Use of phosphosynapsin I-specific antibodies for image analysis of signal transduction in single nerve terminals

Protein Phosphorylation in Depolarized Synaptosomes: Dissecting Primary Effects of Calcium from Synaptic Vesicle Cycling

Synaptic transmission is mediated by the regulated exocytosis of synaptic vesicles. When the presynaptic membrane is depolarized by an incoming action potential, voltage-gated calcium channels open, resulting in the influx of calcium ions that triggers the fusion of synaptic vesicles (SVs) with the plasma membrane. SVs are recycled by endocytosis. Phosphorylation of synaptic proteins plays a major role in these processes, and several studies have shown that the synaptic phosphoproteome changes rapidly in response to depolarization. However, it is unclear which of these changes are directly linked to SV cycling and which might regulate other presynaptic functions that are also controlled by calcium-dependent kinases and phosphatases. To address this question, we analyzed changes in the phosphoproteome using rat synaptosomes in which exocytosis was blocked with botulinum neurotoxins (BoNTs) while depolarization-induced calcium influx remained unchanged. BoNT-treatment significantly alters the response of the synaptic phoshoproteome to depolarization and results in reduced phosphorylation levels when compared with stimulation of synaptosomes by depolarization with KCl alone. We dissect the primary Ca2+-dependent phosphorylation from SV-cycling-dependent phosphorylation and confirm an effect of such SV-cycling-dependent phosphorylation events on syntaxin-1a-T21/T23, synaptobrevin-S75, and cannabinoid receptor-1-S314/T322 on exo- and endocytosis in cultured hippocampal neurons.

Synapsin I deletion reduces neuronal damage and ameliorates clinical progression of experimental autoimmune encephalomyelitis

The classical view of multiple sclerosis (MS) pathogenesis states that inflammation-mediated demyelination is responsible for neuronal damage and loss. However, recent findings show that impairment of neuronal functions and demyelination can be independent events, suggesting the coexistence of other pathogenic mechanisms. Due to the inflammatory milieu, subtle alterations in synaptic function occur, which are probably at the basis of the early cognitive decline that often precedes the neurodegenerative phases in MS patients. In particular, it has been reported that inflammation enhances excitatory synaptic transmission while it decreases GABAergic transmission in vitro and ex vivo. This evidence points to the idea that an excitation/inhibition imbalance occurs in the inflamed MS brain, even though the exact molecular mechanisms leading to this synaptic dysfunction are as yet not completely clear. Along this line, we observed that acute treatment of primary hippocampal neurons in culture with pro-inflammatory cytokines leads to an increased phosphorylation of synapsin I (SynI) by ERK1/2 kinase and to an increase in the frequency of spontaneous synaptic vesicle release events, which is prevented by SynI deletion. In vivo, the ablation of SynI expression is protective in terms of disease progression and neuronal damage in the experimental autoimmune encephalomyelitis mouse model of MS. Our results point to a possible key role in MS pathogenesis of the neuronal protein SynI, a regulator of excitation/inhibition balance in neuronal networks.

Lack of the presynaptic RhoGAP protein oligophrenin1 leads to cognitive disabilities through dysregulation of the cAMP/PKA signalling pathway

Loss-of-function mutations in the gene encoding for the RhoGAP protein of oligophrenin-1 (OPHN1) lead to cognitive disabilities (CDs) in humans, yet the underlying mechanisms are not known. Here, we show that in mice constitutive lack of Ophn1 is associated with dysregulation of the cyclic adenosine monophosphate/phosphate kinase A (cAMP/PKA) signalling pathway in a brain-area-specific manner. Consistent with a key role of cAMP/PKA signalling in regulating presynaptic function and plasticity, we found that PKA-dependent presynaptic plasticity was completely abolished in affected brain regions, including hippocampus and amygdala. At the behavioural level, lack of OPHN1 resulted in hippocampus- and amygdala-related learning disabilities which could be fully rescued by the ROCK/PKA kinase inhibitor fasudil. Together, our data identify OPHN1 as a key regulator of presynaptic function and suggest that, in addition to reported postsynaptic deficits, loss of presynaptic plasticity contributes to the pathophysiology of CDs.

Nerve Terminal GABAA Receptors Activate Ca2+/Calmodulin-dependent Signaling to Inhibit Voltage-gated Ca2+ Influx and Glutamate Release

gamma-Aminobutyric acid type A (GABA(A)) receptors, a family of Cl(-)-permeable ion channels, mediate fast synaptic inhibition as postsynaptically enriched receptors for gamma-aminobutyric acid at GABAergic synapses. Here we describe an alternative type of inhibition mediated by GABA(A) receptors present on neocortical glutamatergic nerve terminals and examine the underlying signaling mechanism(s). By monitoring the activity of the presynaptic CaM kinase II/synapsin I signaling pathway in isolated nerve terminals, we demonstrate that GABA(A) receptor activation correlated with an increase in basal intraterminal [Ca(2+)](i). Interestingly, this activation of GABA(A) receptors resulted in a reduction of subsequent depolarization-evoked Ca(2+) influx, which thereby led to an inhibition of glutamate release. To investigate how the observed GABA(A) receptor-mediated modulation operates, we determined the sensitivity of this process to the Na-K-2Cl cotransporter 1 antagonist bumetanide, as well as substitution of Ca(2+) with Ba(2+), or Ca(2+)/calmodulin inhibition by W7. All of these treatments abolished the modulation by GABA(A) receptors. Application of selective antagonists of voltage-gated Ca(2+) channels (VGCCs) revealed that the GABA(A) receptor-mediated modulation of glutamate release required the specific activity of L- and R-type VGCCs. Crucially, the inhibition of release by these receptors was abolished in terminals isolated from R-type VGCC knock-out mice. Together, our results indicate that a functional coupling between nerve terminal GABA(A) receptors and L- or R-type VGCCs is mediated by Ca(2+)/calmodulin-dependent signaling. This mechanism provides a GABA-mediated control of glutamatergic synaptic activity by a direct inhibition of glutamate release.

The synapsin cycle: a view from the synaptic endocytic zone

Although the synapsin phosphoproteins were discovered more than 30 years ago and are known to play important roles in neurotransmitter release and synaptogenesis, a complete picture of their functions within the nerve terminal is lacking. It has been shown that these proteins play an important role in the clustering of synaptic vesicles (SVs) at active zones and function as modulators of synaptic strength by acting at both pre- and postdocking levels. Recent studies have demonstrated that synapsins migrate to the endocytic zone of central synapses during neurotransmitter release, which suggests that there are additional functions for these proteins in SV recycling.

Expression of a synapsin IIb site 1 phosphorylation mutant in 3T3-L1 adipocytes inhibits basal intracellular retention of Glut4

Glut4 exocytosis in adipocytes uses protein machinery that is shared with other regulated secretory processes. Synapsins are phosphoproteins that regulate a ;reserve pool' of vesicles clustered behind the active zone in neurons. We found that adipocytes (primary cells and the 3T3-L1 cell line) express synapsin IIb mRNA and protein. Synapsin IIb co-localizes with Glut4 in perinuclear vesicle clusters. To test whether synapsin plays a role in Glut4 traffic, a site 1 phosphorylation mutant (S10A synapsin) was expressed in 3T3-L1 adipocytes. Interestingly, expression of S10A synapsin increased basal cell surface Glut4 almost fourfold (50% maximal insulin effect). Insulin caused a further twofold translocation of Glut4 in these cells. Expression of the N-terminus of S10A synapsin (amino acids 1-118) was sufficient to inhibit basal Glut4 retention. Neither wild-type nor S10D synapsin redistributed Glut4. S10A synapsin did not elevate surface levels of the transferrin receptor in adipocytes or Glut4 in fibroblasts. Therefore, S10A synapsin is inhibiting the specialized process of basal intracellular retention of Glut4 in adipocytes, without affecting general endocytic cycling. While mutant forms of many proteins inhibit Glut4 exocytosis in response to insulin, S10A synapsin is one of only a few that specifically inhibits Glut4 retention in basal adipocytes. These data indicate that the synapsins are important regulators of membrane traffic in many cell types.

Glucose/oxygen deprivation induces the alteration of synapsin I and phosphosynapsin

Synapsin I is believed to be involved in regulating neurotransmitter release and in synapse formation. Its interactions with the actin filaments and synaptic vesicles are regulated by phosphorylation. Because exocytosis and synapsin I phosphorylation are a Ca(2+)-dependent process, it is possible that an ischemic insult modifies the presynaptic proteins. However, the neuronal damage and the changes in synapsin I as well as its phosphorylation level as a result of glucose/oxygen deprivation (GOD) and reperfusion in organotypic hippocampal slice cultures have not been established. In this study, the level of synapsin I and phosphosynapsin was measured in organotypic hippocampal slice cultures in order to determine the role of synapsin I in the presynaptic nerve terminals during GOD/reperfusion. Propidium iodide fluorescence was observed in the CA1 area after GOD for 30 min, which could be detected in the whole pyramidal cell layer during reperfusion for 24 h. The immunofluorescence of the neuron specific nuclear protein, NeuN, showed a negative correlation with the PI fluorescence. During GOD/reperfusion, the immunofluorescence of synapsin I increased in the stratum radiatum and the stratum oriens of the CA1 area and the stratum lucidum and the stratum oriens of the CA3 area. The phosphosynapsin level evidently increased in the stratum lucidum of the CA3 area after GOD for 30 min, which was reduced to the control level after reperfusion. These results suggested that the neuronal damage and degenerations were observed as a result of GOD/reperfusion and the increase in synapsin I and its phosphorylation might play a role in modulating the release of neurotransmitters via exocytosis and in the formation of new synapses after brain ischemia.

Fluorescence resonance energy transfer detection of synaptophysin I and vesicle-associated membrane protein 2 interactions during exocytosis from single live synapses

To investigate the molecular interactions of synaptophysin I and vesicle-associated membrane protein 2 (VAMP2)/synaptobrevin II during exocytosis, we have used time-lapse videomicroscopy to measure fluorescence resonance energy transfer in live neurons. For this purpose, fluorescent protein variants fused to synaptophysin I or VAMP2 were expressed in rat hippocampal neurons. We show that synaptophysin I and VAMP2 form both homo- and hetero-oligomers on the synaptic vesicle membrane. When exocytosis is stimulated with alpha-latrotoxin, VAMP2 dissociates from synaptophysin I even in the absence of appreciable exocytosis, whereas synaptophysin I oligomers disassemble only upon incorporation of the vesicle with the plasma membrane. We propose that synaptophysin I has multiple roles in neurotransmitter release, regulating VAMP2 availability for the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex and possibly participating in the late steps of exocytosis.

Persistent defect in transmitter release and synapsin phosphorylation in cerebral cortex after transient moderate ischemic injury

BACKGROUND AND PURPOSE

Synaptic transmission is highly vulnerable to metabolic perturbations. However, the long-term consequences of transient metabolic perturbations on synapses are not clear. We studied the long-lasting changes in synaptic transmission and phosphorylation of presynaptic proteins in penumbral cortical neurons after transient moderate ischemia.

METHODS

Rats were subjected to 1 hour of middle cerebral artery occlusion. After reperfusion, electric activity of neurons in the peri-infarct region was recorded intracellularly and extracellularly in situ. Phosphorylation of synapsin-I and tyrosine residues was studied by immunohistochemistry.

RESULTS

Neurons in the penumbra displayed no postsynaptic potentials 1 to 3 hours after recirculation. However, these cells were able to generate action potentials and were responsive to glutamate, suggesting that postsynaptic excitability was preserved but the synaptic transmission was blocked because of a presynaptic defect. The synaptic transmission was still depressed 24 hours after recirculation in neurons in the peri-infarct area that survived ischemia. The amount of immunoreactive synapsin-I, synaptophysin, and synaptotagmin was not appreciably changed for 72 hours after reperfusion. However, phosphorylation of synapsin-l was significantly decreased, whereas phosphotyrosine immunoreactivity was increased, suggesting a selective defect in synapsin-I phosphorylation.

CONCLUSIONS

These data demonstrate that synaptic transmission may be permanently impaired after transient moderate brain injury. Since postsynaptic excitability is preserved, the transmission failure is likely to be caused by presynaptic mechanisms, one of which may be impaired phosphorylation of presynaptic proteins.