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The Synapsins and the Control of Neuroexocytosis

,* , , , and .

* Corresponding Author: Department of Experimental Medicine, Section of Human Physiology, University of Genova, Viale Benedetto XV, 3, 16132 Genova, Italy. Email

The synapsins have been the first synaptic vesicle-associated proteins to be discovered thanks to their prominent ability to be phosphorylated by a variety of protein kinases. At present, the synapsin family in mammals consists of at least 10 isoforms encoded by three distinct genes and composed by a mosaic of conserved and variable domains. The synapsins are highly conserved evolutionarily and synapsin homologues have been described in invertebrates and lower vertebrates. The synapsins are implicated in multiple interactions with synaptic vesicle proteins and phospholipids, actin and protein kinases. Via these interactions, the synapsins play multiple roles in synaptic transmission, including control of synapse formation, regulation of synaptic vesicle trafficking, neurotransmitter release and expression of short-term synaptic plasticity phenomena. This chapter tries to summarize the main functional features of the synapsins that have emerged in the last 20 years, in order to provide a framework for interpreting the complex role played by these phosphoproteins in synaptic physiology.


The release of classical neurotransmitters (NTs) occurs at specialized sites of the plasma membrane, named active zones, by exocytotic fusion of small synaptic vesicles (SVs). The uniform loading of SVs with a discrete amount of NT8 is reflected by the reproducibility in the size of the postsynaptic response elicited by each exocytotic event referred to a NT quantum. At variance with nonneuronal cells, neuroexocytosis is characterized by: (1) an “explosive” rate of NT release, many orders of magnitude faster than that of nonneuronal cells; (2) the ability to operate at various levels of efficiency depending on the microenviromental conditions and the previous “history” of the neuron; and (3) the ability to sustain repetitive high frequency NT release over a long period of time with strong reliability. The molecular features that confer such properties to neuroexocytosis in neurons are: (i) the high colocalization of Ca2+ channels with fusion competent SVs which allows an extremely rapid Ca2+-dependent exocytosis, (ii) the existence of a strategically localized reserve pool of SV buffering the depletion of the readily released pool during sustained repetitive release and (iii) the presence of efficient recycling mechanisms active at the presynaptic membrane that prevent the rapid depletion of SVs during a sustained repetitive release. Such recycling mechanisms are contributed by a fast and direct endocytotic pathway operating at the active zones (“kiss & run” mechanism) and by a slower clathrin-mediated endocytosis active at periactive zones.

The remarkable properties of neurotransmitter release are generated by the activity of a number of proteins that are localized within the presynaptic terminal and participate in synapse formation, maintenance and function. Among many presynaptic actors which have been identified in the last 20 years, the most abundant phosphoproteins are the synapsins, a highly conserved multigene family of neuron-specific, SV-associated phosphoproteins.

Synapsins exist in all organisms endowed with a nervous system and, in mammals, are encoded by three distinct genes (SYNI, SYNII and SYNIII) located in chromosome X, 3 and 22, respectively.16,19,24 They are composed of a mosaic of individual and shared domains, the latter of which are highly conserved during evolution (Fig. 1). Synapsins I and II are stably expressed at synapses of mature neurons, where they associate with the cytoplasmic surface of small SVs, whereas the expression of synapsin III is developmentally controlled and not strictly confined to synaptic terminals (Fig. 2). Synapsins are excellent substrates for a large array of protein kinases including protein kinase A, Ca2+/calmodulin-dependent protein kinases (CaMK) I, II and IV, mitogen-activate protein (MAP) kinase and cyclin-dependent kinase-1, that phosphorylate them on distinct serine residues. Synapsins interact in vitro with lipid and protein components of SVs, as well as with various cytoskeletal proteins including actin, and control multiple aspects of synapse structure and function, from synaptogenesis and regulation of SV trafficking to modulation of short-term synaptic plasticity. Here, we will describe the functional studies which have outlined the role played by synapsins in regulation of NT release. In the first part, we will focus on those studies that led to the proposed model of a predocking mechanism of action of the synapsins. Then, we will critically summarize the recently growing body of evidence suggesting that synapsins, in addition to their predocking action, directly control the efficiency of synaptic transmission and the rate of NT release by acting at the post-docking level. For further details concerning structure, biochemistry, genetics, cellular and molecular biology and developmental role of the synapsins, the reader is referred to more extensive reviews in references 3, 11, 16, 19, 24.

Figure 1. Evolutionary conservation of the synapsins.

Figure 1

Evolutionary conservation of the synapsins. Synapsins have been cloned from a variety of species, from invertebrates to man. Synapsins are composed of a mosaic of conserved and individual domains that are schematically represented in blocked color form (more...)

Figure 2. Temporal pattern of expression of synapsin I (S I), synapsin II (S II) and synapsin III (S III) in primary hippocampal neurons as a function of the days in vitro (DIV).

Figure 2

Temporal pattern of expression of synapsin I (S I), synapsin II (S II) and synapsin III (S III) in primary hippocampal neurons as a function of the days in vitro (DIV). Expression at various times is shown in percentage of the maximal level of expression (more...)

The Synapsins and the Reserve Pool of Synaptic Vesicles

A large body of experimental evidence obtained in reconstituted nerve terminals has proposed that the synapsins reversibly cross-link SVs to each other and to the actin-based cytoskeletal meshwork. This action is believed to be important for the formation and maintenance of a reserve pool of SVs as well as in the fine regulation of the balance between the reserve pool and a pool of SVs ready to undergo exocytosis in an activity- and phosphorylation-dependent manner.

In order to perturb synapsin function at the nerve terminal and define its functional role in neuroexocytosis, two main experimental approaches have been used: (i) microinjection into large presynaptic terminals of invertebrate or lower vertebrate neurons of exogenous synapsin, antibodies to synapsin or peptides derived from evolutionary conserved synapsin sequences; (ii) deletion of one or more of the synapsin genes in mice by gene knockout (KO) technology. Both techniques have advantages and limitations.

Microinjection studies are potentially the best method to acutely study functional changes induced by perturbation of intraterminal synapsin levels. It allows to interfere directly with synapsin function and follow the effects generated by the injected agent in real-time. However, injection of proteins, peptides or antibodies could have nonspecific effects for the relative high concentrations that are often required and for the possibility that the injected agent undergoes a nonphysiological targeting within the neuron. Genetic studies are probably the best approach to provide answers to the ultimate function of a given protein, i.e., the function that cannot be compensated during development by other genes, but again they have some drawbacks. First of all, very often a change in a single protein promotes a series of homeostatic responses in downstream processes that allow neuronal systems to respond to the initial manipulation with secondary changes, making the interpretation of the phenotype difficult. Moreover, the effects of specific gene deletions are often attenuated by the presence of homologue gene products with redundant functions. This is particularly true in the case of the synapsins that are encoded by three genes. Despite these limitations, the knocking out of synapsin genes represents a fundamental technique to study in vivo the action of synapsin on development, synaptogenesis, maintenance and function of synapses in a long-term time scale.

Pioneer experiments testing the effect of exogenous synapsin I in squid giant synapses showed that the injection of dephosphorylated synapsin I decreased the amplitude and rate of rise of postsynaptic potentials, whereas the injection of either phosphorylated synapsin I or heat-inactivated dephosphorylated synapsin I were ineffective. Conversely, injection of CaMKII increased the rate of rise and the amplitude of postsynaptic potentials.27,28 Analysis of synaptic noise in the same system revealed that dephosphorylated synapsin I reduced the rate of spontaneous and evoked quantal release, whereas the injection of CaMKII increased evoked release without affecting the frequency of spontaneous miniature events.29 Further data obtained in vertebrate goldfish neurons showed that the presynaptic injection of dephosphorylated synapsin I reduced both spontaneous and evoked synaptic transmission.17 Internalization of dephosphorylated synapsin, phosphorylated synapsin or activated CaMKII into rat brain synaptosomes using freeze-thaw permeabilization confirmed the results obtained by in vivo injections.32,33

These data suggested an initial model in which dephosphorylated synapsin I inhibits synaptic transmission without interfering directly with the release process, but recruiting SVs to the reserve pool and inhibiting SV mobilization to the readily releasable pool, a process that can be reverted upon phosphorylation. Studies on the physical distribution of the protein in response to a depolarising stimulus conducted in frog nerve muscle preparation showed that synapsin I dissociation from the SV membrane is not a prerequisite for fusion and that under high frequency electrical stimulation synapsin I partially dissociates from SVs during exocytosis and reassociates with the SV membrane following endocytosis.46,47 In agreement with the latter data, phosphorylation of synapsin I in rat brain synaptosomes treated with depolarising agents is associated with a rapid translocation of the protein from the membrane fraction to the synaptosol.40 These data have been recently confirmed in living hippocampal neurons, in which synapsin was found to disperse in the presynaptic terminal and preterminal axon during depolarization and to recluster at SV sites following return to the resting state.5 In these studies it was also found that the rates of dispersion and reclustering are indeed controlled by synapsin phosphorylation and dephosphorylation, respectively, and that CaMK-mediated phosphorylation controls SV mobilization at low frequency of stimulation, whereas MAP kinase phosphorylation is recruited at both low and high frequencies of stimulation.5,6

Ultrastructural studies were consistent with early functional studies. In living lamprey reticulospinal axons forming en passant synapses, presynaptic injection of an anti-synapsin antibody, specifically recognizing sequences of the synapsin domain E (Fig. 1), caused the disappearance of SVs distal to the synaptic cleft (reserve pool), leaving unaffected the SVs docked at the active zone. The depletion of the reserve pool was in turn associated with a markedly enhanced depression following high, but not low, frequency stimulation.36 Consistently, the presynaptic injection of a highly conserved peptide fragment of the synapsin domain E into the squid giant synapse greatly reduced the number of SVs far from the active zone and increased the rate and extent of synaptic depression, indicating that domain E, present in both synapsin isoforms expressed in squid (Fig. 1), is essential for the synapsin-mediated maintenance and regulation of the SV reserve pool.18 Closely similar results were obtained after the injection of a highly conserved peptide derived from the synapsin domain C.20 Interestingly, both peptides inhibit the synapsin-actin interactions, providing a common mechanism for the physiological and ultrastructural effects of the peptides from domains E and C.

A fundamental contribution to the study of the role of synapsin in NT release derives from genetic experiments in mice in which synapsin genes have been inactivated to generate single, double and triple KO animals.7,9,15,26,37-39,44,45 All strains of KO mice were viable and fertile. Despite the absence of gross defects in brain morphology or behaviour, synapsin I and synapsin II (but not synapsin III) KO mice as well as double synapsin I/II and triple synapsin I/II/III KO mice exhibited early onset spontaneous and sensory stimuli-evoked (audiogenic) epileptic seizures. Attacks consisted of partial, secondarily generalized “grand mal” attacks followed by post-seizure grooming.38 Electroencephalogram analysis showed that subconvulsive electrical stimulation in the amygdala was able to induce seizures when applied to synapsin mutant mice.26 Typically, seizures develop after 2-3 months of age and become more frequent with age. The incidence of seizures is higher in synapsin II than in synapsin I KO mice and is proportional to the number of inactivated synapsin genes. While the synapsin II and I/II KO mice have been reported to have impaired contextual conditioning41 and triple KO mice exhibited impaired motor coordination and defective spatial learning,15 a detailed analysis of the behavioural phenotype of the synapsin KO mice is still lacking.

Ultrastructural and physiological abnormalities observed in adult synapsin mutant mice largely confirmed and validated the data obtained by injection studies. Synapsin I, II and I/II KO mice showed a selective decrease in the total number of SVs, as demonstrated by a decrease in the levels of most SV markers (Fig. 3) and by electron microscopy of central synapses.14,26,38,44 Similarly to what observed with the injection studies,19,36 the nerve terminal ultrastructure showed a dramatic decrease and disassembly of SVs in the reserve pool, while SVs docked at active zones were only poorly affected.26,44 In synapsin I KO mice, SV depletion was accompanied by a strong impairment in glutamate release from cortical synaptosomes and by a greater delay in the recovery of synaptic transmission after NT depletion by high frequency stimulation.26

Figure 3. The specific decrease in SV density in central synapses is reflected by a decrease in the expression of the major synaptic vesicle proteins.

Figure 3

The specific decrease in SV density in central synapses is reflected by a decrease in the expression of the major synaptic vesicle proteins. Homogenate (HOM) and purified SV fractions obtained from wild-type (WT) and synapsin I KO mice were analyzed by immunoblotting (more...)

The study of SV recycling at individual synaptic boutons using FM dyes showed that the number of exocytosed SVs during brief action potential trains and the total recycling SV pool are significantly reduced in synapsin I KO mice, while the kinetics of endocytosis and SV repriming appear normal.39 The results were similar to those obtained in a different strain of synapsin KO mice by an independent laboratory,38 except that (i) in double KO mice the SV depletion was not restricted to the reserve pool, but affected to the same extent the readily releasable pool of SVs and (ii) there was no detectable increase in synaptic depression induced by 30 sec of repetitive stimulation at 10 Hz in synapsin I KO mice. However, depression was increased in synapsin II KO mice and further enhanced in I/II double KO mice, suggesting a participation also of synapsin I in the build-up of depression.

The molecular basis of the epileptic phenotype observed in synapsin deficient mice are still far from being elucidated. It has been hypothesized that synaptic depression during repetitive stimulation contributes to seizure development by causing an imbalance between excitatory and inhibitory systems. This imbalance is attributable to the fact that inhibitory GABAergic interneurons experience high frequency firing that may make GABA release particularly sensitive to the relative SV depletion induced by synapsin deletion. Terada and coworkers45 investigated the impairment of inhibitory transmission in cultured hippocampal synapses from synapsin I KO mice and demonstrated that inhibitory, but not excitatory, synapses become easily fatigued upon repeated application of hypertonic sucrose and recover slowly from depression. Stimulated terminals showed a decrease in the number of SVs in the reserve pool, but not in the readily releasable pool, that was slightly more intense in GABAergic terminals than in glutamatergic ones. However, the young age of the hippocampal neurons used in this study (8 DIV), a stage in which synaptogenesis is in progress and the formed synapses are still immature, suggests that the observed effects could be ascribed, at least in part, to a defect in synaptogenesis rather than to a change in the mature exocytosis machinery.

Taken together, the effects observed in synapsin I, II, and I/II KO mice are in general agreement with the data obtained by injection studies and strongly support the predocking model in which synapsins I and II participate in the formation and maintenance of the reserve pool of SVs (Fig. 4). This pool provides a strategically localized SV reserve, buffering the depletion of the readily released pool when sustained and repetitive release overrides the tonic SV recycling capacity of the terminal through the direct (kiss & stay/kiss & run) or clathrin-mediated endocytosis.16,19

Figure 4. Schematic model of the exo-endocytosis process and of the putative physiological role of the synapsins.

Figure 4

Schematic model of the exo-endocytosis process and of the putative physiological role of the synapsins. Evoked neurotransmitter release is a multi-step process in which SVs, after being released from the reserve pool where they are bound to the actin (more...)

Synapsin III, the most recently identified member of the synapsin family, plays a role in synaptic function and NT release that appears completely distinct from that of synapsin I or II. First of all, synapsin III is expressed early during neuronal development and its expression is downregulated in mature neurons,10 while the product of the other two synapsin genes have an opposite pattern of expression (Fig. 2). Mice lacking synapsin III exhibited a marked delay in neurite outgrowth, no change in SV density, an increase in the size of the recycling pool of SVs and a significant decrease in synaptic depression,9 in sharp contrast with what has been observed in synapsins I and II KO mice.26,39 These data indicate a unique nonredundant role for synapsin III in the regulation of NT release.

One of the most intriguing functions of synapsin III is its ability to limit the size of the recycling pool of SVs that allows more SVs to be recruited for NT release during repetitive stimulation in synapsin III KO mice. It is possible that synapsin III, highly expressed in early stages of synaptogenesis, may serve to tether SVs to the cytoskeleton and keep them from recycling during synaptic activity as previously suggested for synapsins I and II.1,34 However, the marked decrease in the SV population observed in synapsin I and II KO mice indicates that, while synapsins I and II have profound effects on SV clustering and stability,2,35 synapsin III may be devoid of this activity. Although no physiological evidence for a role of ATP binding to synapsins has been provided thus far, the differential effect of Ca2+ on ATP binding to synapsins (Ca2+ inhibits ATP binding to synapsin III and stimulates ATP binding to synapsin I)21 suggests another potential molecular difference between synapsins I and III.

Notwithstanding the absence of an overt or latent epileptic phenotype, synapsin III KO mice also showed an impairment of GABAergic transmission, while excitatory transmission was unaffected. These results leave open the possibility that the function of synapsin III in inhibitory terminals may differ from that at excitatory synapses. A recent study on synapsin I/II/III triple KO mice investigated this possibility in great detail. Excitatory and inhibitory synaptic transmission was differentially altered in these mice: excitatory synapses exhibited normal basal transmission, but decreased number of SV in the reserve pool and marked depression, whereas inhibitory synapses exhibited impaired basal transmission, mild changes in the number of SV and no changes in depression. Although these observations leave completely open the physiological basis of the increased seizure propensity of synapsin I, II, I/II and I/II/III KO mice, but not of synapsin III KO mice, they demonstrate that the synapsins have a critical role in maintaining the balance between excitatory and inhibitory synapses in brain networks.15

The Synapsins and Short-Term Synaptic Plasticity

There are only relatively few data concerning the role played by synapsins on short-term synaptic plasticity and their interpretation is still debated. Field EPSPs recorded in hippocampal slices of synapsin I KO mice exhibited increased paired-pulse facilitation (PPF),37,38 but no effect was observed on post-tetanic potentation (PTP).38 On the other hand, synapsin II and I/II KO mice showed no changes in PPF, but a dramatic decrease of PTP.38 Cultured hippocampal neurons (7-14 DIV) obtained from synapsin III or I/II/III KO mice showed no changes in PPF.9,15

In cholinergic synapses of Aplysia californica, the functional ablation of synapsin by antibody injection produced a virtual disappearance of PTP, that was substituted by an intense post-tetanic depression. In the same study, basal synaptic transmission was not altered, but PPF was significantly decreased at physiological Ca2+ concentrations. However, decreasing the release probability by lowering the Ca2+/Mg2+ ratio to remove synaptic depression revealed that PPF was not affected by synapsin neutralization.22 Finally, presynaptic injection of the peptide fragment of domain E in squid giant synapses dramatically decreased postsynaptic potential in response to a single action potential but did not affect PPF.18 Thus, most of the available data indicate that PPF is not a primary target of synapsin action in excitatory terminals, although the function of synapsin on short-term plasticity of inhibitory synapses remains completely unexplored. Moreover, PPF has an intrinsic kinetics of tens of milliseconds, a time-range much faster than the time necessary for the Ca2+-dependent mobilization of SVs from the reserve to the readily releasable pool to occur.51 Thus, the possibility exists that synapsin affects PPF either through a post-docking mechanism (see below) or by altering the baseline level of transmission and indirectly influencing the magnitude of short-term plasticity, as recently suggested. 9,19,22,39,45 Indeed, at most synapses an increase in the initial probability of NT release decreases the magnitude of synaptic enhancement (lower PPF), and, conversely, a decrease in the probability of release results in larger synaptic enhancement or smaller synaptic depression (higher PPF).51 Although the absence of synapsins seems not to impair PPF, a recent finding indicates that synapsin I is necessary for the increase in PPF promoted by the constitutive activation of the Ras/MAP kinase pathway.25

At variance with PPF, synapsins appear to have a definite role in the presynaptic expression of PTP. In fact, both genetically altered mice and invertebrate synapses exhibit a marked impairment in PTP after genetic deletion or neutralization of synapsin I and/or synapsin II.22,38 Although synapsin I KO mice showed no detectable changes in PTP, the almost double effect on PTP observed in synapsin I/II double KO mice as compared to the single synapsin II KO mice strongly indicates that synapsin I also plays a role in regulating PTP and that the absence of changes in PTP observed in synapsin I KO mice is attributable to the compensatory effect of synapsin II. PTP is characterized by a time-course in the order of seconds in mammals, a time sufficiently long to involve SV mobilization from the reserve pool. Thus, the action of synapsins on PTP are consistent with the predocking mechanism model described above, in which Ca2+ accumulation induced by tetanic stimulation activates a Ca2+-dependent phosphorylation of synapsin releasing SVs from the actin cytoskeleton and increasing their availability for exocytosis.

The basis for the partial disagreement among some of the observed effects of synapsin deletion on short-term plasticity could be several-fold: (i) except for some of the most recent studies,9,15 a separate analysis of inhibitory and excitatory synapses was not carried out; (ii) the data were obtained using different neuronal preparations, i.e., either primary cultures of hippocampal neurons or acute slices; (iii) primary cultures were used at different stages of maturation and, in most studies, before a complete synaptic maturation had occurred; and (iv) it is experimentally very difficult to measure the baseline level of synaptic transmission, especially in slices. For instance, studies of PPF were performed in brain slices of synapsin KO mice through extracellular stimulation evoking a response that was detected with extracellular electrodes (field EPSPs).37,38 Under these conditions, the amplitude of the evoked response does not provide a measure of baseline transmission because it reflects the activation of many presynaptic fibres and depends upon several factors (i.e., position of the electrodes, stimulation intensity, slice viability). Thus, the previous studies did not determine whether the observed changes in plasticity were direct effects on facilitation or depression or whether they were secondary to changes in some quantal parameter characterizing the efficiency of synaptic transmission, such as initial release probability and release rate. Single cell patch-clamp recordings represent a more adequate experimental approach, although this technique has to be used with caution. In particular, using neuronal cultures obtained by KO mice, a change in the amplitude of evoked postsynaptic currents can be due to impaired synaptogenesis and/or neurite elongation that dramatically decrease the number of functioning synapses and consequently the number of SV released in response to presynaptic stimulation. Only a detailed quantal analysis of miniature currents and a noise analysis of evoked postsynaptic currents will provide the quantal parameter of neurotransmission necessary to interpret the effects on short-term plasticity.

The Synapsins and Release Probability and Kinetics

According to the general model of synapsins tethering SVs to the actin cytoskeleton at a distance from the active zones and releasing them upon activity through phosphorylation-dependent dissociation, SVs recruited to the readily releasable pool should be depleted of synapsins. Although this general picture is still valid and accounts for most of the physiological data, it has become clear that synapsins also have a function at the membrane stages of release after SVs have docked to the active zones. Several observations support the latter view: (i) SVs in the readily releasable pool are only partially depleted of synapsins, and about 20-35% of the synapsin molecules associated with SVs in the reserve pool remain associated with actively recycling SVs during high frequency stimulation;47 (ii) while in resting synapses synapsins are preferentially confined to the reserve pool, during synaptic activity synapsins are detected in association with SVs of the readily releasable pool and with uncoated recycled SVs;4 (iii) synapsins colocalize with actin in the dynamic filamentous cytomatrix present in sites of intense SV recycling.4

The possibility that synapsins could play some role in the post-docking stages of neurexocytosis, initially suggested by the uncertain effects on PPF, was recently demonstrated by growing evidence showing that synapsins can directly affect the probability and the rate of NT release. The first functional evidence suggesting a possible post-docking effect of synapsins was obtained by studying SV dynamics with styryl FM dyes. In hippocampal neurons from synapsin I KO mice, the reduction in the total functional recycling SV pool size was found to be associated with a decrease in the total number of SVs which undergo exocytosis during brief trains of action potentials (20 impulses) at individual synaptic boutons.39 While the former observation was in agreement with the decrease in the reserve pool of SVs,26,44 the latter result was rather unexpected, since stimuli in this range would be expected to draw solely upon the readily releasable pool of SVs which appears relatively intact in synapsin I KO mice, and suggests a decrease in release probability.

More recently, the presynaptic injection of a peptide corresponding to the highly conserved region of domain E of squid synapsin into the squid giant synapse completely inhibited NT release in the absence of appreciable changes in the number of docked SV.18 Interestingly, this effect was accompanied by an increase in the rise and decay times of postsynaptic currents. The kinetics of release was also profoundly altered in cholinergic synapses of Aplysia californica injected with a specific antibody to snail synapsins.22 In this study, the rise time of the evoked postsynaptic current was significantly slowed in the absence of any changes in decay time and mean amplitude of postsynaptic response. Closely similar results were obtained with the injection of a conserved peptide derived from the C domain.20

A post-docking action of synapsins is likely to be involved also in the decrease of evoked inhibitory postsynaptic currents (eIPSCs) observed in CA3 pyramidal neurons from hippocampal slices (P10-14) of synapsin I KO mice.45 Mutant mice showed a decrease in the amplitude and an increase in the coefficient of variation of eIPSCs, while the amplitude of miniature IPSCs was not affected, suggesting that synapsin I deficiency reduces the efficiency of inhibitory synaptic transmission by decreasing the number of SV released by a single action potential.

The decrease of eIPSCs observed in cultured hippocampal autaptic neurons (7-9 DIV) from synapsin III KO mice9 could be also attributable to a decreased release probability, although a more detailed electrophysiological analysis is necessary to exclude other possibilities.

Taken together, the data summarized here strongly suggest that the synapsins are also involved in the post-docking steps of release. By directly or indirectly regulating priming and/or fusion reactions, the synapsins may play a role in determining the rate and the amount of docked SVs released in response to the action potential (Fig. 4). Such post-docking action could be accounted for by interactions of the synapsins with the dynamic actin cytoskeleton present at the active and periactive zones and/or with presynaptic proteins involved in the priming/fusion steps. On the one hand, it has recently been shown that the synapsin domain E and domain C peptides have the ability to inhibit the binding of endogenous synapsins to actin,20 suggesting that an interaction with actin at the active zone may play a role in the post-docking effects of synapsin. On the other hand, synapsins have been recently shown to interact with the SV-associated G protein Rab3A and to modulate Rab3 cycling and GTPase activity within nerve terminals.13,14 As Rab 3A has been proposed to limit the amount of NT released in response to the Ca2+ signal in a late step that follows docking and priming,12 in principle the post-docking effect of synapsins can be achieved through the removal of the Rab3-mediated inhibitory constraint on quantal release.

The Synapsins, Long-Term Plasticity and Learning

Previous studies have shown that electrically-induced long-term potentiation (LTP) increases the phosphorylation of synapsin I at its CaM kinase II sites immediately after stimulation, with an effect that persists for over 30 min and is fully blocked by the NMDA receptor antagonist D(2)-2-amino-5-phosphonopentanoic acid (APV).31 Moreover LTP-like potentiation produced by β-adrenergic agonists and protein kinase C activators produces a dose-dependent increase in the phosphorylation of synapsin I at its CaM kinase II sites.30,43

Otherwise, a large body of previous data demonstrated that synapsin I KO mice have normal LTP and normal or slightly impaired learning.37,41,42 However, very recent observations put forward a role for synapsin I in LTP and learning. In fact, deletion of synapsin I blocked the enhancement of LTP, of spatial learning and of contextual fear conditioning associated with a constitutive activation of the H-Ras/ERK signalling pathway.25 These results suggest that synapsin I could be the main presynaptic effector for certain forms of LTP triggered by the activation of the Ras/MAP kinase pathway. A growing body of evidence suggests that BDNF could be the presynaptic activator of H-Ras/ERK/synapsin I signalling pathway. Indeed, BDNF/ trkB signalling can modulate presynaptic function and learning48-50 and BDNF increases synaptosomal glutamate release through an ERK-dependent phosphorylation of synapsin I.23


In this chapter we have summarized and attempted to compose into a unifying frame the numerous physiological observations and hypotheses on synapsin function that have been put forward over the last 15 years in a large array of experimental systems, from reconstituted or isolated nerve terminals to mice bearing deletions in single and multiple synapsin genes. The emerging picture, summarized in Figure 4, is complex, as expected from a complex family of proteins that includes several isoforms with partly redundant functions and distinct developmental and regional patterns of expression and that are targets of multiple signal transduction pathways. Notwithstanding this complexity, the extremely high evolutionary conservation and the overt deficits in synaptic function and neural circuit activity observed in their absence strongly support a central role of the synapsins in the regulation of information transfer among neurons.


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