Phosphorylation of synapsin domain A is required for post-tetanic potentiation

Increased presynaptic excitability in a migraine with aura mutation

Migraine is a common and disabling neurological disorder. The headache and sensory amplifications of migraine are attributed to hyperexcitable sensory circuits, but a detailed understanding remains elusive. A mutation in casein kinase 1 delta (CK1δ) was identified in non-hemiplegic familial migraine with aura and advanced sleep phase syndrome. Mice carrying the CK1δT44A mutation were more susceptible to spreading depolarization (the phenomenon that underlies migraine aura), but mechanisms underlying this migraine-relevant phenotype were not known. We used a combination of whole-cell electrophysiology and multiphoton imaging, in vivo and in brain slices, to compare CK1δT44A mice (adult males) to their wild-type littermates. We found that despite comparable synaptic activity at rest, CK1δT44A neurons were more excitable upon repetitive stimulation than wild-type, with a reduction in presynaptic adaptation at excitatory but not inhibitory synapses. The mechanism of this adaptation deficit was a calcium-dependent enhancement of the size of the readily releasable pool of synaptic vesicles, and a resultant increase in glutamate release, in CK1δT44A compared to wild-type synapses. Consistent with this mechanism, CK1δT44A neurons showed an increase in the cumulative amplitude of excitatory post-synaptic currents, and a higher excitation-to-inhibition ratio during sustained activity compared to wild-type. At a local circuit level, action potential bursts elicited in CK1δT44A neurons triggered an increase in recurrent excitation compared to wild-type, and at a network level, CK1δT44A mice showed a longer duration of 'up state' activity, which is dependent on recurrent excitation. Finally, we demonstrated that the spreading depolarization susceptibility of CK1δT44A mice could be returned to wild-type levels with the same intervention (reduced extracellular calcium) that normalized presynaptic adaptation. Taken together, these findings show a stimulus-dependent presynaptic gain of function at glutamatergic synapses in a genetic model of migraine, that accounts for the increased spreading depolarization susceptibility and may also explain the sensory amplifications that are associated with the disease.

Invertebrate neurons as a simple model to study the hyperexcitable state of epileptic disorders in single cells, monosynaptic connections, and polysynaptic circuits

Epilepsy is a neurological disorder characterized by a hyperexcitable state in neurons from different brain regions. Much is unknown about epilepsy and seizures development, depicting a growing field of research. Animal models have provided important clues about the underlying mechanisms of seizure-generating neuronal circuits. Mammalian complexity still makes it difficult to define some principles of nervous system function, and non-mammalian models have played pivotal roles depending on the research question at hand. Mollusks and the Helix land snail have been used to study epileptic-like behavior in neurons. Neurons from these organisms confer advantages as single-cell identification, isolation, and culture, either as single cells or as physiological relevant monosynaptic or polysynaptic circuits, together with amenability to different protocols and treatments. This review's purpose consists in presenting relevant papers in order to gain a better understanding of Helix neurons, their characteristics, uses, and capabilities for studying the fundamental mechanisms of epileptic disorders and their treatment, to facilitate their more expansive use in epilepsy research.

Presynaptic Short-Term Plasticity Persists in the Absence of PKC Phosphorylation of Munc18-1

Post-tetanic potentiation (PTP) is a form of short-term plasticity that lasts for tens of seconds following a burst of presynaptic activity. It has been proposed that PTP arises from protein kinase C (PKC) phosphorylation of Munc18-1, an SM (Sec1/Munc-18 like) family protein that is essential for release. To test this model, we made a knock-in mouse in which all Munc18-1 PKC phosphorylation sites were eliminated through serine-to-alanine point mutations (Munc18-1SA mice), and we studied mice of either sex. The expression of Munc18-1 was not altered in Munc18-1SA mice, and there were no obvious behavioral phenotypes. At the hippocampal CA3-to-CA1 synapse and the granule cell parallel fiber (PF)-to-Purkinje cell (PC) synapse, basal transmission was largely normal except for small decreases in paired-pulse facilitation that are consistent with a slight elevation in release probability. Phorbol esters that mimic the activation of PKC by diacylglycerol still increased synaptic transmission in Munc18-1SA mice. In Munc18-1SA mice, 70% of PTP remained at CA3-to-CA1 synapses, and the amplitude of PTP was not reduced at PF-to-PC synapses. These findings indicate that at both CA3-to-CA1 and PF-to-PC synapses, phorbol esters and PTP enhance synaptic transmission primarily by mechanisms that are independent of PKC phosphorylation of Munc18-1.SIGNIFICANCE STATEMENT A leading mechanism for a prevalent form of short-term plasticity, post-tetanic potentiation (PTP), involves protein kinase C (PKC) phosphorylation of Munc18-1. This study tests this mechanism by creating a knock-in mouse in which Munc18-1 is replaced by a mutated form of Munc18-1 that cannot be phosphorylated. The main finding is that most PTP at hippocampal CA3-to-CA1 synapses or at cerebellar granule cell-to-Purkinje cell synapses does not rely on PKC phosphorylation of Munc18-1. Thus, mechanisms independent of PKC phosphorylation of Munc18-1 are important mediators of PTP.

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.

Modulation of cPKCγ on Synapsin-Ia/b-Specific Phosphorylation Sites in the Developing Visual Cortex of Mice

Purpose

To explore the role of synapsin-Ia/b in visual cortical plasticity, the dynamic changes in total protein expression (T-) and conventional protein kinase C (cPKC)γ-modulated phosphorylation (P-) levels of synapsin-Ia/b were observed in the developing visual cortex of mice.

Methods

The Western blot analysis was used to determine the levels of T- and P-synapsin-Ia/b at site of Ser9, 549, and 603; the cPKCγ gene wild-type (cPKCγ+/+) and knockout (cPKCγ-/-) mice were applied to explore the modulation of cPKCγ on synapsin-Ia/b phosphorylation status in visual cortex of mice at postnatal 7 to 60 days (P7-P60, n = 6 per group).

Results

The results showed that T-synapsin-Ia/b protein levels significantly increased at P14 to P35 and peaked at P42 to 60 (P < 0.001) in visual cortex when compared with that of P7 cPKCγ+/+ mice, and cPKCγ-/- did not affect this pattern of T-synapsin-Ia/b protein expressions. For synapsin-Ia/b phosphorylation status, the levels of P-Ser9 and 603 synapsin-Ia/b significantly elevated at P21 to P28 (P < 0.05 or 0.001), and then went down and maintained at lower levels at P35 to P60 (P < 0.05 or 0.001) compared with P7 cPKCγ+/+ mice. In addition, the cPKCγ gene knockout could significantly (P < 0.001) inhibit both the increase and decrease of P-Ser9 and 603 synapsin-Ia/b levels when compared with cPKCγ+/+ mice at P7 to P60. However, there were no significant changes of P-Ser549 synapsin-Ia/b in the developing visual cortex of both cPKCγ+/+ and cPKCγ-/- mice at P7 to P60.

Conclusions

These results suggested that both protein expression levels and cPKCγ-modulated phosphorylation status at Ser9 and 603 of synapsin-Ia/b may play important role in developing visual cortex of mice.

Molecular Mechanisms of Short-Term Plasticity: Role of Synapsin Phosphorylation in Augmentation and Potentiation of Spontaneous Glutamate Release

We used genetic and pharmacological approaches to identify the signaling pathways involved in augmentation and potentiation, two forms of activity dependent, short-term synaptic plasticity that enhance neurotransmitter release. Trains of presynaptic action potentials produced a robust increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs). Following the end of the stimulus, mEPSC frequency followed a bi-exponential decay back to basal levels. The time constants of decay identified these two exponential components as the decay of augmentation and potentiation, respectively. Augmentation increased mEPSC frequency by 9.3-fold, while potentiation increased mEPSC frequency by 2.4-fold. In synapsin triple-knockout (TKO) neurons, augmentation was reduced by 83% and potentiation was reduced by 74%, suggesting that synapsins are key signaling elements in both forms of plasticity. To examine the synapsin isoforms involved, we expressed individual synapsin isoforms in TKO neurons. While synapsin IIIa rescued both augmentation and potentiation, none of the other synapsin isoforms produced statistically significant amounts of rescue. To determine the involvement of protein kinases in these two forms of short-term plasticity, we examined the effects of inhibitors of protein kinases A (PKA) and C (PKC). While inhibition of PKC had little effect, PKA inhibition reduced augmentation by 76% and potentiation by 60%. Further, elevation of intracellular cAMP concentration, by either forskolin or IBMX, greatly increased mEPSC frequency and occluded the amount of augmentation and potentiation evoked by electrical stimulation. Finally, mutating a PKA phosphorylation site to non-phosphorylatable alanine largely abolished the ability of synapsin IIIa to rescue both augmentation and potentiation. Together, these results indicate that PKA activation is required for both augmentation and potentiation of spontaneous neurotransmitter release and that PKA-mediated phosphorylation of synapsin IIIa underlies both forms of presynaptic short-term plasticity.

Calcium-Dependent Protein Kinase C Is Not Required for Post-Tetanic Potentiation at the Hippocampal CA3 to CA1 Synapse

Post-tetanic potentiation (PTP) is a widespread form of short-term synaptic plasticity in which a period of elevated presynaptic activation leads to synaptic enhancement that lasts tens of seconds to minutes. A leading hypothesis for the mechanism of PTP is that tetanic stimulation elevates presynaptic calcium that in turn activates calcium-dependent protein kinase C (PKC) isoforms to phosphorylate targets and enhance neurotransmitter release. Previous pharmacological studies have implicated this mechanism in PTP at hippocampal synapses, but the results are controversial. Here we combine genetic and pharmacological approaches to determine the role of classic PKC isoforms in PTP. We find that PTP is unchanged in PKC triple knock-out (TKO) mice in which all calcium-dependent PKC isoforms have been eliminated (PKCα, PKCβ, and PKCγ). We confirm previous studies and find that in wild-type mice 10 μm of the PKC inhibitor GF109203 eliminates PTP and the PKC activator PDBu enhances neurotransmitter release and occludes PTP. However, we find that the same concentrations of GF109203 and PDBu have similar effects in TKO animals. We also show that 2 μm GF109203 does not abolish PTP even though it inhibits the PDBu-dependent phosphorylation of PKC substrates. We conclude that at the CA3 to CA1 synapse Ca(2+)-dependent PKC isoforms do not serve as calcium sensors to mediate PTP.

SIGNIFICANCE STATEMENT

Neurons dynamically regulate neurotransmitter release through many processes known collectively as synaptic plasticity. Post-tetanic potentiation (PTP) is a widespread form of synaptic plasticity that lasts for tens of seconds that may have important computational roles and contribute to short-term memory. According to a leading mechanism, presynaptic calcium activates protein kinase C (PKC) to increase neurotransmitter release. Pharmacological studies have also implicated this mechanism at hippocampal CA3 to CA1 synapses, but there are concerns about the specificity of PKC activators and inhibitors. We therefore used a molecular genetic approach and found that PTP was unaffected when all calcium-dependent PKC isozymes were eliminated. We conclude that PKC isozymes are not the calcium sensors that mediate PTP at the CA3 to CA1 synapse.

Subconvulsant doses of pentylenetetrazol uncover the epileptic phenotype of cultured synapsin-deficient Helix serotonergic neurons in the absence of excitatory and inhibitory inputs

Synapsins are a family of presynaptic proteins related to several processes of synaptic functioning. A variety of reports have linked mutations in synapsin genes with the development of epilepsy. Among the proposed mechanisms, a main one is based on the synapsin-mediated imbalance towards network hyperexcitability due to differential effects on neurotransmitter release in GABAergic and glutamatergic synapses. Along this line, a non-synaptic effect of synapsin depletion increasing neuronal excitability has recently been described in Helix neurons. To further investigate this issue, we examined the effect of synapsin knock-down on the development of pentylenetetrazol (PTZ)-induced epileptic-like activity using single neurons or isolated monosynaptic circuits reconstructed on microelectrode arrays (MEAs). Compared to control neurons, synapsin-silenced neurons showed a lower threshold for the development of epileptic-like activity and prolonged periods of activity, together with the occurrence of spontaneous firing after recurrent PTZ-induced epileptic-like activity. These findings highlight the crucial role of synapsin on neuronal excitability regulation in the absence of inhibitory or excitatory inputs.

SAD-B kinase regulates pre-synaptic vesicular dynamics at hippocampal Schaffer collateral synapses and affects contextual fear memory

Synapses of amphids defective (SAD)-A/B kinases control various steps in neuronal development and differentiation, such as axon specifications and maturation in central and peripheral nervous systems. At mature pre-synaptic terminals, SAD-B is associated with synaptic vesicles and the active zone cytomatrix; however, how SAD-B regulates neurotransmission and synaptic plasticity in vivo remains unclear. Thus, we used SAD-B knockout (KO) mice to study the function of this pre-synaptic kinase in the brain. We found that the paired-pulse ratio was significantly enhanced at Shaffer collateral synapses in the hippocampal CA1 region in SAD-B KO mice compared with wild-type littermates. We also found that the frequency of the miniature excitatory post-synaptic current was decreased in SAD-B KO mice. Moreover, synaptic depression following prolonged low-frequency synaptic stimulation was significantly enhanced in SAD-B KO mice. These results suggest that SAD-B kinase regulates vesicular release probability at pre-synaptic terminals and is involved in vesicular trafficking and/or regulation of the readily releasable pool size. Finally, we found that hippocampus-dependent contextual fear learning was significantly impaired in SAD-B KO mice. These observations suggest that SAD-B kinase plays pivotal roles in controlling vesicular release properties and regulating hippocampal function in the mature brain. Synapses of amphids defective (SAD)-A/B kinases control various steps in neuronal development and differentiation, but their roles in mature brains were only partially known. Here, we demonstrated, at mature pre-synaptic terminals, that SAD-B regulates vesicular release probability and synaptic plasticity. Moreover, hippocampus-dependent contextual fear learning was significantly impaired in SAD-B KO mice, suggesting that SAD-B kinase plays pivotal roles in controlling vesicular release properties and regulating hippocampal function in the mature brain.

Knock-down of synapsin alters cell excitability and action potential waveform by potentiating BK and voltage-gated Ca(2+) currents in Helix serotonergic neurons

Synapsins (Syns) are an evolutionarily conserved family of presynaptic proteins crucial for the fine-tuning of synaptic function. A large amount of experimental evidences has shown that Syns are involved in the development of epileptic phenotypes and several mutations in Syn genes have been associated with epilepsy in humans and animal models. Syn mutations induce alterations in circuitry and neurotransmitter release, differentially affecting excitatory and inhibitory synapses, thus causing an excitation/inhibition imbalance in network excitability toward hyperexcitability that may be a determinant with regard to the development of epilepsy. Another approach to investigate epileptogenic mechanisms is to understand how silencing Syn affects the cellular behavior of single neurons and is associated with the hyperexcitable phenotypes observed in epilepsy. Here, we examined the functional effects of antisense-RNA inhibition of Syn expression on individually identified and isolated serotonergic cells of the Helix land snail. We found that Helix synapsin silencing increases cell excitability characterized by a slightly depolarized resting membrane potential, decreases the rheobase, reduces the threshold for action potential (AP) firing and increases the mean and instantaneous firing rates, with respect to control cells. The observed increase of Ca(2+) and BK currents in Syn-silenced cells seems to be related to changes in the shape of the AP waveform. These currents sustain the faster spiking in Syn-deficient cells by increasing the after hyperpolarization and limiting the Na(+) and Ca(2+) channel inactivation during repetitive firing. This in turn speeds up the depolarization phase by reaching the AP threshold faster. Our results provide evidence that Syn silencing increases intrinsic cell excitability associated with increased Ca(2+) and Ca(2+)-dependent BK currents in the absence of excitatory or inhibitory inputs.

Synapsin knockdown is associated with decreased neurite outgrowth, functional synaptogenesis impairment, and fast high-frequency neurotransmitter release

Synapsins (Syns) are an evolutionarily conserved family of synaptic vesicle-associated proteins related to fine tuning of synaptic transmission. Studies with mammals have partially clarified the different roles of Syns; however, the presence of different genes and isoforms and the development of compensatory mechanisms hinder accurate data interpretation. Here, we use a simple in vitro monosynaptic Helix neuron connection, reproducing an in vivo physiological connection as a reliable experimental model to investigate the effects of Syn knockdown. Cells overexpressing an antisense construct against Helix Syn showed a time-dependent decrease of Syn immunostaining, confirming protein loss. At the morphological level, Syn-silenced cells showed a reduction in neurite linear outgrowth and branching and in the size and number of synaptic varicosities. Functionally, Syn-silenced cells presented a reduced ability to form synaptic connections; however, functional chemical synapses showed similar basal excitatory postsynaptic potentials and similar short-term plasticity paradigms. In addition, Syn-silenced cells presented faster neurotransmitter release and decreased postsynaptic response toward the end of long tetanic presynaptic stimulations, probably related to an impairment of the synaptic vesicle trafficking resulting from a different vesicle handling, with an increased readily releasable pool and a compromised reserve pool.

The effect of α7 nicotinic receptor activation on glutamatergic transmission in the hippocampus

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory, however the cellular mechanism of these actions remains elusive. With help from newly developed biosensors and optogenetic tools, recent studies provide new insights on signaling mechanisms involved in the activation of nAChRs. Here we will review α7 nAChR's action in the tri-synaptic pathway in the hippocampus. The effects of α7 nAChR activation via either exogenous compounds or endogenous cholinergic innervation are detailed for spontaneous and evoked glutamatergic synaptic transmission and synaptic plasticity, as well as the underlying signaling mechanisms. In summary, α7 nAChRs trigger intracellular calcium rise and calcium-dependent signaling pathways to enhance glutamate release and induce glutamatergic synaptic plasticity.

Activation of α7 nicotinic acetylcholine receptors increases intracellular cAMP levels via activation of AC1 in hippocampal neurons

The activation of α7 nAChRs has been shown to improve hippocampal-dependent learning and memory. However, the molecular mechanism of α7 nAChRs' action remains elusive. We previously reported that activation of α7 nAChRs induced a prolonged enhancement of glutamatergic synaptic transmission in a PKA-dependent manner. Here, we investigated any connection between the activation of the α7 nAChR and cAMP signaling in hippocampal neurons. To address this question, we employed a FRET-based biosensor to measure the intracellular cAMP levels directly via live cell imaging. We found that application of the α7 nAChR-selective agonist choline, in the presence of the α7 nAChR positive allosteric modulator PNU-120596, induced a significant change in emission ratio of F535/F470, which indicated an increase in intracellular cAMP levels. This choline-induced increase was abolished by the α7 nAChR antagonist MLA and the calcium chelator BAPTA, suggesting that the cAMP increase depends on the α7 nAChR activation and subsequent intracellular calcium rise. The selective AC1 inhibitor CB-6673567 and siRNA-mediated deletion of AC1 both blocked the choline-induced cAMP increase, suggesting that calcium-dependent AC1 is required for choline's action. Furthermore, α7 nAChR activation stimulated the phosphorylation of synapsin, which serves as a downstream effector to regulate neurotransmitter release. Our findings provide the first direct evidence to link activation of α7 nAChRs to a cAMP rise via AC1, which defines a new signaling pathway employed by α7 nAChRs. Our study sheds light into potential molecular mechanisms of the positive cognitive actions of α7 nAChR agonists and development of therapeutic treatments for cognitive impairments.

Central vagal afferent endings mediate reduction of food intake by melanocortin-3/4 receptor agonist

Injection of the melanocortin-3/4 receptor agonist melanotan-II (MTII) into the nucleus of the solitary tract (NTS) produces rapid and sustained reduction of food intake. Melanocortin-4 receptors (MC4Rs) are expressed by vagal afferent endings in the NTS, but it is not known whether these endings participate in MTII-induced reduction of food intake. In experiments described here, we evaluated the contribution of central vagal afferent endings in MTII-induced reduction of food intake. Examination of rat hindbrain sections revealed that neuronal processes expressing immunoreactivity for the endogenous MC4R agonist α-melanoctyte-stimulating hormone course parallel and wrap around anterogradely labeled vagal afferent endings in the NTS and thus are aptly positioned to activate vagal afferent MC4Rs. Furthermore, MTII and endogenous MC4R agonists increased protein kinase A (PKA)-catalyzed phosphorylation of synapsin I in vagal afferent endings, an effect known to increase synaptic strength by enhancing neurotransmitter release in other neural systems. Hindbrain injection of a PKA inhibitor, KT5720, significantly attenuated MTII-induced reduction of food intake and the increase in synapsin I phosphorylation. Finally, unilateral nodose ganglion removal, resulting in degeneration of vagal afferent endings in the ipsilateral NTS, abolished MTII-induced synapsin I phosphorylation ipsilateral to nodose ganglion removal. Moreover, reduction of food intake following MTII injection into the NTS ipsilateral to nodose ganglion removal was significantly attenuated, whereas the response to MTII was not diminished when injected into the contralateral NTS. Altogether, our results suggest that reduction of food intake following hindbrain MC4R activation is mediated by central vagal afferent endings.

Theoretical models of synaptic short term plasticity

Short term plasticity is a highly abundant form of rapid, activity-dependent modulation of synaptic efficacy. A shared set of mechanisms can cause both depression and enhancement of the postsynaptic response at different synapses, with important consequences for information processing. Mathematical models have been extensively used to study the mechanisms and roles of short term plasticity. This review provides an overview of existing models and their biological basis, and of their main properties. Special attention will be given to slow processes such as calcium channel inactivation and the effect of activation of presynaptic autoreceptors.

Selective modulation of chemical and electrical synapses of Helix neuronal networks during in vitro development

Background

A large number of invertebrate models, including the snail Helix, emerged as particularly suitable tools for investigating the formation of synapses and the specificity of neuronal connectivity. Helix neurons can be individually identified and isolated in cell culture, showing well-conserved size, position, biophysical properties, synaptic connections, and physiological functions. Although we previously showed the potential usefulness of Helix polysynaptic circuits, a full characterization of synaptic connectivity and its dynamics during network development has not been performed.

Results

In this paper, we systematically investigated the in vitro formation of polysynaptic circuits, among Helix B2 and the serotonergic C1 neurons, from a morphological and functional point of view. Since these cells are generally silent in culture, networks were chemically stimulated with either high extracellular potassium concentrations or, alternatively, serotonin. Potassium induced a transient depolarization of all neurons. On the other hand, we found prolonged firing activity, selectively maintained following the first serotonin application. Statistical analysis revealed no significant changes in neuronal dynamics during network development. Moreover, we demonstrated that the cell-selective effect of serotonin was also responsible for short-lasting alterations in C1 excitability, without long-term rebounds.

Estimation of the functional connections by means of cross-correlation analysis revealed that networks under elevated KCl concentrations exhibited strongly correlated signals with short latencies (about 5 ms), typical of electrically coupled cells. Conversely, neurons treated with serotonin were weakly connected with longer latencies (exceeding 20 ms) between the interacting neurons. Finally, we clearly demonstrated that these two types of correlations (in terms of strength/latency) were effectively related to the presence of electrical or chemical connections, by comparing Micro-Electrode Array (MEA) signal traces with intracellularly recorded cell pairs.

Conclusions

Networks treated with either potassium or serotonin were predominantly interconnected through electrical or chemical connections, respectively. Furthermore, B2 response and short-term increase in C1 excitability induced by serotonin is sufficient to trigger spontaneous activity with chemical connections, an important requisite for long-term maintenance of firing activity.

Pentylenetetrazol-induced epileptiform activity affects basal synaptic transmission and short-term plasticity in monosynaptic connections

Epileptic activity is generally induced in experimental models by local application of epileptogenic drugs, including pentylenetetrazol (PTZ), widely used on both vertebrate and invertebrate neurons. Despite the high prevalence of this neurological disorder and the extensive research on it, the cellular and molecular mechanisms underlying epileptogenesis still remain unclear. In this work, we examined PTZ-induced neuronal changes in Helix monosynaptic circuits formed in vitro, as a simpler experimental model to investigate the effects of epileptiform activity on both basal release and post-tetanic potentiation (PTP), a form of short-term plasticity. We observed a significant enhancement of basal synaptic strength, with kinetics resembling those of previously described use-dependent forms of plasticity, determined by changes in estimated quantal parameters, such as the readily releasable pool and the release probability. Moreover, these neurons exhibited a strong reduction in PTP expression and in its decay time constant, suggesting an impairment in the dynamic reorganization of synaptic vesicle pools following prolonged stimulation of synaptic transmission. In order to explain this imbalance, we determined whether epileptic activity is related to the phosphorylation level of synapsin, which is known to modulate synaptic plasticity. Using western blot and immunocytochemical staining we found a PTZ-dependent increase in synapsin phosphorylation at both PKA/CaMKI/IV and MAPK/Erk sites, both of which are important for modulating synaptic plasticity. Taken together, our findings suggest that prolonged epileptiform activity leads to an increase in the synapsin phosphorylation status, thereby contributing to an alteration of synaptic strength in both basal condition and tetanus-induced potentiation.

Myotonic dystrophy CTG expansion affects synaptic vesicle proteins, neurotransmission and mouse behaviour

Myotonic dystrophy type 1 is a complex multisystemic inherited disorder, which displays multiple debilitating neurological manifestations. Despite recent progress in the understanding of the molecular pathogenesis of myotonic dystrophy type 1 in skeletal muscle and heart, the pathways affected in the central nervous system are largely unknown. To address this question, we studied the only transgenic mouse line expressing CTG trinucleotide repeats in the central nervous system. These mice recreate molecular features of RNA toxicity, such as RNA foci accumulation and missplicing. They exhibit relevant behavioural and cognitive phenotypes, deficits in short-term synaptic plasticity, as well as changes in neurochemical levels. In the search for disease intermediates affected by disease mutation, a global proteomics approach revealed RAB3A upregulation and synapsin I hyperphosphorylation in the central nervous system of transgenic mice, transfected cells and post-mortem brains of patients with myotonic dystrophy type 1. These protein defects were associated with electrophysiological and behavioural deficits in mice and altered spontaneous neurosecretion in cell culture. Taking advantage of a relevant transgenic mouse of a complex human disease, we found a novel connection between physiological phenotypes and synaptic protein dysregulation, indicative of synaptic dysfunction in myotonic dystrophy type 1 brain pathology.

Adaptive regulation maintains posttetanic potentiation at cerebellar granule cell synapses in the absence of calcium-dependent PKC

Posttetanic potentiation (PTP) is a transient, calcium-dependent increase in the efficacy of synaptic transmission following elevated presynaptic activity. The calcium-dependent protein kinase C (PKC(Ca)) isoforms PKCα and PKCβ mediate PTP at the calyx of Held synapse, with PKCβ contributing significantly more than PKCα. It is not known whether PKC(Ca) isoforms play a conserved role in PTP at other synapses. We examined this question at the parallel fiber → Purkinje cell (PF→PC) synapse, where PKC inhibitors suppress PTP. We found that PTP is preserved when single PKC(Ca) isoforms are knocked out and in PKCα/β double knock-out (dko) mice, even though in the latter all PKC(Ca) isoforms are eliminated from granule cells. However, in contrast to wild-type and single knock-out animals, PTP in PKCα/β dko animals is not suppressed by PKC inhibitors. These results indicate that PKC(Ca) isoforms mediate PTP at the PF→PC synapse in wild-type and single knock-out animals. However, unlike the calyx of Held, at the PF→PC synapse either PKCα or PKCβ alone is sufficient to mediate PTP, and if both isoforms are eliminated a compensatory PKC-independent mechanism preserves the plasticity. These results suggest that a feedback mechanism allows granule cells to maintain the normal properties of short-term synaptic plasticity even when the mechanism that mediates PTP in wild-type mice is eliminated.

Short-term presynaptic plasticity

Different types of synapses are specialized to interpret spike trains in their own way by virtue of the complement of short-term synaptic plasticity mechanisms they possess. Numerous types of short-term, use-dependent synaptic plasticity regulate neurotransmitter release. Short-term depression is prominent after a single conditioning stimulus and recovers in seconds. Sustained presynaptic activation can result in more profound depression that recovers more slowly. An enhancement of release known as facilitation is prominent after single conditioning stimuli and lasts for hundreds of milliseconds. Finally, tetanic activation can enhance synaptic strength for tens of seconds to minutes through processes known as augmentation and posttetantic potentiation. Progress in clarifying the properties, mechanisms, and functional roles of these forms of short-term plasticity is reviewed here.

Site-specific synapsin I phosphorylation participates in the expression of post-tetanic potentiation and its enhancement by BDNF

A large amount of experimental evidence has highlighted the rapid changes in synaptic efficacy induced by high-frequency stimulation and BDNF at central excitatory synapses. We clarified the quantal mechanisms and the involvement of Synapsin I (SynI) phosphorylation in the expression of post-tetanic potentiation (PTP) and in its modulation by BDNF in mouse glutamatergic autapses. We found that PTP is associated with an elevation in the probability of release and a concomitant increase in the size of the readily releasable pool (RRP). The latter component was virtually absent in SynI knock-out (KO) neurons, which indeed displayed impaired PTP. PTP was fully rescued by the expression of wild-type SynI, but not of its dephosphomimetic mutants in the phosphorylation sites for cAMP-dependent protein kinase and Ca²⁺/calmodulin-dependent protein kinases I/II. BDNF potently enhanced PTP through a further increase in the RRP size, which was missing in SynI KO neurons. In these neurons, the BDNF-induced PTP enhancement was rescued by the expression of wild-type SynI, but not of its dephosphomimetic mutant at the mitogen-dependent protein kinase sites. The results indicate that the increase in RRP size necessary for the full expression of PTP, and its sensitivity to BDNF, involve phosphorylation of SynI at distinct sites, thus implicating SynI as an essential downstream effector for the expression of PTP and for its enhancement by BDNF.

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.

Calcium-dependent isoforms of protein kinase C mediate posttetanic potentiation at the calyx of Held

High-frequency stimulation leads to a transient increase in the amplitude of evoked synaptic transmission that is known as posttetanic potentiation (PTP). Here we examine the roles of the calcium-dependent protein kinase C isoforms PKCα and PKCβ in PTP at the calyx of Held synapse. In PKCα/β double knockouts, 80% of PTP is eliminated, whereas basal synaptic properties are unaffected. PKCα and PKCβ produce PTP by increasing the size of the readily releasable pool of vesicles evoked by high-frequency stimulation and by increasing the fraction of this pool released by the first stimulus. PKCα and PKCβ do not facilitate presynaptic calcium currents. The small PTP remaining in double knockouts is mediated partly by an increase in miniature excitatory postsynaptic current amplitude and partly by a mechanism involving myosin light chain kinase. These experiments establish that PKCα and PKCβ are crucial for PTP and suggest that long-lasting presynaptic calcium increases produced by tetanic stimulation may activate these isoforms to produce PTP.

A novel form of presynaptic CaMKII-dependent short-term potentiation between Lymnaea neurons

Short-term plasticity is thought to form the basis for working memory, the cellular mechanisms of which are the least understood in the nervous system. In this study, using in vitro reconstructed synapses between the identified Lymnaea neuron visceral dorsal 4 (VD4) and left pedal dorsal 1 (LPeD1), we demonstrate a novel form of short-term potentiation (STP) which is 'use'- but not time-dependent, unlike most previously defined forms of short-term synaptic plasticity. Using a triple-cell configuration we demonstrate for the first time that a single presynaptic neuron can reliably potentiate both inhibitory and excitatory synapses. We further demonstrate that, unlike previously described forms of STP, the synaptic potentiation between Lymnaea neurons does not involve postsynaptic receptor sensitization or presynaptic residual calcium. Finally, we provide evidence that STP at the VD4-LPeD1 synapse requires presynaptic calcium/calmodulin dependent kinase II (CaMKII). Taken together, our study identifies a novel form of STP which may provide the basis for both short- and long-term potentiation, in the absence of any protein synthesis-dependent steps, and involve CaMKII activity exclusively in the presynaptic cell.

Short-term forms of presynaptic plasticity

Synapses exhibit several forms of short-term plasticity that play a multitude of computational roles. Short-term depression suppresses neurotransmitter release for hundreds of milliseconds to tens of seconds; facilitation and post-tetanic potentiation lead to synaptic enhancement lasting hundreds of milliseconds to minutes. Recent advances have provided insight into the mechanisms underlying these forms of plasticity. Vesicle depletion, as well as inactivation of both release sites and calcium channels, contribute to synaptic depression. Mechanisms of short-term enhancement include calcium channel facilitation, local depletion of calcium buffers, increases in the probability of release downstream of calcium influx, altered vesicle pool properties, and increases in quantal size. Moreover, there is a growing appreciation of the heterogeneity of vesicles and release sites and how they can contribute to use-dependent plasticity.

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.

MAPK/Erk-dependent phosphorylation of synapsin mediates formation of functional synapses and short-term homosynaptic plasticity

MAPK/Erk is a protein kinase activated by neurotrophic factors involved in synapse formation and plasticity, which acts at both the nuclear and cytoplasmic level. Synapsin proteins are synaptic-vesicle-associated proteins that are well known to be MAPK/Erk substrates at phylogenetically conserved sites. However, the physiological role of MAPK/Erk-dependent synapsin phosphorylation in regulating synaptic formation and function is poorly understood. Here, we examined whether synapsin acts as a physiological effector of MAPK/Erk in synaptogenesis and plasticity. To this aim, we developed an in vitro model of soma-to-soma paired Helix B2 neurons, that establish bidirectional excitatory synapses. We found that the formation and activity-dependent short-term plasticity of these synapses is dependent on the MAPK/Erk pathway. To address the role of synapsin in this pathway, we generated non-phosphorylatable and pseudo-phosphorylated Helix synapsin mutants at the MAPK/Erk sites. Overexpression experiments revealed that both mutants interfere with presynaptic differentiation, synapsin clustering, and severely impair post-tetanic potentiation, a form of short-term homosynaptic plasticity. Our findings show that MAPK/Erk-dependent synapsin phosphorylation has a dual role both in the establishment of functional synaptic connections and their short-term plasticity, indicating that some of the multiple extranuclear functions of MAPK/Erk in neurons can be mediated by the same multifunctional presynaptic target.

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.

Differential regulation of synapsin phosphorylation by monocular deprivation in juveniles and adults

The rodent visual cortex retains significant ocular dominance plasticity beyond the traditional postnatal critical period. However, the intracellular mechanisms that underlie the cortical response to monocular deprivation are predicted to be different in juveniles and adults. Here we show monocular deprivation in adult, but not juvenile rats, induced an increase in the phosphorylation of the prominent presynaptic effecter protein synapsin at two key sites known to regulate synapsin function. Monocular deprivation in adults induced an increase in synapsin phosphorylation at the PKA consensus site (site 1) and the CaMKII consensus site (site 3) in the visual cortex ipsilateral to the deprived eye, which is dominated by non-deprived eye input. The increase in synapsin phosphorylation was observed in total cortical homogenate, but not synaptoneurosomes, suggesting that the pool of synapsin targeted by monocular deprivation in adults does not co-fractionate with excitatory synapses. Phosphorylation of sites 1 and 3 stimulates the release of synaptic vesicles from a reserve pool and increases in the probability of evoked neurotransmitter release, which may contribute to the strengthening of the non-deprived input characteristic of ocular dominance plasticity in adults.

Helix neuronal ensembles with controlled cell type composition and placement develop functional polysynaptic circuits on Micro-Electrode Arrays

Neuronal cell cultures on Micro-Electrode Arrays (MEAs) provide an essential experimental tool for studying the connectivity and long-term activity of complex neuronal networks. MEA studies are generally based on the analysis of mixed neuronal populations constituted by a large number of cultured cells with cell type composition and connectivity patterns which are quite unpredictable a priori. In this work, we propose a different approach which consists of assembling on MEAs neuronal circuits formed by individually identifiable C1, C3, and B2 Helix neurons. Cells were plated under conditions of controlled number and position to form neuronal networks of defined composition. We performed multi-site electrophysiological recordings, and we characterized the firing dynamics. By means of cross-correlation analysis, we studied the electrophysiological properties of MEA-coupled microcircuits and characterized their activity patterns. We showed how the synaptic connectivity, actually observed in polysynaptic circuits of C1, C3 and B2 neurons, correlates well with the expected connectivity of C1-B2, B2-B2 and B2-C3 cell pairs as previously reported in conventional electrophysiological studies in culture.

Nitric oxide and cyclic nucleotide signal transduction modulates synaptic vesicle turnover in human model neurons

The human Ntera2 (NT2) teratocarcinoma cell line can be induced to differentiate into post-mitotic neurons. Here, we report that the human NT2 neurons generated by a spherical aggregate cell culture method express increasing levels of typical pre-synaptic proteins (synapsin and synaptotagmin I) along the neurite depending on the length of in vitro culture. By employing an antibody directed against the luminal domain of synaptotagmin I and the fluorescent dye N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide, we show that depolarized NT2 neurons display calcium-dependent exo-endocytotic synaptic vesicle recycling. NT2 neurons express the neuronal isoform of neuronal nitric oxide synthase and soluble guanylyl cyclase (sGC), the major receptor for nitric oxide (NO). We tested whether NO signal transduction modulates synaptic vesicle turnover in human NT2 neurons. NO donors and cylic guanosine-monophosphate analogs enhanced synaptic vesicle recycling while a sGC inhibitor blocked the effect of NO donors. Two NO donors, sodium nitroprusside, and and N-Ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydrazino) ethanamine evoked vesicle exocytosis which was partially blocked by the sGC inhibitor. The activator of adenylyl cyclase, forskolin, and a cAMP analog induced synaptic vesicle recycling and exocytosis via a parallel acting protein kinase A pathway. Our data from NT2 neurons suggest that NO/cyclic nucleotide signaling pathways may facilitate neurotransmitter release in human brain cells.

Characterization and role of Helix contactin-related proteins in cultured Helix pomatia neurons

We report on the structural and functional properties of the Helix contactin-related proteins (HCRPs), a family of closely related glycoproteins previously identified in the nervous system of the land snail Helix pomatia through antibodies against the mouse F3/contactin glycoprotein. We focus on HCRP1 and HCRP2, soluble FNIII domains-containing proteins of 90 and 45 kD bearing consensus motifs for both N- and O-glycosylation. Using the anti-HCRPs serum, we find secreted HCRPs in Helix nervous tissue isotonic extracts and in culture medium conditioned by Helix ganglia. In addition, we demonstrate expression of HCRPs on neuronal soma and on neurite extensions. Functionally, in Helix neurons, the antisense HCRP2 mRNA counteracts neurite elongation, and the recombinant HCRP2 protein exerts a strong positive effect on neurite growth when used as substrate. These data point to HCRPs as novel neurite growth-promoting molecules expressed in invertebrate nervous tissue.

Synapsin Regulates Basal Synaptic Strength, Synaptic Depression, and Serotonin-Induced Facilitation of Sensorimotor Synapses in Aplysia

Synapsin is a synaptic vesicle-associated protein implicated in the regulation of vesicle trafficking and transmitter release, but its role in heterosynaptic plasticity remains elusive. Moreover, contradictory results have obscured the contribution of synapsin to homosynaptic plasticity. We previously reported that the neuromodulator serotonin (5-HT) led to the phosphorylation and redistribution of Aplysia synapsin, suggesting that synapsin may be a good candidate for the regulation of vesicle mobilization underlying the short-term synaptic plasticity induced by 5-HT. This study examined the role of synapsin in homosynaptic and heterosynaptic plasticity. Overexpression of synapsin reduced basal transmission and enhanced homosynaptic depression. Although synapsin did not affect spontaneous recovery from depression, it potentiated 5-HT–induced dedepression. Computational analysis showed that the effects of synapsin on plasticity could be adequately simulated by altering the rate of Ca2+-dependent vesicle mobilization, supporting the involvement of synapsin not only in homosynaptic but also in heterosynaptic forms of plasticity by regulating vesicle mobilization.