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

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.

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.

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.

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

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

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.

Effects of pregnenolone intervention on the cholinergic system and synaptic protein 1 in aged rats

OBJECTIVE

To observe the effect of pregnenolone (PREG) intervention on the cholinergic system function and the synaptic protein 1 (SYP1) expression in different brain regions of aged rats.

METHOD

Twenty-four-month-old male Sprague Dawley rats intraperitoneally injected every other day for one month were divided into blank control group, solvent control group, PREG (0.5 mg/kg) intervention group and PREG (2.0 mg/kg) intervention group. The rats were sacrificed 2 d after the intervention and the corresponding regions of brain tissue were separated and cryopreserved. Western blot analysis was used to detect the expression level of choline acetyltransferase (ChAT), SYP1, serum PREG and the activity of ChAT and acetylcholinesterase (AChE) in different brain regions. In addition, the semiquantitative changes in the expression level of ChAT and SYP1 in frontal lobe and hippocampus were tested by immunohistochemistry.

RESULT

Western blot and immunohistochemistry analysis showed that PREG (2.0 mg/kg) administration led to a significant increase of ChAT and SYP1 expressions in frontal lobe, temporal lobe, and hippocampus regions (p < 0.05). The result of enzyme-linked immunosorbent assay showed that PREG (2.0 mg/kg) administration significantly increased ChAT activity and serum PREG levels and caused a decrease in AChE activity (p < 0.05); while PREG (0.5 mg/kg) only elevated levels of serum PREG.

CONCLUSION

PREG significantly improved the synaptic plasticity of memory-related brain areas of aged rats, significantly increased brain cholinergic activity and thus helps to improve learning and memory in aged rats.

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.

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.

Clustering of excess growth resources within leading growth cones underlies the recurrent "deposition" of varicosities along developing neurites

Varicosities (VRs) are ubiquitous neuronal structures that are considered to serve as presynaptic structures. The mechanisms of their assembly are unknown. Using cultured Aplysia neurons, we found that in the absence of postsynaptic targets, VRs form at the leading edge of extending neurites when anterogradely transported organelles accumulate within the palm of the growth cone (GC) at a rate that exceeds their utilization by the GC machinery. The aggregation of excess organelles at the palm of the GC leads to slowdown of the GC's advance. As the size of the organelle clusters increases, the rate of organelle sequestration diminishes and the supply of building blocks to the GC resumes. The GCs' advance is re-initiated, "leaving behind" an organelle-loaded nascent VR. These mechanisms account for the recurrent "deposition" of almost equally spaced VRs by advancing GCs. Consistent with the view that VRs serve as "ready-to-go" presynaptic terminals, we found that a short train of action potentials leads to exocytosis of labeled vesicles within the varicosities. We propose that the formation and spacing of VRs by advancing GCs is the default outcome of the balance between the rate of supply of growth-supporting resources and the usage of these resources by the GC's machinery at the leading edges of specific neurites.

Abnormality of synaptic vesicular associated proteins in cerebral cortex and hippocampus after microwave exposure

Studies were performed to determine the effects of microwave on synaptic vesicles and the expression of synaptic vesicular associated proteins including synapsin I, VAMP-2, syntaxin, and synaptophysin. 25 Wistar rats were exposed to microwave which the average power density was 30 mW/cm(2), and whole body average specific absorption rate was 14.1 W/kg for 5 min. Synaptosome preparations in the cerebral cortex and hippocampus were obtained by isotonic Percoll/sucrose discontinuous gradients at 6 h, 1, 3, and 7 days after radiation. The expression of synaptic vesicular associated proteins was measured using Western blots and image analysis. The interaction between VAMP-2 and syntaxin was examined by coimmunoprecipitation analysis. Synapsin I in the cerebral cortex were decreased at 3 days (P < 0.01) after radiation and in the hippocampus increased at 1 day (P < 0.01), decreased at 3 days (P < 0.01), increased again at 7 days (P < 0.01) after exposure, compared with the sham-treated controls. Synaptophysin were increased in 1-7 days (P < 0.01) after exposure in the cerebral cortex and hippocampus. VAMP-2 were decreased at 1 and 3 days (P < 0.01) and syntaxin were decreased in 6 h to 3 days (P < 0.01) after radiation in the cerebral cortex and hippocampus. The interactions between VAMP-2 and syntaxin were decreased at 3-7 days (P < 0.01) after radiation in the cerebral cortex and hippocampus, compared with the sham-treated controls. These results suggest that 30 mW/cm(2) (SAR 14.1 W/kg) microwave radiation can result in the perturbation of the synaptic vesicles associated proteins: synapsin I, synaptophysin, VAMP-2, and syntaxin. The perturbation could induce the deposit of synaptic vesicle, which might be relative to the dysfunction of the synaptic transmission, even the cognition deficit.

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.

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

Post-tetanic potentiation (PTP) is a form of homosynaptic plasticity important for information processing and short-term memory in the nervous system. The synapsins, a family of synaptic vesicle (SV)-associated phosphoproteins, have been implicated in PTP. Although several synapsin functions are known to be regulated by phosphorylation by multiple protein kinases, the role of individual phosphorylation sites in synaptic plasticity is poorly understood. All the synapsins share a phosphorylation site in the N-terminal domain A (site 1) that regulates neurite elongation and SV mobilization. Here, we have examined the role of phosphorylation of synapsin domain A in PTP and other forms of short-term synaptic enhancement (STE) at synapses between cultured Helix pomatia neurons. To this aim, we cloned H. pomatia synapsin (helSyn) and overexpressed GFP-tagged wild-type helSyn or site-1-mutant helSyn mutated in the presynaptic compartment of C1-B2 synapses. We found that PTP at these synapses depends both on Ca2+/calmodulin-dependent and cAMP-dependent protein kinases, and that overexpression of the non-phosphorylatable helSyn mutant, but not wild-type helSyn, specifically impairs PTP, while not altering facilitation and augmentation. Our findings show that phosphorylation of site 1 has a prominent role in the expression of PTP, thus defining a novel role for phosphorylation of synapsin domain A in short-term homosynaptic plasticity.

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.

Synapsin-regulated synaptic transmission from readily releasable synaptic vesicles in excitatory hippocampal synapses in mice

The effects of synapsin proteins on synaptic transmission from vesicles in the readily releasable vesicle pool have been examined by comparing excitatory synaptic transmission in hippocampal slices from mice devoid of synapsins I and II and from wild-type control animals. Application of stimulus trains at variable frequencies to the CA3-to-CA1 pyramidal cell synapse suggested that, in both genotypes, synaptic responses obtained within 2 s stimulation originated from readily releasable vesicles. Detailed analysis of the responses during this period indicated that stimulus trains at 2-20 Hz enhanced all early synaptic responses in the CA3-to-CA1 pyramidal cell synapse, but depressed all early responses in the medial perforant path-to-granule cell synapse. The synapsin-dependent part of these responses, i.e. the difference between the results obtained in the transgene and the wild-type preparations, showed that in the former synapse, the presence of synapsins I and II minimized the early responses at 2 Hz, but enhanced the early responses at 20 Hz, while in the latter synapse, the presence of synapsins I and II enhanced all responses at both stimulation frequencies. The results indicate that synapsins I and II are necessary for full expression of both enhancing and decreasing modulatory effects on synaptic transmission originating from the readily releasable vesicles in these excitatory synapses.

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

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

Disrupted synaptic development in the hypoxic newborn brain

Infants born prematurely risk significant life-long cognitive disability, representing a major pediatric health crisis. The neuropathology of this cohort is accurately modeled in mice subjected to sublethal postnatal hypoxia. Massively parallel transcriptome analysis using cDNA microchips (9,262 genes), combined with immunohistochemical and protein assays, reveals that sublethal hypoxia accentuates genes subserving presynaptic function, and it suppresses genes involved with synaptic maturation, postsynaptic function, and neurotransmission. Other significantly affected pathways include those involved with glial maturation, vasculogenesis, and components of the cortical and microtubular cytoskeleton. These patterns reveal a global dysynchrony in the maturation programs of the hypoxic developing brain, and offer insights into the vulnerabilities of processes that guide early postnatal cerebral maturation.

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

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