Vesicle pools and synapsins: New insights into old enigmas

Inhibition of Glutamate Release from Rat Cortical Nerve Terminals by Dehydrocorydaline, an Alkaloid from Corydalis yanhusuo

Excessive release of glutamate induces excitotoxicity and causes neuronal damage in several neurodegenerative diseases. Natural products have emerged as potential neuroprotective agents for preventing and treating neurological disorders. Dehydrocorydaline (DHC), an active alkaloid compound isolated from Corydalis yanhusuo, possesses neuroprotective capacity. The present study investigated the effect of DHC on glutamate release using a rat brain cortical synaptosome model. Our results indicate that DHC inhibited 4-aminopyridine (4-AP)-evoked glutamate release and elevated intrasynaptosomal calcium levels. The inhibitory effect of DHC on 4-AP-evoked glutamate release was prevented in the presence of the vesicular transporter inhibitor bafilomycin A1 and the N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC but not the intracellular inhibitor of Ca²⁺ release dantrolene or the mitochondrial Na⁺/Ca²⁺ exchanger inhibitor CGP37157. Moreover, the inhibitory effect of DHC on evoked glutamate release was prevented by the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) inhibitor PD98059. Western blotting data in synaptosomes also showed that DHC significantly decreased the level of ERK1/2 phosphorylation and synaptic vesicle-associated protein synapsin I, the main presynaptic target of ERK. Together, these results suggest that DHC inhibits presynaptic glutamate release from cerebrocortical synaptosomes by suppressing presynaptic voltage-dependent Ca²⁺ entry and the MAPK/ERK/synapsin I signaling pathway.

Quantitative Proteomics Reveals the Dynamic Pathophysiology Across Different Stages in a Rat Model of Severe Traumatic Brain Injury

Background

Severe traumatic brain injury (TBI) has become a global health problem and causes a vast worldwide societal burden. However, distinct mechanisms between acute and subacute stages have not been systemically revealed. The present study aimed to identify differentially expressed proteins in severe TBI from the acute to subacute phase.

Methods

Sixty Sprague Dawley (SD) rats were randomly divided into sham surgery and model groups. The severe TBI models were induced by the controlled cortical impact (CCI) method. We evaluated the neurological deficits through the modified neurological severity score (NSS). Meanwhile, H&E staining and immunofluorescence were performed to assess the injured brain tissues. The protein expressions of the hippocampus on the wounded side of CCI groups and the same side of Sham groups were analyzed by the tandem mass tag-based (TMT) quantitative proteomics on the third and fourteenth days. Then, using the gene ontology (GO), Kyoto encyclopedia of genes and genomes (KEGG), and protein–protein interaction (PPI), the shared and stage-specific differentially expressed proteins (DEPs) were screened, analyzed, and visualized. Eventually, target proteins were further verified by Western blotting (WB).

Results

In the severe TBI, the neurological deficits always exist from the acute stage to the subacute stage, and brain parenchyma was dramatically impaired in either period. Of the significant DEPs identified, 312 were unique to the acute phase, 76 were specific to the subacute phase, and 63 were shared in both. Of the 375 DEPs between Sham-a and CCI-a, 240 and 135 proteins were up-regulated and down-regulated, respectively. Of 139 DEPs, 84 proteins were upregulated, and 55 were downregulated in the Sham-s and CCI-s. Bioinformatics analysis revealed that the differential pathophysiology across both stages. One of the most critical shared pathways is the complement and coagulation cascades. Notably, three pathways associated with gastric acid secretion, insulin secretion, and thyroid hormone synthesis were only enriched in the acute phase. Amyotrophic lateral sclerosis (ALS) was significantly enriched in the subacute stage. WB experiments confirmed the reliability of the TMT quantitative proteomics results.

Conclusion

Our findings highlight the same and different pathological processes in the acute and subacute phases of severe TBI at the proteomic level. The results of potential protein biomarkers might facilitate the design of novel strategies to treat TBI.

Dynamic Foot Stimulations During Short-Term Hindlimb Unloading Prevent Dysregulation of the Neurotransmission in the Hippocampus of Rats

Spaceflight and simulated microgravity both affect learning and memory, which are mostly controlled by the hippocampus. However, data about molecular alterations in the hippocampus in real or simulated microgravity conditions are limited. Adult Wistar rats were recruited in the experiments. Here we analyzed whether short-term simulated microgravity caused by 3-day hindlimb unloading (HU) will affect the glutamatergic and GABAergic systems of the hippocampus and how dynamic foot stimulation (DFS) to the plantar surface applied during HU can contribute in the regulation of hippocampus functioning. The results demonstrated a decreased expression of vesicular glutamate transporters 1 and 2 (VGLUT1/2) in the hippocampus after 3 days of HU, while glutamate decarboxylase 67 (GAD67) expression was not affected. HU also significantly induced Akt signaling and transcriptional factor CREB that are supposed to activate the neuroprotective mechanisms. On the other hand, DFS led to normalization of VGLUT1/2 expression and activity of Akt and CREB. Analysis of exocytosis proteins revealed the inhibition of SNAP-25, VAMP-2, and syntaxin 1 expression in DFS group proposing attenuation of excitatory neurotransmission. Thus, we revealed that short-term HU causes dysregulation of glutamatergic system of the hippocampus, but, at the same time, stimulates neuroprotective Akt-dependent mechanism. In addition, most importantly, we demonstrated positive effect of DFS on the hippocampus functioning that probably depends on the regulation of neurotransmitter exocytosis.

Typhaneoside Suppresses Glutamate Release Through Inhibition of Voltage-Dependent Calcium Entry in Rat Cerebrocortical Nerve Terminals

Glutamate is the major excitatory neurotransmitter in the brain and is involved in many brain functions. In this study, we investigated whether typhaneoside, a flavonoid from Typhae angustifolia pollen, affects endogenous glutamate release from rat cortical synaptosomes. Using a one-line enzyme-coupled fluorometric assay, glutamate release stimulated by the K⁺ channel blocker 4-aminopyridine was monitored to explore the possible underlying mechanisms. The vesicular transporter inhibitor bafilomycin A1 and chelation of extracellular Ca²⁺ ions with EGTA suppressed the effect of typhaneoside on the induced glutamate release. Nevertheless, the typhaneoside activity has not been affected by the glutamate transporter inhibitor dl-threo-beta-benzyloxyaspartate. The synaptosomal plasma membrane potential was assayed using a membrane potential-sensitive dye DiSC₃(5), and cytosolic Ca²⁺ concentrations ([Ca²⁺]_C) was monitored by a Ca²⁺ indicator Fura-2. Results showed that typhaneoside did not alter the synaptosomal membrane potential but lowered 4-aminopyridine-induced increases in [Ca²⁺]_C. Furthermore, the Ca_v2.2 (N-type) channel blocker ω-conotoxin GVIA blocked Ca²⁺ entry and inhibited the effect of typhaneoside on 4-aminopyridine-induced glutamate release. However, the inhibitor of intracellular Ca²⁺ release dantrolene and the mitochondrial Na⁺/Ca²⁺ exchanger blocker 7-chloro-5-(2-chloropheny)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one have no effect on the suppression of glutamate release mediated by typhaneoside. Moreover, inhibition of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) prevented the inhibitory effect of typhaneoside on induced glutamate release. Typhaneoside reduced 4-aminopyridine-induced phosphorylation of ERK1/2 and the major presynaptic ERK target synapsin I, which is a synaptic vesicle-associated protein. In conclusion, these findings suggest a role for typhaneoside in modulating glutamate release by suppressing voltage-dependent Ca²⁺ channel mediated presynaptic Ca²⁺ influx and the MAPK/ERK/synapsin I signaling cascade.

LRRK2 at the pre-synaptic site: A 16-years perspective

Parkinson's disease is a common neurodegenerative disorder and is clinically characterized by bradykinesia, rigidity, and resting tremor. Missense mutations in the leucine-rich repeat protein kinase-2 gene (LRRK2) are a recognized cause of inherited Parkinson's disease. The physiological and pathological impact of LRRK2 is still obscure, but accumulating evidence indicates that LRRK2 orchestrates diverse aspects of membrane trafficking, such as membrane fusion and vesicle formation and transport along actin and tubulin tracks. In the present review, we focus on the special relation between LRRK2 and synaptic vesicles. LRRK2 binds and phosphorylates key actors within the synaptic vesicle cycle. Accordingly, alterations in dopamine and glutamate transmission have been described upon LRRK2 manipulations. However, the different modeling strategies and phenotypes observed require a critical approach to decipher the outcome of LRRK2 at the pre-synaptic site.

Silymarin Inhibits Glutamate Release and Prevents against Kainic Acid-Induced Excitotoxic Injury in Rats

Silymarin, a polyphenoic flavonoid derived from the seeds of milk thistle (Silybum marianum), exhibits neuroprotective effects. In this study, we used a model of rat cerebrocortical synaptosomes to investigate whether silymarin affects the release of glutamate, an essential neurotransmitter involved in excitotoxicity. Its possible neuroprotective effect on a rat model of kainic acid (KA)-induced excitotoxicity was also investigated. In rat cortical synaptosomes, silymarin reduced glutamate release and calcium elevation evoked by the K⁺ channel blocker 4-aminopyridine but did not affect glutamate release caused by the Na⁺ channel activator veratridine or the synaptosomal membrane potential. Decreased glutamate release by silymarin was prevented by removal of extracellular calcium and blocking of N- and P/Q-type Ca²⁺ channel or extracellular signal-regulated kinase 1/2 (ERK1/2) but not by blocking of intracellular Ca²⁺ release. Immunoblotting assay results revealed that silymarin reduced 4-aminopyridine-induced phosphorylation of ERK1/2. Moreover, systemic treatment of rats with silymarin (50 or 100 mg/kg) 30 min before systemic KA (15 mg/kg) administration attenuated KA-induced seizures, glutamate concentration elevation, neuronal damage, glial activation, and heat shock protein 70 expression as well as upregulated KA-induced decrease in Akt phosphorylation in the rat hippocampus. Taken together, the present study demonstrated that silymarin depressed synaptosomal glutamate release by suppressing voltage-dependent Ca²⁺ entry and ERK1/2 activity and effectively prevented KA-induced in vivo excitotoxicity.

1H, 13C, and 15N chemical shift assignment of human PACSIN1/syndapin I SH3 domain in solution

Human neuron-specific PACSIN1 plays a key role in synaptic vesicle recycling and endocytosis, as well as reorganization of the microtubule dynamics to maintain axonal plasticity. PACSIN1 contains a highly conserved C-terminal SH3 domain and an F-bar domain at its N-terminus. Due to its remarkable interaction network, PACSIN1 plays a central role in key neuronal functions. Here, we present a robust backbone and side-chain assignment of PACSIN1 SH3 domain based on 2D [¹H,¹⁵N] HSQC or HMQC, and 3D BEST-HNCO, -HNCACB, -HN(CO)CACB, -HN(CA)CO, and standard (H)CC(CO)NH, HN(CA)NNH, HN(COCA)NH, HBHANNH, HNHA, HBHA(CO)NH, H(CC)(CO)NH, HCCH-TOCSY, that covers 96% for all ¹³CO, ¹³C_α and ¹³C_β, 28% of ¹³C_γδε, and 95% of ¹HN and ¹⁵N chemical shifts. Modelling based on sequence homology with a known related structure, and chemical shift-based secondary structure predictions, identified the presence of five β-strands linked by flexible loops. Taken together, these results open up new avenues to investigate and develop new therapeutic strategies.

A Critical LRRK at the Synapse? The Neurobiological Function and Pathophysiological Dysfunction of LRRK2

Since the discovery of LRRK2 mutations causal to Parkinson's disease (PD) in the early 2000s, the LRRK2 protein has been implicated in a plethora of cellular processes in which pathogenesis could occur, yet its physiological function remains elusive. The development of genetic models of LRRK2 PD has helped identify the etiological and pathophysiological underpinnings of the disease, and may identify early points of intervention. An important role for LRRK2 in synaptic function has emerged in recent years, which links LRRK2 to other genetic forms of PD, most notably those caused by mutations in the synaptic protein α-synuclein. This point of convergence may provide useful clues as to what drives dysfunction in the basal ganglia circuitry and eventual death of substantia nigra (SN) neurons. Here, we discuss the evolution and current state of the literature placing LRRK2 at the synapse, through the lens of knock-out, overexpression, and knock-in animal models. We hope that a deeper understanding of LRRK2 neurobiology, at the synapse and beyond, will aid the eventual development of neuroprotective interventions for PD, and the advancement of useful treatments in the interim.

Drosophila enabled promotes synapse morphogenesis and regulates active zone form and function

Background

Recent studies of synapse form and function highlight the importance of the actin cytoskeleton in regulating multiple aspects of morphogenesis, neurotransmission, and neural plasticity. The conserved actin-associated protein Enabled (Ena) is known to regulate development of the Drosophila larval neuromuscular junction through a postsynaptic mechanism. However, the functions and regulation of Ena within the presynaptic terminal has not been determined.

Methods

Here, we use a conditional genetic approach to address a presynaptic role for Ena on presynaptic morphology and ultrastructure, and also examine the pathway in which Ena functions through epistasis experiments.

Results

We find that Ena is required to promote the morphogenesis of presynaptic boutons and branches, in contrast to its inhibitory role in muscle. Moreover, while postsynaptic Ena is regulated by microRNA-mediated mechanisms, presynaptic Ena relays the output of the highly conserved receptor protein tyrosine phosphatase Dlar and associated proteins including the heparan sulfate proteoglycan Syndecan, and the non-receptor Abelson tyrosine kinase to regulate addition of presynaptic varicosities. Interestingly, Ena also influences active zones, where it restricts active zone size, regulates the recruitment of synaptic vesicles, and controls the amplitude and frequency of spontaneous glutamate release.

Conclusion

We thus show that Ena, under control of the Dlar pathway, is required for presynaptic terminal morphogenesis and bouton addition and that Ena has active zone and neurotransmission phenotypes. Notably, in contrast to Dlar, Ena appears to integrate multiple pathways that regulate synapse form and function.

Early effects of 16O radiation on neuronal morphology and cognition in a murine model

Astronauts exposed to high linear energy transfer radiation may experience cognitive injury. The pathogenesis of this injury is unknown but may involve glutamate receptors or modifications to dendritic structure and/or dendritic spine density and morphology. Glutamate is the major excitatory neurotransmitter in the central nervous system, where it acts on ionotropic and metabotropic glutamate receptors located at the presynaptic terminal and in the postsynaptic membrane at synapses in the hippocampus. Dendritic spines are sites of excitatory synaptic transmission, and changes in spine structure and dendrite morphology are thought to be morphological correlates of altered brain function associated with hippocampal-dependent learning and memory. The aim of the current study is to assess whether behavior, glutamate receptor gene expression, and dendritic structure in the hippocampus are altered in mice after early exposure to ¹⁶O radiation in mice. Two weeks post-irradiation, animals were tested for hippocampus-dependent cognitive performance in the Y-maze. During Y-maze testing, mice exposed to 0.1 Gy and 0.25 Gy radiation failed to distinguish the novel arm, spending approximately the same amount of time in all 3 arms during the retention trial. Exposure to ¹⁶O significantly reduced the expression of Nr1 and GluR1 in the hippocampus and modulated spine morphology in the dentate gyrus and cornu Ammon 1 within the hippocampus. The present data provide evidence that ¹⁶O radiation has early deleterious effects on mature neurons that are associated with hippocampal learning and memory.

The LRRK2 G2385R variant is a partial loss-of-function mutation that affects synaptic vesicle trafficking through altered protein interactions

Mutations in the Leucine-rich repeat kinase 2 gene (LRRK2) are associated with familial Parkinson's disease (PD). LRRK2 protein contains several functional domains, including protein-protein interaction domains at its N- and C-termini. In this study, we analyzed the functional features attributed to LRRK2 by its N- and C-terminal domains. We combined TIRF microscopy and synaptopHluorin assay to visualize synaptic vesicle trafficking. We found that N- and C-terminal domains have opposite impact on synaptic vesicle dynamics. Biochemical analysis demonstrated that different proteins are bound at the two extremities, namely β3-Cav2.1 at N-terminus part and β-Actin and Synapsin I at C-terminus domain. A sequence variant (G2385R) harboured within the C-terminal WD40 domain increases the risk for PD. Complementary biochemical and imaging approaches revealed that the G2385R variant alters strength and quality of LRRK2 interactions and increases fusion of synaptic vesicles. Our data suggest that the G2385R variant behaves like a loss-of-function mutation that mimics activity-driven events. Impaired scaffolding capabilities of mutant LRRK2 resulting in perturbed vesicular trafficking may arise as a common pathophysiological denominator through which different LRRK2 pathological mutations cause disease.

Synaptic function is modulated by LRRK2 and glutamate release is increased in cortical neurons of G2019S LRRK2 knock-in mice

Mutations in Leucine-Rich Repeat Kinase-2 (LRRK2) result in familial Parkinson's disease and the G2019S mutation alone accounts for up to 30% in some ethnicities. Despite this, the function of LRRK2 is largely undetermined although evidence suggests roles in phosphorylation, protein interactions, autophagy and endocytosis. Emerging reports link loss of LRRK2 to altered synaptic transmission, but the effects of the G2019S mutation upon synaptic release in mammalian neurons are unknown. To assess wild type and mutant LRRK2 in established neuronal networks, we conducted immunocytochemical, electrophysiological and biochemical characterization of >3 week old cortical cultures of LRRK2 knock-out, wild-type overexpressing and G2019S knock-in mice. Synaptic release and synapse numbers were grossly normal in LRRK2 knock-out cells, but discretely reduced glutamatergic activity and reduced synaptic protein levels were observed. Conversely, synapse density was modestly but significantly increased in wild-type LRRK2 overexpressing cultures although event frequency was not. In knock-in cultures, glutamate release was markedly elevated, in the absence of any change to synapse density, indicating that physiological levels of G2019S LRRK2 elevate probability of release. Several pre-synaptic regulatory proteins shown by others to interact with LRRK2 were expressed at normal levels in knock-in cultures; however, synapsin 1 phosphorylation was significantly reduced. Thus, perturbations to the pre-synaptic release machinery and elevated synaptic transmission are early neuronal effects of LRRK2 G2019S. Furthermore, the comparison of knock-in and overexpressing cultures suggests that one copy of the G2019S mutation has a more pronounced effect than an ~3-fold increase in LRRK2 protein. Mutant-induced increases in transmission may convey additional stressors to neuronal physiology that may eventually contribute to the pathogenesis of Parkinson's disease.

Maggot learning and Synapsin function

Drosophila larvae are focused on feeding and have few neurons. Within these bounds, however, there still are behavioural degrees of freedom. This review is devoted to what these elements of flexibility are, and how they come about. Regarding odour-food associative learning, the emerging working hypothesis is that when a mushroom body neuron is activated as a part of an odour-specific set of mushroom body neurons, and coincidently receives a reinforcement signal carried by aminergic neurons, the AC-cAMP-PKA cascade is triggered. One substrate of this cascade is Synapsin, and therefore this review features a general and comparative discussion of Synapsin function. Phosphorylation of Synapsin ensures an alteration of synaptic strength between this mushroom body neuron and its target neuron(s). If the trained odour is encountered again, the pattern of mushroom body neurons coding this odour is activated, such that their modified output now allows conditioned behaviour. However, such an activated memory trace does not automatically cause conditioned behaviour. Rather, in a process that remains off-line from behaviour, the larvae compare the value of the testing situation (based on gustatory input) with the value of the odour-activated memory trace (based on mushroom body output). The circuit towards appetitive conditioned behaviour is closed only if the memory trace suggests that tracking down the learned odour will lead to a place better than the current one. It is this expectation of a positive outcome that is the immediate cause of appetitive conditioned behaviour. Such conditioned search for reward corresponds to a view of aversive conditioned behaviour as conditioned escape from punishment, which is enabled only if there is something to escape from - much in the same way as we only search for things that are not there, and run for the emergency exit only when there is an emergency. One may now ask whether beyond 'value' additional information about reinforcement is contained in the memory trace, such as information about the kind and intensity of the reinforcer used. The Drosophila larva may allow us to develop satisfyingly detailed accounts of such mnemonic richness - if it exists.

Increased vesicular γ-GABA transporter and decreased phosphorylation of synapsin I in the rostral preoptic area is associated with decreased gonadotrophin-releasing hormone and c-Fos coexpression in middle-aged female mice

Hypothalamic glutamate (Glu) and γ-GABA neurotransmission are involved in the ovarian hormone-induced gonadotrophin-releasing hormone (GnRH)/luteinising hormone (LH) surge in rodents. Studies have shown that reduced Glu and increased γ-GABA in the rostral preoptic area (rPOA) of the hypothalamus, where most activated GnRH neurones are located, play a key role in decreasing the reproductive function of female rats. However, the mechanism underlying the altered balance of these neurotransmitters is poorly understood. In the present study, we observed a decline in the function of GnRH neurones in the rPOA at the time of the GnRH/LH surge in middle-aged intact female mice with regular oestrous cycles. In young mice, there is an increase of vesicular Glu transporter 2 on the pro-oestrus afternoon, which is not observed in middle-aged mice. By contrast, vesicular γ-GABA transporter levels in young mice decrease at the time of the LH surge, whereas they increase in middle-aged mice. Of note, we found that, in middle-aged mice at the time of the GnRH/LH surge, the phosphorylation of synapsin I at Ser603 and Ca(2+) /calmodulin-dependent kinase IIα was significantly lower than in young mice. These data suggest that, in middle-aged mice, higher levels of presynaptic stores of GABA, a lack of increase of Glu and a decreased ability of synaptic vesicle mobilisation could account for the imbalance of Glu and GABA in the rPOA, which decreases the activation of GnRH neurones.

Ketamine regulates the presynaptic release machinery in the hippocampus

In the search for new drug targets, that may help point the way to develop fast-acting treatments for mood disorders, we have explored molecular pathways regulated by ketamine, an NMDA receptor antagonist, which has consistently shown antidepressant response within a few hours of administration. Using Sprague-Dawley rats we investigated the effects of ketamine on the presynaptic release machinery responsible for neurotransmitter release at 1, 2 and 4 h as well as 7 days after administration of a single subanesthetic dose of ketamine (15 mg/kg). A large reduction in the accumulation of SNARE complexes was observed in hippocampal synaptic membranes after 1, 2 and 4 h of ketamine administration. In parallel, we found a selective reduction in the expression of the synaptic vesicle protein synaptotagmin I and an increase in the levels of synapsin I in hippocampal synaptosomes suggesting a mechanism by which ketamine reduces SNARE complex formation, in part, by regulating the number of synaptic vesicles in the nerve terminals. Moreover, ketamine reduced Thr(286)-phosphorylated αCaMKII and its interaction with syntaxin 1A, which identifies CaMKII as a potential target for second messenger-mediated actions of ketamine. In addition, despite previous reports of ketamine-induced inhibition of GSK-3, we were unable to detect regulation of its activity after ketamine administration. Our findings demonstrate that ketamine rapidly induces changes in the hippocampal presynaptic machinery similar to those that are obtained only with chronic treatments with traditional antidepressants. This suggests that reduction of neurotransmitter release in the hippocampus has possible relevance for the rapid antidepressant effect of ketamine.

Berberine Inhibits the Release of Glutamate in Nerve Terminals from Rat Cerebral Cortex

Berberine, an isoquinoline plant alkaloid, protects neurons against neurotoxicity. An excessive release of glutamate is considered to be one of the molecular mechanisms of neuronal damage in several neurological diseases. In this study, we investigated whether berberine could affect endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes) and explored the possible mechanism. Berberine inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP), and this phenomenon was prevented by the chelating extracellular Ca²⁺ ions and the vesicular transporter inhibitor bafilomycin A1, but was insensitive to the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate. Inhibition of glutamate release by berberine was not due to it decreasing synaptosomal excitability, because berberine did not alter 4-AP-mediated depolarization. The inhibitory effect of berberine on glutamate release was associated with a reduction in the depolarization-induced increase in cytosolic free Ca²⁺ concentration. Involvement of the Ca_v2.1 (P/Q-type) channels in the berberine action was confirmed by blockade of the berberine-mediated inhibition of glutamate release by the Ca_v2.1 (P/Q-type) channel blocker ω-agatoxin IVA. In addition, the inhibitory effect of berberine on evoked glutamate release was prevented by the mitogen-activated/extracellular signal-regulated kinase kinase (MEK) inhibitors. Berberine decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synapsin I, the main presynaptic target of ERK; this decrease was also blocked by the MEK inhibition. Moreover, the inhibitory effect of berberine on evoked glutamate release was prevented in nerve terminals from mice lacking synapsin I. Together, these results indicated that berberine inhibits glutamate release from rats cortical synaptosomes, through the suppression of presynaptic Cav2.1 channels and ERK/synapsin I signaling cascade. This finding may provide further understanding of the mode of berberine action in the brain and highlights the therapeutic potential of this compound in the treatment of a wide range of neurological disorders.

Tanshinone IIA, a constituent of Danshen, inhibits the release of glutamate in rat cerebrocortical nerve terminals

ETHNOPHARMACOLOGICAL RELEVANCE

Danshen is a commonly used traditional Chinese medicine and has received considerable attention due to their beneficial effects on the health, including prevention of cardiovascular disease, and cancer. Tanshinone IIA, a major active constituent of Danshen, has been reported to have a neuroprotective profile.

AIM OF THE STUDY

An excessive release of glutamate is considered to be related to neuropathology of several neurological diseases. In this study, we investigated whether tanshinone IIA could affect endogenous glutamate release and explored the possible mechanism.

MATERIALS AND METHODS

The experimental model was the isolated nerve terminals (synaptosomes) purified from the rat cerebral cortex. The release of glutamate was evoked by the K(+) channel blocker 4-aminopyridine (4-AP) and measured by one-line enzyme-coupled fluorometric assay. We also used a membrane potential-sensitive dye to assay nerve terminal excitability and depolarization, and a Ca(2+) indicator, Fura-2-acetoxymethyl ester, to monitor cytosolic Ca(2+) concentrations ([Ca(2+)]C).

RESULTS

Tanshinone IIA inhibited the release of glutamate evoked by 4-AP in a concentration-dependent manner. Inhibition of glutamate release by tanshinone IIA was prevented by the chelating the extracellular Ca(2+) ions, and by the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate did not have any effect on the action of tanshinone IIA. Tanshinone IIA decreased the depolarization-induced increase in [Ca(2+)]C, whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. Furthermore, the effect of tanshinone IIA on evoked glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC, but not by the ryanodine receptor blocker dantrolene or the mitochondrial Na(+)/Ca(2+) exchanger blocker CGP37157. Mitogen-activated protein kinase (MEK) inhibition also prevented the inhibitory effect of tanshinone IIA on evoked glutamate release.

CONCLUSION

These results show that tanshinone IIA inhibits glutamate release from cortical synaptosomes in rats through the suppression of presynaptic voltage-dependent Ca(2+) entry and MEK signaling cascade.

Perinatal exposure to bisphenol-A inhibits synaptogenesis and affects the synaptic morphological development in offspring male mice

Our previous study indicated that perinatal exposure to low-dose BPA, one of the most common environmental endocrine disrupters, alters behavioral development in offspring mice. Given that synaptic structure of the hippocampus is closely related to behaviors, in the present study, we examined the effects of perinatal exposure to BPA (0.04, 0.4, and 4.0 mg kg(-1) day(-1)) on the synaptic density and the synaptic structural modification of pyramidal cells in hippocampus region CA1 and the expressions of synaptic proteins such as synapsin I and PSD-95 and glutamate NMDA and AMPA receptors in male offspring mice on postnatal day (PND) 14, 21, and 56. The results of electron microscope measurement showed that BPA significantly reduced the numeric synaptic density and altered the structural modification of synaptic interface of pyramidal cells with the enlarged synaptic cleft, the shortened active zone, and the thinned postsynaptic density (PSD) on PND 14, 21, and 56 and the increased curvature of synaptic interface on PND 14 and 21. Further analyses of Western blot indicated that BPA markedly reduced the levels of synapsin I and PSD-95 on PND 14, 21, and 56 and down-regulated NMDA receptor subunit NR1 and AMPA receptor subunit GluR1 during development and young adulthood. These results suggest that perinatal exposure to low level of BPA inhibits synaptogenesis and affects synaptic structural modification after birth. The reduced expressions of synaptic proteins synapsin I and PSD-95 and glutamate NMDA and AMPA receptors may be involved in the negative changes in the synaptic plasticity.

Synapsin II gene expression in the dorsolateral prefrontal cortex of brain specimens from patients with schizophrenia and bipolar disorder: effect of lifetime intake of antipsychotic drugs

Synapsins are neuronal phosphoproteins crucial to regulating the processes required for normal neurotransmitter release. Synapsin II, in particular, has been implied as a candidate gene for schizophrenia. This study investigated synapsin II mRNA expression, using Real Time RT-PCR, in coded dorsolateral prefrontal cortical samples provided by the Stanley Foundation Neuropathology Consortium. Synapsin IIa was decreased in patients with schizophrenia when compared to both healthy subjects and patients with bipolar disorder, whereas the synapsin IIb was only significantly reduced in patients with schizophrenia when compared to healthy subjects, but not patients with bipolar disorder. Furthermore, lifetime antipsychotic drug use was positively associated with synapsin IIa expression in patients with schizophrenia. Results suggest that impairment of synaptic transmission by synapsin II reduction may contribute to dysregulated convergent molecular mechanisms which result in aberrant neural circuits that characterize schizophrenia, while implicating involvement of synapsin II in therapeutic mechanisms of currently prescribed antipsychotic drugs.

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.

Coenzyme Q10 inhibits the release of glutamate in rat cerebrocortical nerve terminals by suppression of voltage-dependent calcium influx and mitogen-activated protein kinase signaling pathway

This study investigates the effects and possible mechanism of coenzyme Q10 (CoQ10) on endogenous glutamate release in the cerebral cortex nerve terminals of rats. CoQ10 inhibited the release of glutamate evoked by the K+ channel blocker 4-aminopyridine (4-AP). CoQ10 reduced the depolarization-induced increase in cytosolic [Ca2+]c but did not alter the 4-AP-mediated depolarization. The effect of CoQ10 on evoked glutamate release was abolished by blocking the Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels and mitogen-activated protein kinase kinase (MEK). In addition, CoQ10 decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synaptic vesicle-associated protein synapsin I, a major presynaptic substrate for ERK. Moreover, the inhibition of glutamate release by CoQ10 was strongly attenuated in mice without synapsin I. These results suggest that CoQ10 inhibits glutamate release from cortical synaptosomes in rats through the suppression of the presynaptic voltage-dependent Ca2+ entry and ERK/synapsin I signaling pathway.

Curcumin inhibits glutamate release from rat prefrontal nerve endings by affecting vesicle mobilization

Curcumin, one of the major constituents of Curcuma longa, has been shown to inhibit depolarization-evoked glutamate release from rat prefrontocortical nerve terminals by reducing voltage-dependent Ca²⁺ entry. This study showed that curcumin inhibited ionomycin-induced glutamate release and KCl-evoked FM1-43 release, suggesting that some steps after Ca²⁺ entry are regulated by curcumin. Furthermore, disrupting the cytoskeleton organization using cytochalasin D abolished the inhibitory action of curcumin on ionomycin-induced glutamate release. Mitogen-activated protein kinase kinase (MEK) inhibition also prevented the inhibitory effect of curcumin on ionomycin-induced glutamate release. Western blot analyses showed that curcumin decreased the ionomycin-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synaptic vesicle-associated protein synapsin I, the main presynaptic target of ERK. These results show that curcumin-mediated inhibition of glutamate release involves modulating downstream events by controlling synaptic vesicle recruitment and exocytosis, possibly through a decrease of MAPK/ERK activation and synapsin I phosphorylation, thereby decreasing synaptic vesicle availability for exocytosis.

Behavioral effects of non-viral mediated RNA interference of synapsin II in the medial prefrontal cortex of the rat

Synapsin II is a synaptic vesicle-associated phosphoprotein that has been implicated in the pathophysiology of schizophrenia. Researchers have demonstrated reductions in synapsin II mRNA and protein in post-mortem prefrontal cortex and hippocampus samples from patients with schizophrenia. Synapsin II protein expression has been shown to be regulated by dopamine D(1) and D(2) receptor activation. Furthermore, behavioral testing of the synapsin II knockout mouse has revealed a schizophrenic-like behavioral phenotype in this mutant strain, suggesting a relationship between dysregulated and/or reduced synapsin II and schizophrenia. However, it remains unknown the specific regions of the brain of which perturbations in synapsin II play a role in the pathophysiology of this disease. The aim of this project was to evaluate animals with a selective knock-down of synapsin II in the medial prefrontal cortex through the use of siRNA technology. Two weeks after continuous infusion of synapsin II siRNAs, animals were examined for the presence of a schizophrenic-like behavioral phenotype. Our results reveal that rats with selective reductions in medial prefrontal cortical synapsin II demonstrate deficits in sensorimotor gating (prepulse inhibition), hyperlocomotion, and reduced social behavior. These results implicate a role for decreased medial prefrontal cortical synapsin II levels in the pathophysiology of schizophrenia and the mechanisms of aberrant prefrontal cortical circuitry, and suggest that increasing synapsin II levels in the medial prefrontal cortex may potentially serve as a novel therapeutic target for this devastating disorder.

Inhibition of exocytosis or endocytosis blocks activity-dependent redistribution of synapsin

The synaptic vesicle cycle encompasses the pre-synaptic events that drive neurotransmission. Influx of calcium leads to the fusion of synaptic vesicles with the plasma membrane and the release of neurotransmitter, closely followed by endocytosis. Vacated release sites are repopulated with vesicles which are then primed for release. When activity is intense, reserve vesicles may be mobilized to counteract an eventual decline in transmission. Recently, interplay between endocytosis and repopulation of the readily releasable pool of vesicles has been identified. In this study, we show that exo-endocytosis is necessary to enable detachment of synapsin from reserve pool vesicles during synaptic activity. We report that blockage of exocytosis in cultured mouse hippocampal neurons, either by tetanus toxin or by the deletion of munc13, inhibits the activity-dependent redistribution of synapsin from the pre-synaptic terminal into the axon. Likewise, perturbation of endocytosis with dynasore or by a dynamin dominant-negative mutant fully prevents synapsin redistribution. Such inhibition of synapsin redistribution occurred despite the efficient phosphorylation of synapsin at its protein kinase A/CaMKI site, indicating that disengagement of synapsin from the vesicles requires exocytosis and endocytosis in addition to phosphorylation. Our results therefore reveal hitherto unidentified feedback within the synaptic vesicle cycle involving the synapsin-managed reserve pool.

Effects of docosahexaenoic acid on mouse brain synaptic plasma membrane proteome analyzed by mass spectrometry and (16)O/(18)O labeling

Docosahexenoic acid (DHA, 22:6n-3) plays an important role in development of proper brain function in mammals. We have previously reported that DHA promotes synaptogenesis and synaptic function in hippocampal neurons while DHA-depletion in the brain due to n-3 fatty acid deficiency produces opposite effects. To gain insight into underlying molecular mechanisms, we investigated whether the brain DHA status affects the synaptic plasma membrane (SPM) proteome by using nanoLC-ESI-MS/MS and (16)O/(18)O labeling. The DHA level in mouse brains was lowered by dietary depletion of n-3 fatty acids, and SPM was prepared by differential centrifugation followed by osmotic shock. SPM proteins from DHA-adequate and depleted brains were analyzed by nanoLC-ESI-MS/MS after SDS-PAGE, in-gel digestion, and differential O(18)/O(16) labeling. This strategy allowed comparative quantitation of more than 200 distinct membrane or membrane-associated proteins from DHA-adequate or depleted brains. We found that 18 pre- and postsynaptic proteins that are relevant to synaptic physiology were significantly down-regulated in DHA-depleted mouse brains. The protein network analysis suggests involvement of CREB and caspase-3 pathways in the DHA-dependent modulation of synaptic proteome. Reduction of specific synaptic proteins due to brain DHA-depletion may be an important mechanism for the suboptimal brain function associated with n-3 fatty acid deficiency.

Modulation of neuronal signal transduction and memory formation by synaptic zinc

The physiological role of synaptic zinc has remained largely enigmatic since its initial detection in hippocampal mossy fibers over 50 years ago. The past few years have witnessed a number of studies highlighting the ability of zinc ions to regulate ion channels and intracellular signaling pathways implicated in neuroplasticity, and others that shed some light on the elusive role of synaptic zinc in learning and memory. Recent behavioral studies using knock-out mice for the synapse-specific zinc transporter ZnT-3 indicate that vesicular zinc is required for the formation of memories dependent on the hippocampus and the amygdala, two brain centers that are prominently innervated by zinc-rich fibers. A common theme emerging from this research is the activity-dependent regulation of the Erk1/2 mitogen-activated-protein kinase pathway by synaptic zinc through diverse mechanisms in neurons. Here we discuss current knowledge on how synaptic zinc may play a role in cognition through its impact on neuronal signaling.

Medial prefrontal cortical synapsin II knock-down induces behavioral abnormalities in the rat: examining synapsin II in the pathophysiology of schizophrenia

Synapsin II is a synaptic vesicle-associated phosphoprotein that has been implicated in the pathophysiology of schizophrenia. Studies have demonstrated reductions in synapsin II mRNA and protein in medial prefrontal cortical post-mortem samples from patients with schizophrenia, genetic associations between synapsin II and schizophrenia, and synapsin II protein regulation by dopamine receptor activation. Collectively, this research indicates a relationship between synapsin II dysregulation and schizophrenia; however, it remains unknown whether perturbations in synapsin II play a role in the pathophysiology of this disease. The aim of this project was to evaluate animals with selective knock-down of synapsin II in the medial prefrontal cortex. After continuous infusion of synapsin II antisense sequences, animals were examined for the presence of schizophrenic-like behavioral phenotypes and assessed on the response to clinically relevant antipsychotic drugs. Our results indicate that rats with selective reductions in medial prefrontal cortical synapsin II demonstrate deficits in sensorimotor gating (prepulse inhibition), reduced social behavior, and hyperlocomotion, which are corrected by the atypical antipsychotic drug olanzapine. Additionally, synapsin II knock-down disrupts serial search efficiency. These behavioral changes are accompanied by reductions in vesicular neurotransmitter transporter protein concentrations for glutamate (VGLUT1 and VGLUT2) and GABA (VGAT), without affecting dopamine (VMAT2). These results implicate a causal role for decreased synapsin II in the medial prefrontal cortex in the pathophysiology of schizophrenia and the mechanisms of aberrant prefrontal cortical circuitry, and suggest that synapsin II may potentially serve as a novel therapeutic target for this disorder.

Prenatal exposure to perfluorooctanesulfonate in rat resulted in long-lasting changes of expression of synapsins and synaptophysin

Both animal and human studies have demonstrated that exposure to chemical pollutants during critical developmental period causes adverse consequences later in life. In uterus, perfluorooctanesulfonate (PFOS) exposure has been known to cause developmental neurotoxicity, such as increased motor activity, reduced habitation and impaired cognitive function. The possible mechanism of the impaired cognitive function induced by prenatal PFOS exposure was evaluated in this study. Pregnant Sprague Dawley (SD) rats were given 0.1, 0.6, and 2.0 mg kg(-1) birth weight (bw) d(-1) by gavage from gestation day (GD) 0 to GD20. Control received 0.5% Tween-20 vehicle (4 ml kg(-1) bw d(-1)). PFOS concentration in hippocampus of offspring was observed on postnatal day (PND) 0 and PND21. The ultrastructure of hippocampus and the gene expression of synaptic vesicle associated proteins in offspring hippocampus, which were important for the neurotransmitter release, were investigated. The transmission electron photomicrographs of the offspring hippocampus from PFOS-treated maternal groups showed the ultrastructure of synapses was negatively affected. The offspring from PFOS-treated maternal groups also differed significantly from controls with respect to the expression of synaptic vesicle associated proteins. The mRNA levels of synapsin1 (Syn1), synapsin2 (Syn2), and synaptophysin (Syp) were decreased in treated groups either on PND0 or on PND21. However, the mRNA level of synapsin3 (Syn3) decreased in 0.6- and 2.0-mg kg(-1) group on PND0, and showed no significant difference among control group and all treated groups on PND21. These results indicate that the impairment of cognitive function induced by PFOS may be attributed to the lower mRNA levels of synaptic vesicle associated proteins and the change of synaptic ultrastructure in hippocampus.

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.

Taking a back seat: synaptic vesicle clustering in presynaptic terminals

Central inter-neuronal synapses employ various molecular mechanisms to sustain neurotransmitter release during phases of high-frequency synaptic activity. One of the features ensuring this property is the presence of a pool of synaptic vesicles (SVs) in the presynaptic terminal. At rest and low rates of stimulation, most of the vesicles composing this pool remain in a tight cluster. They are actively utilized when neurons fire action potentials at higher rates and the capability of the recycling machinery is limited. In addition, SV clusters are capable of migrating between release sites and reassemble into clusters at neighboring active zones (AZs). Within the cluster, thin "tethers" interconnect SVs. These dynamic filamentous structures are reorganized during stimulation thereby releasing SVs from the cluster. So far, one protein family, the synapsins, which bind actin filaments and vesicles in a phosphorylation-dependent manner, has been implicated in SV clustering in vertebrate synapses. As evident from recent studies, many endocytic proteins reside in the SV cluster in addition to synapsin. Here we discuss alternative possible mechanisms involved in the organization of this population of SVs. We propose a model in which synapsins together with other synaptic proteins, a large proportion of which is involved in SV recycling, form a dynamic proteinaceous "matrix" which limits the mobility of SVs. Actin filaments, however, do not seem to contribute to SV crosslinking within the SV cluster, but instead they are present peripherally to it, at sites of neurotransmitter release, and at sites of SV recycling.

Binding of protein kinase inhibitors to synapsin I inferred from pair-wise binding site similarity measurements

Predicting off-targets by computational methods is getting increasing importance in early drug discovery stages. We herewith present a computational method based on binding site three-dimensional comparisons, which prompted us to investigate the cross-reaction of protein kinase inhibitors with synapsin I, an ATP-binding protein regulating neurotransmitter release in the synapse. Systematic pair-wise comparison of the staurosporine-binding site of the proto-oncogene Pim-1 kinase with 6,412 druggable protein-ligand binding sites suggested that the ATP-binding site of synapsin I may recognize the pan-kinase inhibitor staurosporine. Biochemical validation of this hypothesis was realized by competition experiments of staurosporine with ATP-gamma(35)S for binding to synapsin I. Staurosporine, as well as three other inhibitors of protein kinases (cdk2, Pim-1 and casein kinase type 2), effectively bound to synapsin I with nanomolar affinities and promoted synapsin-induced F-actin bundling. The selective Pim-1 kinase inhibitor quercetagetin was shown to be the most potent synapsin I binder (IC50 = 0.15 microM), in agreement with the predicted binding site similarities between synapsin I and various protein kinases. Other protein kinase inhibitors (protein kinase A and chk1 inhibitor), kinase inhibitors (diacylglycerolkinase inhibitor) and various other ATP-competitors (DNA topoisomerase II and HSP-90alpha inhibitors) did not bind to synapsin I, as predicted from a lower similarity of their respective ATP-binding sites to that of synapsin I. The present data suggest that the observed downregulation of neurotransmitter release by some but not all protein kinase inhibitors may also be contributed by a direct binding to synapsin I and phosphorylation-independent perturbation of synapsin I function. More generally, the data also demonstrate that cross-reactivity with various targets may be detected by systematic pair-wise similarity measurement of ligand-annotated binding sites.

Synapsins differentially control dopamine and serotonin release

Synapsins are a family of synaptic vesicle proteins that are important for neurotransmitter release. Here we have used triple knock-out (TKO) mice lacking all three synapsin genes to determine the roles of synapsins in the release of two monoamine neurotransmitters, dopamine and serotonin. Serotonin release evoked by electrical stimulation was identical in substantia nigra pars reticulata slices prepared from TKO and wild-type mice. In contrast, release of dopamine in response to electrical stimulation was approximately doubled in striatum of TKO mice, both in vivo and in striatal slices, in comparison to wild-type controls. This was due to loss of synapsin III, because deletion of synapsin III alone was sufficient to increase dopamine release. Deletion of synapsins also increased the sensitivity of dopamine release to extracellular calcium ions. Although cocaine did not affect the release of serotonin from nigral tissue, this drug did enhance dopamine release. Cocaine-induced facilitation of dopamine release was a function of external calcium, an effect that was reduced in TKO mice. We conclude that synapsins play different roles in the control of release of dopamine and serotonin, with release of dopamine being negatively regulated by synapsins, specifically synapsin III, while serotonin release appears to be relatively independent of synapsins. These results provide further support for the concept that synapsin function in presynaptic terminals varies according to the neurotransmitter being released.

Human Alzheimer's disease synaptic O-GlcNAc site mapping and iTRAQ expression proteomics with ion trap mass spectrometry

Neuronal synaptic functional deficits are linked to impaired learning and memory in Alzheimer's disease (AD). We recently demonstrated that O-GlcNAc, a novel cytosolic and nuclear carbohydrate post-translational modification, is enriched at neuronal synapses and positively regulates synaptic plasticity linked to learning and memory in mice. Reduced levels of O-GlcNAc have been observed in AD, suggesting a possible link to deficits in synaptic plasticity. Using lectin enrichment and mass spectrometry, we mapped several human cortical synaptic O-GlcNAc modification sites. Overlap in patterns of O-GlcNAcation between mouse and human appears to be high, as previously mapped mouse synaptic O-GlcNAc sites in Bassoon, Piccolo, and tubulin polymerization promoting protein p25 were identified in human. Novel O-GlcNAc modification sites were identified on Mek2 and RPN13/ADRM1. Mek2 is a signaling component of the Erk 1/2 pathway involved in synaptic plasticity. RPN13 is a component of the proteasomal degradation pathway. The potential interplay of phosphorylation with mapped O-GlcNAc sites, and possible implication of those sites in synaptic plasticity in normal versus AD states is discussed. iTRAQ is a powerful differential isotopic quantitative approach in proteomics. Pulsed Q dissociation (PQD) is a recently introduced fragmentation strategy that enables detection of low mass iTRAQ reporter ions in ion trap mass spectrometry. We optimized LTQ ion trap settings for PQD-based iTRAQ quantitation and demonstrated its utility in O-GlcNAc site mapping. Using iTRAQ, abnormal synaptic expression levels of several proteins previously implicated in AD pathology were observed in addition to novel changes in synaptic specific protein expression including Synapsin II.

Synaptic homeostasis is consolidated by the cell fate gene gooseberry, a Drosophila pax3/7 homolog

In a large-scale screening effort, we identified the gene gooseberry (gsb) as being necessary for synaptic homeostasis at the Drosophila neuromuscular junction. The gsb gene encodes a pair-rule transcription factor that participates in embryonic neuronal cell fate specification. Here, we define a new postembryonic role for gooseberry. We show that gsb becomes widely expressed in the postembryonic CNS, including within mature motoneurons. Loss of gsb does not alter neuromuscular growth, morphology, or the distribution of essential synaptic proteins. However, gsb function is required postembryonically for the sustained expression of synaptic homeostasis. In GluRIIA mutant animals, miniature EPSP (mEPSP) amplitudes are significantly decreased, and there is a compensatory homeostatic increase in presynaptic release that restores normal muscle excitation. Loss of gsb significantly impairs the homeostatic increase in presynaptic release in the GluRIIA mutant. Interestingly, gsb is not required for the rapid induction of synaptic homeostasis. Furthermore, gsb seems to be specifically involved in the mechanisms responsible for a homeostatic increase in presynaptic release, since it is not required for the homeostatic decrease in presynaptic release observed following an increase in mEPSP amplitude. Finally, Gsb has been shown to antagonize Wingless signaling during embryonic fate specification, and we present initial evidence that this activity is conserved during synaptic homeostasis. Thus, we have identified a gene (gsb) that distinguishes between rapid induction versus sustained expression of synaptic homeostasis and distinguishes between the mechanisms responsible for homeostatic increase versus decrease in synaptic vesicle release.

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.

Regulation of markers of synaptic function in mouse models of depression: chronic mild stress and decreased expression of VGLUT1

Depression has been linked to failure in synaptic plasticity originating from environmental and/or genetic risk factors. The chronic mild stress model regulates the expression of synaptic markers of neurotransmitter function and associated depressive-like behaviour. Moreover, mice heterozygous for the synaptic vesicle protein vesicular glutamate transporter 1 (VGLUT1), have been proposed as a genetic model of deficient glutamate function linked to depressive-like behaviour. Here, we aimed to identify, in these two experimental models, mechanisms of failure in synaptic plasticity, common to stress and impaired glutamate function. First, we show that chronic mild stress induced a transient decrease of different plasticity markers (VGLUT1, synapsin 1, sinaptophysin, rab3A and activity regulated cytoskeletal protein - Arc) but a long-lasting decrease of the brain derived neurotrophic factor as well as depressive-like behaviour. The immediate early gene Arc was also down-regulated in VGLUT1+/- heterozygous mice. In contrast, an opposite regulation of synapsin 1 was observed. Finally, both models showed a marked increase of cortical Arc response to novelty. Increased Arc response to novelty could be suggested as a molecular mechanism underlying failure to adapt to environmental changes, common to chronic stress and altered glutamate function. Further studies should investigate whether these changes are associated to depressive-like behaviour both in animal models and in depressed patients.