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

Long-Term Synaptic Plasticity Tunes the Gain of Information Channels through the Cerebellum Granular Layer

A central hypothesis on brain functioning is that long-term potentiation (LTP) and depression (LTD) regulate the signals transfer function by modifying the efficacy of synaptic transmission. In the cerebellum, granule cells have been shown to control the gain of signals transmitted through the mossy fiber pathway by exploiting synaptic inhibition in the glomeruli. However, the way LTP and LTD control signal transformation at the single-cell level in the space, time and frequency domains remains unclear. Here, the impact of LTP and LTD on incoming activity patterns was analyzed by combining patch-clamp recordings in acute cerebellar slices and mathematical modeling. LTP reduced the delay, increased the gain and broadened the frequency bandwidth of mossy fiber burst transmission, while LTD caused opposite changes. These properties, by exploiting NMDA subthreshold integration, emerged from microscopic changes in spike generation in individual granule cells such that LTP anticipated the emission of spikes and increased their number and precision, while LTD sorted the opposite effects. Thus, akin with the expansion recoding process theoretically attributed to the cerebellum granular layer, LTP and LTD could implement selective filtering lines channeling information toward the molecular and Purkinje cell layers for further processing.

The different clinical facets of SYN1-related neurodevelopmental disorders

Synapsin-I (SYN1) is a presynaptic phosphoprotein crucial for synaptogenesis and synaptic plasticity. Pathogenic SYN1 variants are associated with variable X-linked neurodevelopmental disorders mainly affecting males. In this study, we expand on the clinical and molecular spectrum of the SYN1-related neurodevelopmental disorders by describing 31 novel individuals harboring 22 different SYN1 variants. We analyzed newly identified as well as previously reported individuals in order to define the frequency of key features associated with these disorders. Specifically, behavioral disturbances such as autism spectrum disorder or attention deficit hyperactivity disorder are observed in 91% of the individuals, epilepsy in 82%, intellectual disability in 77%, and developmental delay in 70%. Seizure types mainly include tonic-clonic or focal seizures with impaired awareness. The presence of reflex seizures is one of the most representative clinical manifestations related to SYN1. In more than half of the cases, seizures are triggered by contact with water, but other triggers are also frequently reported, including rubbing with a towel, fever, toothbrushing, fingernail clipping, falling asleep, and watching others showering or bathing. We additionally describe hyperpnea, emotion, lighting, using a stroboscope, digestive troubles, and defecation as possible triggers in individuals with SYN1 variants. The molecular spectrum of SYN1 variants is broad and encompasses truncating variants (frameshift, nonsense, splicing and start-loss variants) as well as non-truncating variants (missense substitutions and in-frame duplications). Genotype-phenotype correlation revealed that epileptic phenotypes are enriched in individuals with truncating variants. Furthermore, we could show for the first time that individuals with early seizures onset tend to present with severe-to-profound intellectual disability, hence highlighting the existence of an association between early seizure onset and more severe impairment of cognitive functions. Altogether, we present a detailed clinical description of the largest series of individuals with SYN1 variants reported so far and provide the first genotype-phenotype correlations for this gene. A timely molecular diagnosis and genetic counseling are cardinal for appropriate patient management and treatment.

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.

Synapsin II Regulation of GABAergic Synaptic Transmission Is Dependent on Interneuron Subtype

Synapsins are epilepsy susceptibility genes that encode phosphoproteins reversibly associated with synaptic vesicles. Synapsin II (SynII) gene deletion produces a deficit in inhibitory synaptic transmission, and this defect is thought to cause epileptic activity. We systematically investigated how SynII affects synchronous and asynchronous release components of inhibitory transmission in the CA1 region of the mouse hippocampus. We found that the asynchronous GABAergic release component is diminished in SynII-deleted (SynII(-)) slices. To investigate this defect at different interneuron subtypes, we selectively blocked either N-type or P/Q-type Ca²⁺ channels. SynII deletion suppressed the asynchronous release component at synapses dependent on N-type Ca²⁺ channels but not at synapses dependent on P/Q-type Ca²⁺ channels. We then performed paired double-patch recordings from inhibitory basket interneurons connected to pyramidal neurons and used cluster analysis to classify interneurons according to their spiking and synaptic parameters. We identified two cell subtypes, presumably parvalbumin (PV) and cholecystokinin (CCK) expressing basket interneurons. To validate our interneuron classification, we took advantage of transgenic animals with fluorescently labeled PV interneurons and confirmed that their spiking and synaptic parameters matched the parameters of presumed PV cells identified by the cluster analysis. The analysis of the release time course at the two interneuron subtypes demonstrated that the asynchronous release component was selectively reduced at SynII(-) CCK interneurons. In contrast, the transmission was desynchronized at SynII(-) PV interneurons. Together, our results demonstrate that SynII regulates the time course of GABAergic release, and that this SynII function is dependent on the interneuron subtype.SIGNIFICANCE STATEMENT Deletion of the neuronal protein synapsin II (SynII) leads to the development of epilepsy, probably due to impairments in inhibitory synaptic transmission. We systematically investigated SynII function at different subtypes of inhibitory neurons in the hippocampus. We discovered that SynII affects the time course of GABA release, and that this effect is interneuron subtype specific. Within one of the subtypes, SynII deficiency synchronizes the release and suppresses the asynchronous release component, while at the other subtype SynII deficiency suppresses the synchronous release component. These results reveal a new SynII function in the regulation of the time course of GABA release and demonstrate that this function is dependent on the interneuron subtype.

Alteration in synaptic junction proteins following traumatic brain injury

Extensive research and scientific efforts have been focused on the elucidation of the pathobiology of cellular and axonal damage following traumatic brain injury (TBI). Conversely, few studies have specifically addressed the issue of synaptic dysfunction. Synaptic junction proteins may be involved in post-TBI alterations, leading to synaptic loss or disrupted plasticity. A Synapse Protein Database on synapse ontology identified 109 domains implicated in synaptic activities and over 5000 proteins, but few of these demonstrated to play a role in the synaptic dysfunction after TBI. These proteins are involved in neuroplasticity and neuromodulation and, most importantly, may be used as novel neuronal markers of TBI for specific intervention.

Functional role of ATP binding to synapsin I in synaptic vesicle trafficking and release dynamics

Synapsins (Syns) are synaptic vesicle (SV)-associated proteins involved in the regulation of synaptic transmission and plasticity, which display a highly conserved ATP binding site in the central C-domain, whose functional role is unknown. Using molecular dynamics simulations, we demonstrated that ATP binding to SynI is mediated by a conformational transition of a flexible loop that opens to make the binding site accessible; such transition, prevented in the K269Q mutant, is not significantly affected in the absence of Ca(2+) or by the E373K mutation that abolishes Ca(2+)-binding. Indeed, the ATP binding to SynI also occurred under Ca(2+)-free conditions and increased its association with purified rat SVs regardless of the presence of Ca(2+) and promoted SynI oligomerization. However, although under Ca(2+)-free conditions, SynI dimerization and SV clustering were enhanced, Ca(2+) favored the formation of tetramers at the expense of dimers and did not affect SV clustering, indicating a role of Ca(2+)-dependent dimer/tetramer transitions in the regulation of ATP-dependent SV clustering. To elucidate the role of ATP/SynI binding in synaptic physiology, mouse SynI knock-out hippocampal neurons were transduced with either wild-type or K269Q mutant SynI and inhibitory transmission was studied by patch-clamp and electron microscopy. K269Q-SynI expressing inhibitory synapses showed increased synaptic strength due to an increase in the release probability, an increased vulnerability to synaptic depression and a dysregulation of SV trafficking, when compared with wild-type SynI-expressing terminals. The results suggest that the ATP-SynI binding plays predocking and postdocking roles in the modulation of SV clustering and plasticity of inhibitory synapses.

Biochemical characterization of sirtuin 6 in the brain and its involvement in oxidative stress response

Sirtuin 6 (SIRT6) is a member of nicotinamide adenine dinucleotide-dependent deacetylase protein family and has been implicated in the control of glucose and lipid metabolism, cancer, genomic stability and DNA repair. Moreover, SIRT6 regulates the expression of a large number of genes involved in stress response and aging. The role of SIRT6 in brain function and neuronal survival is largely unknown. Here, we biochemically characterized SIRT6 in brain tissues and primary neuronal cultures and found that it is highly expressed in cortical and hippocampal regions and enriched in the synaptosomal membrane fraction. Immunoblotting analysis on cortical and hippocampal neurons showed that SIRT6 is downregulated during maturation in vitro, reaching the lowest expression at 11 days in vitro. In addition, SIRT6 overexpression in terminally differentiated cortical and hippocampal neurons, mediated by a neuron-specific recombinant adeno-associated virus, downregulated cell viability under oxidative stress condition. By contrast, under control condition, SIRT6 overexpression had no detrimental effect. Overall these results suggest that SIRT6 may play a role in synaptic function and neuronal maturation and it may be implicated in the regulation of neuronal survival.

Involvement of synaptic genes in the pathogenesis of autism spectrum disorders: the case of synapsins

Autism spectrum disorders (ASDs) are heterogeneous neurodevelopmental disorders characterized by deficits in social interaction and social communication, restricted interests, and repetitive behaviors. Many synaptic protein genes are linked to the pathogenesis of ASDs, making them prototypical synaptopathies. An array of mutations in the synapsin (Syn) genes in humans has been recently associated with ASD and epilepsy, diseases that display a frequent comorbidity. Syns are pre-synaptic proteins regulating synaptic vesicle traffic, neurotransmitter release, and short-term synaptic plasticity. In doing so, Syn isoforms control the tone of activity of neural circuits and the balance between excitation and inhibition. As ASD pathogenesis is believed to result from dysfunctions in the balance between excitatory and inhibitory transmissions in neocortical areas, Syns are novel ASD candidate genes. Accordingly, deletion of single Syn genes in mice, in addition to epilepsy, causes core symptoms of ASD by affecting social behavior, social communication, and repetitive behaviors. Thus, Syn knockout mice represent a good experimental model to define synaptic alterations involved in the pathogenesis of ASD and epilepsy.

Synapsin II desynchronizes neurotransmitter release at inhibitory synapses by interacting with presynaptic calcium channels

In the central nervous system, most synapses show a fast mode of neurotransmitter release known as synchronous release followed by a phase of asynchronous release, which extends over tens of milliseconds to seconds. Synapsin II (SYN2) is a member of the multigene synapsin family (SYN1/2/3) of synaptic vesicle phosphoproteins that modulate synaptic transmission and plasticity, and are mutated in epileptic patients. Here we report that inhibitory synapses of the dentate gyrus of Syn II knockout mice display an upregulation of synchronous neurotransmitter release and a concomitant loss of delayed asynchronous release. Syn II promotes γ-aminobutyric acid asynchronous release in a Ca²⁺-dependent manner by a functional interaction with presynaptic Ca²⁺ channels, revealing a new role in synaptic transmission for synapsins.

The arrival of action potentials at nerve terminals often leads to synchronous neurotransmitter release. Medrihan and colleagues use electrophysiology on mouse hippocampal neurons to show that the vesicle protein Synapsin II promotes GABAergic asynchronous release by interacting with calcium channels.

Synapsins contribute to the dynamic spatial organization of synaptic vesicles in an activity-dependent manner

The precise subcellular organization of synaptic vesicles (SVs) at presynaptic sites allows for rapid and spatially restricted exocytotic release of neurotransmitter. The synapsins (Syns) are a family of presynaptic proteins that control the availability of SVs for exocytosis by reversibly tethering them to each other and to the actin cytoskeleton in a phosphorylation-dependent manner. Syn ablation leads to reduction in the density of SV proteins in nerve terminals and increased synaptic fatigue under high-frequency stimulation, accompanied by the development of an epileptic phenotype. We analyzed cultured neurons from wild-type and Syn I,II,III(-/-) triple knock-out (TKO) mice and found that SVs were severely dispersed in the absence of Syns. Vesicle dispersion did not affect the readily releasable pool of SVs, whereas the total number of SVs was considerably reduced at synapses of TKO mice. Interestingly, dispersion apparently involved exocytosis-competent SVs as well; it was not affected by stimulation but was reversed by chronic neuronal activity blockade. Altogether, these findings indicate that Syns are essential to maintain the dynamic structural organization of synapses and the size of the reserve pool of SVs during intense SV recycling, whereas an additional Syn-independent mechanism, whose molecular substrate remains to be clarified, targets SVs to synaptic boutons at rest and might be outpaced by activity.

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.

Synapsin I is an oligomannose-carrying glycoprotein, acts as an oligomannose-binding lectin, and promotes neurite outgrowth and neuronal survival when released via glia-derived exosomes

Oligomannosidic glycans play important roles in nervous system development and function. By performing a phage display screening with oligomannose-specific antibodies, we identified an oligomannose-mimicking peptide that was functionally active in modulating neurite outgrowth and neuron-astrocyte adhesion. Using the oligomannose-mimicking peptide in crosslinking experiments, synapsin I was identified as a novel oligomannose-binding protein in mouse brain. Further analyses not only verified that synapsin I is an oligomannose-binding lectin, but also indicated that it is a glycoprotein carrying oligomannose and Lewis(x). We also found that synapsin I is expressed in glia-enriched cultures and is released from glial cells via exosomes. Incubation of glial-derived exosomes in the presence of high KCl concentrations or subjecting glial cell cultures to either oxygen/glucose deprivation or hydrogen peroxide resulted in release of synapsin I from exosomes. Application of synapsin I promoted neurite outgrowth from hippocampal neurons and increased survival of cortical neurons upon hydrogen peroxide treatment or oxygen/glucose deprivation. Coculture experiments using wild-type hippocampal neurons and wild-type or synapsin-deficient glial cells showed enhanced neurite outgrowth when synapsin was expressed by glial cells. Synapsin-induced neurite outgrowth was dependent on oligomannose on synapsin I and the neural cell adhesion molecule NCAM at the neuronal cell surface. The data indicate that, under conditions of high neuronal activity and/or oxidative stress, synapsin can be released from glial-derived exosomes and promotes neurite outgrowth and neuronal survival by modulating the interactions between glia and neurons.

SYN1 loss-of-function mutations in autism and partial epilepsy cause impaired synaptic function

Several genes predisposing to autism spectrum disorders (ASDs) with or without epilepsy have been identified, many of which are implicated in synaptic function. Here we report a Q555X mutation in synapsin 1 (SYN1), an X-linked gene encoding for a neuron-specific phosphoprotein implicated in the regulation of neurotransmitter release and synaptogenesis. This nonsense mutation was found in all affected individuals from a large French-Canadian family segregating epilepsy and ASDs. Additional mutations in SYN1 (A51G, A550T and T567A) were found in 1.0 and 3.5% of French-Canadian individuals with autism and epilepsy, respectively. The majority of these SYN1 mutations were clustered in the proline-rich D-domain which is substrate of multiple protein kinases. When expressed in synapsin I (SynI) knockout (KO) neurons, all the D-domain mutants failed in rescuing the impairment in the size and trafficking of synaptic vesicle pools, whereas the wild-type human SynI fully reverted the KO phenotype. Moreover, the nonsense Q555X mutation had a dramatic impact on phosphorylation by MAPK/Erk and neurite outgrowth, whereas the missense A550T and T567A mutants displayed impaired targeting to nerve terminals. These results demonstrate that SYN1 is a novel predisposing gene to ASDs, in addition to epilepsy, and strengthen the hypothesis that a disturbance of synaptic homeostasis underlies the pathogenesis of both diseases.

Motor deficits in neurofibromatosis type 1 mice: the role of the cerebellum

Neurofibromatosis type 1 (NF1) is an autosomal dominantly inherited disease, characterized by various neurocutaneous symptoms, cognitive impairments and problems in fine and gross motor performance. Although cognitive deficits in NF1 have been attributed to increased release of the inhibitory neurotransmitter γ-amino butyric acid (GABA) in the hippocampus, the origin of the motor deficits is unknown. Cerebellar Purkinje cells, the sole output neurons of the cerebellar cortex, are GABAergic neurons and express neurofibromin at high levels, suggesting an important role for the cerebellum in the observed motor deficits in NF1. To test this, we determined the cerebellar contribution to motor problems in Nf1(+/-) mice, a validated mouse model for NF1. Using the Rotarod, a non-specific motor performance test, we confirmed that, like NF1 patients, Nf1(+/-) mice have motor deficits. Next, to evaluate the role of the cerebellum in these deficits, mice were subjected to cerebellum-specific motor performance and learning tests. Nf1(+/-) mice showed no impairment on the Erasmus ladder, as step time and number of missteps were not different. Furthermore, when compensatory eye movements were tested, no performance deficits were found in the optokinetic reflex and vestibulo-ocular reflex in the dark (VOR) or in the light (VVOR). Finally, Nf1(+/-) mice successfully completed short- and long-term VOR adaptation paradigms, tests that both depend on cerebellar function. Thus, despite the confirmed presence of motor performance problems in Nf1(+/-) mice, we found no indication of a cerebellar component. These results, combined with recent clinical data, suggest that cerebellar function is not overtly affected in NF1 patients.

A novel form of presynaptic plasticity based on the fast reactivation of release sites switched off during low-frequency depression

Repetitive firing of neurons at a low frequency often leads to a decrease in synaptic strength. The mechanism of this low-frequency depression (LFD) is poorly understood. Here, LFD was studied at Aplysia cholinergic synapses. The absence of a significant change in the paired-pulse ratio during LFD, together with the facts that neither the time course nor the extent of LFD were affected by the initial release probability, suggests that LFD is not related to a depletion of the ready-to-fuse synaptic vesicles (SVs) or to a decrease in the release probability, but results from the silencing of a subpopulation of release sites. A subset of SVs or release sites, which acquired a high release probability status during LFD, permits synapses to rapidly and temporarily recover the initial synaptic strength when the stimulation is stopped. However, the recovery of the full capacity of the synapse to sustain repetitive stimulations is slow and involves spontaneous reactivation of the silent release sites. Application of tetanic stimulations accelerates this recovery by immediately switching on the silent sites. This high-frequency-dependent phenomenon underlies a new form of synaptic plasticity that allows resetting of presynaptic efficiency independently of the recent history of the synapse. Microinjection of a mutated Aplysia synapsin that cannot be phosphorylated by cAMP-dependent protein kinase (PKA), or a PKA inhibitor both prevented high-frequency-dependent awakening of release sites. Changes in the firing pattern of neurons appear to be able to regulate the on-off status of release sites via a molecular cascade involving PKA-dependent phosphorylation of synapsin.

Distinct changes in neuronal and astrocytic amino acid neurotransmitter metabolism in mice with reduced numbers of synaptic vesicles

The relations between glutamate and GABA concentrations and synaptic vesicle density in nerve terminals were examined in an animal model with 40-50% reduction in synaptic vesicle numbers caused by inactivation of the genes encoding synapsin I and II. Concentrations and synthesis of amino acids were measured in extracts from cerebrum and a crude synaptosomal fraction by HPLC and (13)C nuclear magnetic resonance spectroscopy (NMRS), respectively. Analysis of cerebrum extracts, comprising both neurotransmitter and metabolic pools, showed decreased concentration of GABA, increased concentration of glutamine and unchanged concentration of glutamate in synapsin I and II double knockout (DKO) mice. In contrast, both glutamate and GABA concentrations were decreased in crude synaptosomes isolated from synapsin DKO mice, suggesting that the large metabolic pool of glutamate in the cerebral extracts may overshadow minor changes in the transmitter pool. (13)C NMRS studies showed that the changes in amino acid concentrations in the synapsin DKO mice were caused by decreased synthesis of GABA (20-24%) in cerebral neurons and increased synthesis of glutamine (36%) in astrocytes. In a crude synaptosomal fraction, the glutamate synthesis was reduced (24%), but this reduction could not be detected in cerebrum extracts. We suggest that lack of synaptic vesicles causes down-regulation of neuronal GABA and glutamate synthesis, with a concomitant increase in astrocytic synthesis of glutamine, in order to maintain normal neurotransmitter concentrations in the nerve terminal cytosol.

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 highly conserved synapsin domain E mediates synapsin dimerization and phospholipid vesicle clustering

Synapsins are abundant SV (synaptic vesicle)-associated phosphoproteins that regulate synapse formation and function. The highly conserved C-terminal domain E was shown to contribute to several synapsin functions, ranging from formation of the SV reserve pool to regulation of the kinetics of exocytosis and SV cycling, although the molecular mechanisms underlying these effects are unknown. In the present study, we used a synthetic 25-mer peptide encompassing the most conserved region of domain E (Pep-E) to analyse the role of domain E in regulating the interactions between synapsin I and liposomes mimicking the phospholipid composition of SVs (SV-liposomes) and other pre-synaptic protein partners. In affinity-chromatography and cross-linking assays, Pep-E bound to endogenous and purified exogenous synapsin I and strongly inhibited synapsin dimerization, indicating a role in synapsin oligomerization. Consistently, Pep-E (but not its scrambled version) counteracted the ability of holo-synapsin I to bind and coat phospholipid membranes, as analysed by AFM (atomic force microscopy) topographical scanning, and significantly decreased the clustering of SV-liposomes induced by holo-synapsin I in FRET (Förster resonance energy transfer) assays, suggesting a causal relationship between synapsin oligomerization and vesicle clustering. Either Pep-E or a peptide derived from domain C was necessary and sufficient to inhibit both dimerization and vesicle clustering, indicating the participation of both domains in these activities of synapsin I. The results provide a molecular explanation for the effects of domain E in nerve terminal physiology and suggest that its effects on the size and integrity of SV pools are contributed by the regulation of synapsin dimerization and SV clustering.

Cognitive impairment in Gdi1-deficient mice is associated with altered synaptic vesicle pools and short-term synaptic plasticity, and can be corrected by appropriate learning training

The GDI1 gene, responsible in human for X-linked non-specific mental retardation, encodes alphaGDI, a regulatory protein common to all GTPases of the Rab family. Its alteration, leading to membrane accumulation of different Rab GTPases, may affect multiple steps in neuronal intracellular traffic. Using electron microscopy and electrophysiology, we now report that lack of alphaGDI impairs several steps in synaptic vesicle (SV) biogenesis and recycling in the hippocampus. Alteration of the SV reserve pool (RP) and a 50% reduction in the total number of SV in adult synapses may be dependent on a defective endosomal-dependent recycling and may lead to the observed alterations in short-term plasticity. As predicted by the synaptic characteristics of the mutant mice, the short-term memory deficit, observed when using fear-conditioning protocols with short intervals between trials, disappeared when the Gdi1 mutants were allowed to have longer intervals between sessions. Likewise, previously observed deficits in radial maze learning could be corrected by providing less challenging pre-training. This implies that an intact RP of SVs is necessary for memory processing under challenging conditions in mice. The possibility to correct the learning deficit in mice may have clinical implication for future studies in human.

Synapsin-I- and synapsin-II-null mice display an increased age-dependent cognitive impairment

Synapsin I (SynI) and synapsin II (SynII) are major synaptic vesicle (SV) proteins that function in the regulation of the availability of SVs for release in mature neurons. SynI and SynII show a high level of sequence similarity and share many functions in vivo, although distinct physiological roles for the two proteins have been proposed. Both SynI–/– and SynII–/– mice have a normal lifespan, but exhibit a decreased number of SVs and synaptic depression upon high-frequency stimulation. Because of the role of the synapsin proteins in synaptic organization and plasticity, we studied the long-lasting effects of synapsin deletion on the phenotype of SynI–/– and SynII–/– mice during aging. Both SynI–/– and SynII–/– mice displayed behavioural defects that emerged during aging and involved emotional memory in both mutants, and spatial memory in SynII–/– mice. These abnormalities, which were more pronounced in SynII–/– mice, were associated with neuronal loss and gliosis in the cerebral cortex and hippocampus. The data indicate that SynI and SynII have specific and non-redundant functions, and that synaptic dysfunctions associated with synapsin mutations negatively modulate cognitive performances and neuronal survival during senescence.

Oxidative stress and modification of synaptic proteins in hippocampus after traumatic brain injury

Oxidative stress, an imbalance between oxidants and antioxidants, contributes to the pathogenesis of traumatic brain injury (TBI). Oxidative neurodegeneration is a key mediator of exacerbated morphological responses and deficits in behavioral recoveries. The present study assessed early hippocampal sequential imbalance to possibly enhance antioxidant therapy. Young adult male Sprague-Dawley rats were subjected to a unilateral moderate cortical contusion. At various times post-TBI, animals were killed and the hippocampus was analyzed for antioxidants (GSH, GSSG, glutathione peroxidase, glutathione reductase, glutathione-S-transferase, glucose-6-phosphate dehydrogenase, superoxide dismutase, and catalase) and oxidants (acrolein, 4-hydroxynonenal, protein carbonyl, and 3-nitrotyrosine). Synaptic markers (synapsin I, postsynaptic density protein 95, synapse-associated protein 97, growth-associated protein 43) were also analyzed. All values were compared with those for sham-operated animals. Significant time-dependent changes in antioxidants were observed as early as 3 h posttrauma and paralleled increases in oxidants (4-hydroxynonenal, acrolein, and protein carbonyl), with peak values obtained at 24-48 h. Time-dependent changes in synaptic proteins (synapsin I, postsynaptic density protein 95, and synapse-associated protein 97) occurred well after levels of oxidants peaked. These results indicate that depletion of antioxidant systems following trauma could adversely affect synaptic function and plasticity. Early onset of oxidative stress suggests that the initial therapeutic window following TBI appears to be relatively short, and it may be necessary to stagger selective types of antioxidant therapy to target specific oxidative components.

Synapsin-dependent development of glutamatergic synaptic vesicles and presynaptic plasticity in postnatal mouse brain

Inactivation of the genes encoding the neuronal, synaptic vesicle-associated proteins synapsin I and II leads to severe reductions in the number of synaptic vesicles in the CNS. We here define the postnatal developmental period during which the synapsin I and/or II proteins modulate synaptic vesicle number and function in excitatory glutamatergic synapses in mouse brain. In wild-type mice, brain levels of both synapsin I and synapsin IIb showed developmental increases during synaptogenesis from postnatal days 5-20, while synapsin IIa showed a protracted increase during postnatal days 20-30. The vesicular glutamate transporters (VGLUT) 1 and VGLUT2 showed synapsin-independent development during postnatal days 5-10, following which significant reductions were seen when synapsin-deficient brains were compared with wild-type brains following postnatal day 20. A similar, synapsin-dependent developmental profile of vesicular glutamate uptake occurred during the same age periods. Physiological analysis of the development of excitatory glutamatergic synapses, performed in the CA1 stratum radiatum of the hippocampus from the two genotypes, showed that both the synapsin-dependent part of the frequency facilitation and the synapsin-dependent delayed response enhancement were restricted to the period after postnatal day 10. Our data demonstrate that while both synaptic vesicle number and presynaptic short-term plasticity are essentially independent of synapsin I and II prior to postnatal day 10, maturation and function of excitatory synapses appear to be strongly dependent on synapsin I and II from postnatal day 20.

Lack of synapsin I reduces the readily releasable pool of synaptic vesicles at central inhibitory synapses

Synapsins (Syns) are synaptic vesicle (SV) phosphoproteins that play a role in neurotransmitter release and synaptic plasticity by acting at multiple steps of exocytosis. Mutation of SYN genes results in an epileptic phenotype in mouse and man suggesting a role of Syns in the control of network excitability. We have studied the effects of the genetic ablation of the SYN1 gene on inhibitory synaptic transmission in primary hippocampal neurons. Inhibitory neurons lacking SynI showed reduced amplitude of IPSCs evoked by isolated action potentials. The impairment in inhibitory transmission was caused by a decrease in the size of the SV readily releasable pool, rather than by changes in release probability or quantal size. The reduction of the readily releasable pool was caused by a decrease in the number of SVs released by single synaptic boutons in response to the action potential, in the absence of variations in the number of synaptic contacts between couples of monosynaptically connected neurons. The deletion of SYN1 did not affect paired-pulse depression or post-tetanic potentiation, but was associated with a moderate increase of synaptic depression evoked by trains of action potentials, which became apparent at high stimulation frequencies and was accompanied by a slow down of recovery from depression. The decreased size of the SV readily releasable pool, coupled with a decreased SV recycling rate and refilling by the SV reserve pool, may contribute to the epileptic phenotype of SynI knock-out mice.

Fast changes in the functional status of release sites during short-term plasticity: involvement of a frequency-dependent bypass of Rac at Aplysia synapses

Synaptic transmission can be described as a stochastic quantal process defined by three main parameters: N, the number of functional release sites; P, the release probability; and Q, the quantum of response. Many changes in synaptic strength that are observed during expression of short term plasticity rely on modifications in P. Regulation of N has been also suggested. We have investigated at identified cholinergic inhibitory Aplysia synapses the cellular mechanism of post-tetanic potentiation (PTP) expressed under control conditions or after N has been depressed by applying lethal toxin (LT) from Clostridium sordellii or tetanus toxin (TeNT). The analysis of the Ca(2+) dependency, paired-pulse ratio and variance to mean amplitude relationship of the postsynaptic responses elicited at distinct extracellular [Ca(2+)]/[Mg(2+)] elicited during control post-tetanic potentiation (PTP(cont)) indicated that PTP(cont) is mainly driven by an increase in release probability, P. The PTP expressed at TeNT-treated synapses (PTP(TeNT)) was found to be similar to PTP(cont), but scaled to the extent of reduction in N produced by TeNT. Despite LT inducing a decrease in N as TeNT does, the PTP expressed at LT-treated synapses (PTP(LT)) was characterized by exceptionally large amplitude and bi-exponential time course, as compared to PTP(cont) or the PTP(TeNT). Analysis of the Ca(2+) dependency of PTP(LT), paired-pulse ratio and fluctuations in amplitude of the postsynaptic responses elicited during PTP(LT) or the variance to mean amplitude relationship of time-locked postsynaptic responses in a series of subsequent PTP(LT) indicated that an N-driven change is involved in the early phase (1 s time scale) of PTP(LT), while at a later stage PTP(LT) is composed of both N and P increases. Our results suggest that fast switching on of the functional status of the release sites occurs also during the early events of PTP(cont). The early N-driven phase of PTP(LT) is likely to be a functional recovery of the release sites silenced by Rac inactivation. This effect did not appear to result from reversion of LT inhibitory action but from bypassing the step regulated by Rac. Altogether the data suggest that Rac and the intracellular pathway which allows the bypassing of Rac are key players in new forms of short-term plasticity that rely on fast, activity-dependent changes in the functional status of the release sites.

Mass Spectrometric Studies on Mouse Hippocampal Synapsins Ia, IIa, and IIb and Identification of a Novel Phosphorylation Site at Serine-546

Synapsins are key phosphoproteins in the mammalian brain, and structural research on synapsins is still holding center stage. Proteins were extracted from hippocampal tissue and separated on two-dimensional gel electrophoresis (2-DE), and the spots were analyzed by MALDI-TOF-TOF and nano-LC-ESI-MS/MS. Synapsins Ia, IIa, and IIb were unambiguously identified and represented by 15 individual spots on 2-DE. Several serine phosphorylation sites were confirmed, and a novel phosphorylation site was observed at Ser-546 in synapsin IIa in all gels analyzed.

Influence of synapsin I on synaptic vesicles: an analysis by force-volume mode of the atomic force microscope and dynamic light scattering

Synaptic vesicles (SVs) are small neuronal organelles that store neurotransmitters and release them by exocytosis into the synaptic cleft for signal transmission between nerve cells. They consist of a highly curved membrane composed of different lipids containing several proteins with specific functions. A family of abundant extrinsic SV proteins, the synapsins, interact with SV proteins and phospholipids and play an important role in the regulation of SV trafficking and stability. We investigated the interactions of one these proteins with the SV membrane using atomic force microscope and dynamic light scattering. We examined SVs isolated from rat forebrain both under native conditions and after depletion of endogenous synapsin I. We used the atomic force microscope in two modes: imaging mode for characterizing the shape and size of SVs, and force-volume mode for characterizing their stiffness. Synapsin-depleted SVs were larger in size and showed a higher tendency to aggregate than native vesicles, although their stiffness was not significantly different. Because synapsins are believed to cross-link SV to each other and to the actin cytoskeleton, we also measured the SV aggregation kinetics induced by synapsin I by dynamic light scattering and atomic force microscopy and found that the addition of synapsin I promotes a rapid aggregation of SVs. The data indicate that synapsin directly affects SV stability and aggregation state and support the physiological role of synapsins in the assembly and regulation of SV pools within nerve terminals.

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.