter release. In order to investigate the role of phosphorylation in this modulatory action of synapsins, we examined the substrate properties of the snail synapsin orthologue recently cloned in Aplysia (apSyn) for various protein kinases and compared the effects of the intracellular injection of wild-type apSyn with those of its phosphorylation site mutants. ApSyn was found to be an excellent in vitro substrate for cAMP-dependent protein kinase, which phosphorylated it at high stoichiometry on a single site (Ser-9) in the highly conserved domain A, unlike the other kinases reported to phosphorylate mammalian synapsins, which phosphorylated apSyn to a much lesser extent. The functional effect of apSyn phosphorylation by cAMP-dependent protein kinase on neurotransmitter release was studied by injecting wild-type or Ser-9 mutated apSyn into the soma of Helix serotonergic C1 neurons cultured under low-release conditions, i.e. in contact with the non-physiological target neuron C3. In this model of impaired neurotransmitter release, the injection of wild-type apSyn induced a significant enhancement of release. This enhancement was virtually absent after injection of the non-phosphorylatable mutant (Ser-9→Ala), but it was maintained after injection of the pseudophosphorylated mutant (Ser-9→Asp). These functional effects of apSyn injection were paralleled by marked ultrastructural changes in the C1 neuron, with the formation of extensive interdigitations of neurite-like processes containing an increased complement of C1 dense core vesicles at the sites of cell-to-cell contact. This structural rearrangement was virtually absent in mock-injected C1 neurons or after injection of the non-phosphorylatable apSyn mutant. These data indicate that phosphorylation of synapsin domain A is essential for the synapsin-induced enhancement of neurotransmitter release and suggest that endogenous kinases phosphorylating this domain play a central role in the regulation of the efficiency of the exocytotic machinery.

Flies lacking all synapsins are unexpectedly healthy but are impaired in complex behaviour

Vertebrate synapsins are abundant synaptic vesicle phosphoproteins that have been proposed to fine-regulate neurotransmitter release by phosphorylation-dependent control of synaptic vesicle motility. However, the consequences of a total lack of all synapsin isoforms due to a knock-out of all three mouse synapsin genes have not yet been investigated. In Drosophila a single synapsin gene encodes several isoforms and is expressed in most synaptic terminals. Thus the targeted deletion of the synapsin gene of Drosophila eliminates the possibility of functional knock-out complementation by other isoforms. Unexpectedly, synapsin null mutant flies show no obvious defects in brain morphology, and no striking qualitative changes in behaviour are observed. Ultrastructural analysis of an identified 'model' synapse of the larval nerve muscle preparation revealed no difference between wild-type and mutant, and spontaneous or evoked excitatory junction potentials at this synapse were normal up to a stimulus frequency of 5 Hz. However, when several behavioural responses were analysed quantitatively, specific differences between mutant and wild-type flies are noted. Adult locomotor activity, optomotor responses at high pattern velocities, wing beat frequency, and visual pattern preference are modified. Synapsin mutant flies show faster habituation of an olfactory jump response, enhanced ethanol tolerance, and significant defects in learning and memory as measured using three different paradigms. Larval behavioural defects are described in a separate paper. We conclude that Drosophila synapsins play a significant role in nervous system function, which is subtle at the cellular level but manifests itself in complex behaviour.

Expression of synapsin III in nerve terminals and neurogenic regions of the adult brain

We have examined the distribution of synapsin III in the adult mouse brain. Expression of synapsin III was observed in puncta throughout the brain, but demonstrated greater regional variation than that of synapsins I or II. This punctate staining is typical for synaptic vesicle proteins located at nerve terminals. These findings are also consistent with the well-established role for synapsins in regulating neurotransmitter release. However, unexpectedly, synapsin III was also highly expressed in the cell body and processes of immature neurons in neurogenic regions of the adult brain, such as the hippocampal dentate gyrus, rostral migratory stream, and olfactory bulb. Many synapsin III-positive neurons also reacted with an antibody directed toward polysialylated-neuronal cell adhesion molecule, a marker of immature, migrating neurons. These results suggest that synapsin III may also play a role in adult neurogenesis.

Muscarinic acetylcholine receptors induce neurite outgrowth and activate the synapsin I gene promoter in neuroblastoma clones

The possible role of acetylcholine as a modulator of neuronal differentiation has been tested using a neuroblastoma cell line (N18TG2), which does not synthesize any neurotransmitter. Acetylcholine synthesis has been activated in this line by transfection with a construct containing a choline acetyltransferase (ChAT) cDNA; ChAT-positive clones share a higher ability to grow fibers and an activation of synapsin I expression compared to the parental cells. Atropine, a muscarinic antagonist, abolishes the higher ability to grow fibers of ChAT-positive transfected clones, and the cholinergic agonist carbachol induces higher neurite outgrowth in the parental line. In transient transfections of ChAT-positive clones, the expression of a reporter gene under the control of synapsin I promoter is considerably reduced by atropine, while it is not modified by carbachol; in contrast, in the parental cells, which do not synthesize acetylcholine, the reporter gene expression is induced by carbachol and this effect is abolished by atropine. The data presented provide evidence for the existence of a direct modulation of fiber outgrowth and synapsin I expression by muscarinic receptor activation, which may be related to early growth response gene-1 (EGR-1) levels.

Spatial and temporal regulation of Ca2+/calmodulin-dependent protein kinase II activity in developing neurons

We have studied Ca2+/calmodulin-dependent protein kinase II (CaMKII) isoform distribution and activity in embryonic hippocampal neurons developing in culture. We have found a strong correlation between the expression of the alpha subunit of the enzyme and the ability to undergo depolarization-dependent phosphorylation, which in young neurons is limited to the somatodendritic pool of the kinase. The lack of responsiveness of the axons of young alphaCaMKII-positive neurons is not caused by a lower Ca2+ influx but rather by a differential balance between kinase and phosphatase activities in this compartment. After the establishment of synaptic contacts, the presynaptic pool of the kinase displays an increasing level of activity and acquires the parallel ability to phosphorylate synapsin I, which represents one of the major CaMKII presynaptic targets in mature nerve terminals. In contrast, the activity of the postsynaptic pool of the kinase remains constant throughout synaptogenesis. In the presence of a nearly homogeneous subcellular distribution, this highly regionalized regulation of activity may reflect the multifunctional roles of CaMKII in both developing and mature neurons.

Neuronal differentiation of cultured human NTERA-2cl.D1 cells leads to increased expression of synapsins

The synapsin family consists of three neuronal-specific phosphoproteins associated with dynamic reorganization of the neuronal cytoskeleton. Synapsin I and II are implicated in axonal and synaptic differentiation, formation and maintenance, whereas the function of synapsin III is not as well defined. We report a significant transcriptional upregulation of all three synapsins (synapsin I, 2.1-fold; synapsin II, 2.6-fold; and synapsin III, 5.5-fold) by retinoic acid-induced differentiation of NTera-2cl.D1 cells, a human paradigm for neuronal differentiation. The observed stronger regulation of synapsin III might be due to still active neurite elongation and a rather early state of presynaptic maturation at the time-point investigated, as synapsin III was previously found to be highly enriched in growth cones and during early synaptic development.

Intracellular injection of synapsin I induces neurotransmitter release in C1 neurons of Helix pomatia contacting a wrong target

The contact with the postsynaptic target induces structural and functional modifications in the serotonergic cell C1 of Helix pomatia. In previous studies we have found that the presence of a non-physiological target down-regulates the number of presynaptic varicosities formed by cultured C1 neurons and has a strong inhibitory effect on the action potential-evoked Ca(2+) influx and neurotransmitter release at C1 terminals. Since a large body of experimental evidence implicates the synapsins in the development and functional maturation of synaptic connections, we have investigated whether the injection of exogenous synapsin I into the presynaptic neuron C1 could affect the inhibitory effect of the wrong target on neurotransmitter release. C1 neurons were cultured with the wrong target neuron C3 for three to five days and then injected with either dephosphorylated or Ca(2+)/calmodulin-dependent protein kinase II-phosphorylated Cy3-labeled synapsin I. The subcellular distribution of exogenous synapsin I, followed by fluorescence videomicroscopy, revealed that only synapsin I phosphorylated by Ca(2+)/calmodulin-dependent protein kinase II diffused in the cytoplasm and reached the terminal arborizations of the axon, while the dephosphorylated form did not diffuse beyond the cell body. Evoked neurotransmitter release was measured during C1 stimulation using a freshly dissociated neuron B2 (sniffer) micromanipulated in close contact with the terminals of C1. A three-fold increase in the amplitude of the sniffer depolarization with respect to the pre-injection amplitude (190+/-29% increase, n=10, P<0.006) was found 5 min after injection of Ca(2+)/calmodulin-dependent protein kinase II-phosphorylated synapsin I that lasted for about 30 min. No significant change was observed after injection of buffer or dephosphorylated synapsin I. These data indicate that the presence of synapsin I induces a fast increase in neurotransmitter release that overcomes the inhibitory effect of the non-physiological target and suggest that the expression of synapsins may play a role in the modulation of synaptic strength and neural connectivity.

Synapsin controls both reserve and releasable synaptic vesicle pools during neuronal activity and short-term plasticity in Aplysia

Neurotransmitter release is a highly efficient secretory process exhibiting resistance to fatigue and plasticity attributable to the existence of distinct pools of synaptic vesicles (SVs), namely a readily releasable pool and a reserve pool from which vesicles can be recruited after activity. Synaptic vesicles in the reserve pool are thought to be reversibly tethered to the actin-based cytoskeleton by the synapsins, a family of synaptic vesicle-associated phosphoproteins that have been shown to play a role in the formation, maintenance, and regulation of the reserve pool of synaptic vesicles and to operate during the post-docking step of the release process. In this paper, we have investigated the physiological effects of manipulating synapsin levels in identified cholinergic synapses of Aplysia californica. When endogenous synapsin was neutralized by the injection of specific anti-synapsin antibodies, the amount of neurotransmitter released per impulse was unaffected, but marked changes in the secretory response to high-frequency stimulation were observed, including the disappearance of post-tetanic potentiation (PTP) that was substituted by post-tetanic depression (PTD), and increased rate and extent of synaptic depression. Opposite changes on post-tetanic potentiation were observed when synapsin levels were increased by injecting exogenous synapsin I. Our data demonstrate that the presence of synapsin-dependent reserve vesicles allows the nerve terminal to release neurotransmitter at rates exceeding the synaptic vesicle recycling capacity and to dynamically change the efficiency of release in response to conditioning stimuli (e.g., post-tetanic potentiation). Moreover, synapsin-dependent regulation of the fusion competence of synaptic vesicles appears to be crucial for sustaining neurotransmitter release during short periods at rates faster than the replenishment kinetics and maintaining synchronization of quanta in evoked release.

Lithium ions Up-regulate mRNAs encoding dense-core vesicle proteins in nerve growth factor-differentiated PC12 cells

We recently reported that lithium ions induced an up-regulation of cysteine string protein (CSP) gene expression in nerve growth factor (NGF)-differentiated PC12 cells but not in undifferentiated cells. Concomitantly, expression of two other proteins of regulated secretory pathways, synaptophysin (SY) and SNAP-25, was unaffected by lithium. To assess further the specificity of this effect of lithium, we used cDNA arrays. Our data indicate that lithium ions increase the level of mRNA for proteins such as secretogranin II and vesicular monoamine transporter 1 that are preferentially associated with large densecore secretory vesicles (LDCVs) without affecting mRNAs for proteins predominantly affiliated with small synaptic-like vesicles, including the vesicular acetylcholine transporter and SY. This action of lithium is detected in NGF-differentiated PC12 cells but not in undifferentiated cells. These observations suggest that lithium ions modulate the turnover of LDCVs, and this may play a role in mediating the therapeutic action of lithium in manic-depressive illness.

Synapsin III: developmental expression, subcellular localization, and role in axon formation

We have investigated the developmental expression and subcellular localization of synapsin III, the newest member of the synapsin family, in cultured mouse hippocampal neurons. Our results indicate that synapsin III is expressed early during development, with levels peaking 7 d after plating and declining thereafter. Synapsin III is highly concentrated in growth cones. Using specific antisense oligonucleotides, we have also examined the effect of depleting synapsin III on neurite elongation and synaptogenesis. When synapsin III was suppressed immediately after plating, hippocampal neurons extended minor processes but failed to differentiate one of them as the axon. The suppression of synapsin III after axonal elongation did not affect the time course of synapse formation. The results indicate that synapsin III has a developmental time course, a subcellular localization, and a developmental function very different from those of synapsin I and synapsin II.

Cloning of cDNAs encoding human synapsins IIa and IIb

The synapsins are a family of neuronal phosphoproteins that are specifically associated with the cytoplasmic surface of synaptic vesicles. In mammals, distinct genes for synapsins I, II, and III give rise to members of the synapsin family. The synapsins are implicated in neurotransmitter release and synaptogenesis, processes believed to be aberrant in several neuropsychiatric diseases. The characterization of human synapsins is therefore important for evaluating the possible role of synapsins in human neuropathology. In this report, we describe the cloning and sequence of human synapsins IIa and IIb, products of the synapsin II gene. Human synapsins IIa and IIb conform to the previously described domain model of the synapsins, and the most conserved protein domains are A, C, and E.

Protein-protein interactions and protein modules in the control of neurotransmitter release

Information transfer among neurons is operated by neurotransmitters stored in synaptic vesicles and released to the extracellular space by an efficient process of regulated exocytosis. Synaptic vesicles are organized into two distinct functional pools, a large reserve pool in which vesicles are restrained by the actin-based cytoskeleton, and a quantitatively smaller releasable pool in which vesicles approach the presynaptic membrane and eventually fuse with it on stimulation. Both synaptic vesicle trafficking and neurotransmitter release depend on a precise sequence of events that include release from the reserve pool, targeting to the active zone, docking, priming, fusion and endocytotic retrieval of synaptic vesicles. These steps are mediated by a series of specific interactions among cytoskeletal, synaptic vesicle, presynaptic membrane and cytosolic proteins that, by acting in concert, promote the spatial and temporal regulation of the exocytotic machinery. The majority of these interactions are mediated by specific protein modules and domains that are found in many proteins and are involved in numerous intracellular processes. In this paper, the possible physiological role of these multiple protein-protein interactions is analysed, with ensuing updating and clarification of the present molecular model of the process of neurotransmitter release.

Overexpression of rat synapsins in NG108-15 neuronal cells enhances functional synapse formation with myotubes

The rate of functional synapse formation in NG108-15 neuronal cells transiently transfected with cDNAs of rat synapsin Ia, Ib, IIa and IIb significantly increased during the late phase of coculture with myotubes. The result shows that four synapsins may function equally well in the facilitation of NG108-15-myotube synapse formation.

A third member of the synapsin gene family

Synapsins are a family of neuron-specific synaptic vesicle-associated phosphoproteins that have been implicated in synaptogenesis and in the modulation of neurotransmitter release. In mammals, distinct genes for synapsins I and II have been identified, each of which gives rise to two alternatively spliced isoforms. We have now cloned and characterized a third member of the synapsin gene family, synapsin III, from human DNA. Synapsin III gives rise to at least one protein isoform, designated synapsin IIIa, in several mammalian species. Synapsin IIIa is associated with synaptic vesicles, and its expression appears to be neuron-specific. The primary structure of synapsin IIIa conforms to the domain model previously described for the synapsin family, with domains A, C, and E exhibiting the highest degree of conservation. Synapsin IIIa contains a novel domain, termed domain J, located between domains C and E. The similarities among synapsins I, II, and III in domain organization, neuron-specific expression, and subcellular localization suggest a possible role for synapsin III in the regulation of neurotransmitter release and synaptogenesis. The human synapsin III gene is located on chromosome 22q12-13, which has been identified as a possible schizophrenia susceptibility locus. On the basis of this localization and the well established neurobiological roles of the synapsins, synapsin III represents a candidate gene for schizophrenia.