A role for cGMP-dependent protein kinase II in AMPA receptor trafficking and synaptic plasticity

Background: Trafficking of AMPA receptors (AMPARs) underlies the activity-dependent modification of synaptic strength and is regulated by specific interactions of AMPAR subunits with other proteins. We have reported (Serulle et al., 2007) 1 that the AMPAR subunit GluR1 binds the cGMP-dependent kinase type II (cGKII) adjacent to the kinase catalytic site, and that this interaction is increased by cGMP. In this complex, cGKII phosphorylates GluR1 at serine 845 (S845) leading to an increase of GluR1 on the plasma membrane. In neurons, cGMP is produced by soluble guanylate cyclase (sGC), which is activated by nitric oxide (NO), which is produced by nNOS under the control of the NMDA receptor. Results: To distinguish the mechanism, we have measured the rate of exogenous GluR1 endocytosis in cultured primary neurons, either the wild type or the S845A or S845D mutants (Figure 1). We find that the S845A mutant (which cannot be phosphorylated) is endocytosed at rate of the wild type, while S845D (phosphomimetic) is endocytosed at a lower rate. Also, cGMP treatment, which elevates the endogenous GluR1 plasma membrane levels (Figure 2A), reduced the rate of of GluR1 endocytosis (Figure 2B). Figure 1 Endocytosis of HAGluR1 is time dependent and is inhibited by the phosphomimetic mutation, S845D Endocytosis of HAGluR1 is time dependent and is inhibited by the phosphomimetic mutation, S845D. Figure 2 Elevation of surface GluR1 and reduction of GluR1 endocytosis by cGMP Elevation of surface GluR1 and reduction of GluR1 endocytosis by cGMP. A. cGMP, elevates surface GluR1. B. cGMP decreases endocytosis of GluR1. Conclusion: These data suggest that S845 phosphorylation increases the plasma membrane levels of GluR1 by reducing the rate of endocytosis

Time-dependent reversal of synaptic plasticity induced by physiological concentrations of oligomeric Aβ42: an early index of Alzheimer’s disease

The oligomeric amyloid-β (Aβ) peptide is thought to contribute to the subtle amnesic changes in Alzheimer’s disease (AD) by causing synaptic dysfunction. Here, we examined the time course of synaptic changes in mouse hippocampal neurons following exposure to Aβ42 at picomolar concentrations, mimicking its physiological levels in the brain. We found opposite effects of the peptide with short exposures in the range of minutes enhancing synaptic plasticity, and longer exposures lasting several hours reducing it. The plasticity reduction was concomitant with an increase in the basal frequency of spontaneous neurotransmitter release, a higher basal number of functional presynaptic release sites, and a redistribution of synaptic proteins including the vesicle-associated proteins synapsin I, synaptophysin, and the post-synaptic glutamate receptor I. These synaptic alterations were mediated by cytoskeletal changes involving actin polymerization and p38 mitogen-activated protein kinase. These in vitro findings were confirmed in vivo with short hippocampal infusions of picomolar Aβ enhancing contextual memory and prolonged infusions impairing it. Our findings provide a model for initiation of synaptic dysfunction whereby exposure to physiologic levels of Aβ for a prolonged period of time causes microstructural changes at the synapse which result in increased transmitter release, failure of synaptic plasticity, and memory loss

Ephrin-A5 and EphA5 Interaction Induces Synaptogenesis during Early Hippocampal Development

Synaptogenesis is a fundamental step in neuronal development. For spiny glutamatergic synapses in hippocampus and cortex, synaptogenesis involves adhesion of pre and postsynaptic membranes, delivery and anchorage of pre and postsynaptic structures including scaffolds such as PSD-95 and NMDA and AMPA receptors, which are glutamate-gated ion channels, as well as the morphological maturation of spines. Although electrical activity-dependent mechanisms are established regulators of these processes, the mechanisms that function during early development, prior to the onset of electrical activity, are unclear. The Eph receptors and ephrins provide cell contact-dependent pathways that regulate axonal and dendritic development. Members of the ephrin-A family are glycosyl-phosphatidylinositol-anchored to the cell surface and activate EphA receptors, which are receptor tyrosine kinases.Here we show that ephrin-A5 interaction with the EphA5 receptor following neuron-neuron contact during early development of hippocampus induces a complex program of synaptogenic events, including expression of functional synaptic NMDA receptor-PSD-95 complexes plus morphological spine maturation and the emergence of electrical activity. The program depends upon voltage-sensitive calcium channel Ca2+ fluxes that activate PKA, CaMKII and PI3 kinase, leading to CREB phosphorylation and a synaptogenic program of gene expression. AMPA receptor subunits, their scaffolds and electrical activity are not induced. Strikingly, in contrast to wild type, stimulation of hippocampal slices from P6 EphA5 receptor functional knockout mice yielded no NMDA receptor currents.These studies suggest that ephrin-A5 and EphA5 signals play a necessary, activity-independent role in the initiation of the early phases of synaptogenesis. The coordinated expression of the NMDAR and PSD-95 induced by eprhin-A5 interaction with EphA5 receptors may be the developmental switch that induces expression of AMPAR and their interacting proteins and the transition to activity-dependent synaptic regulation

Involvement of dopamine D₂ and 5-HT_1A receptors in roxindole-induced antinociception

234-237Roxindole, a DA D2 receptor aganist (2-16 mg/kg) produced dose-dependent increase in percentage antinociception. The effect which was blocked by DA D2 antagonist (-)sulpiride (50 mg/kg) and 5-HT1A receptor antagonist (-) pin dolol (5 mg/kg). Roxindole (4 and 8 mg/kg) reversed both naloxone (20 mg/kg)-induced hyperalgesia and reserpine (2 mg/kg)-induced hyperalgesia. This reversal was sensitive to blockade by both (-)sulpiride (50 mg/kg) and (-) pindolol (5mg/kg). The present study suggests that roxindole-induced antinociception is mediated by postsynaptic DA D2 and 5-HT1A receptors

Biphasic modulation of NMDA-induced responses in pyramidal cells of the medial prefrontal cortex by Y-931, a potential atypical antipsychotic drug

Similar to the effects produced by the atypical antipsychotic drugs (APDs) clozapine and olanzapine, Y-931 {8-fluoro-12(4-methylpiperazin-1-yl)-6H-[1]benzothieno[2,3-b][1,5] benzodiazepine maleate, a purported atypical APD} effectively facilitated N-methyl-D-aspartate (NMDA)-induced, but not (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-evoked, responses in pyramidal cells of the rat medial prefrontal cortex (mPFC). Similar to olanzapine and clozapine, the concentration-response curve of Y-931 in these experiments was biphasic. At present, the mechanisms behind the biphasic modulatory actions of Y-931 and olanzapine on NMDA-induced currents in the mPFC are not clear. In addition to augmenting NMDA responses, Y-931 prevented the phencyclidine (PCP)-induced block of the NMDA responses and increased the amplitudes and durations of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of the forceps minor. Overall, our findings suggest that APDs, particularly the atypical ones, share a common property in that they facilitate NMDA receptor-mediated transmission in the mPFC and perhaps other functionally related limbic structures as well, which could be the cellular basis for their ability to alleviate some schizophrenic negative symptoms and cognitive dysfunctions. © 2001 Wiley-Liss, Inc

Early presynaptic changes during plasticity in cultured hippocampal neurons

Long-lasting increase in synaptic strength is thought to underlie learning. An explosion of data has characterized changes in postsynaptic (pstS) AMPA receptor cycling during potentiation. However, changes occurring within the presynaptic (prS) terminal remain largely unknown. We show that appearance of new release sites during potentiation between cultured hippocampal neurons is due to (a) conversion of nonrecycling sites to recycling sites, (b) formation of new releasing sites from areas containing diffuse staining for the prS marker Vesicle-Associated Membrane Protein-2 and (c) budding of new recycling sites from previously existing recycling sites. In addition, potentiation is accompanied by a release probability increase in pre-existing boutons depending upon their individual probability. These prS changes precede and regulate fluorescence increase for pstS GFP-tagged-AMPA-receptor subunit GluR1. These results suggest that potentiation involves early changes in the prS terminal including remodeling and release probability increase of pre-existing synapses

Definition of a Bidirectional Activity-Dependent Pathway Involving BDNF and Narp

One of the cardinal features of neural development and adult plasticity is the contribution of activity-dependent signaling pathways. However, the interrelationships between different activity-dependent genes are not well understood. The immediate early gene neuronal-activity-regulated pentraxin (NPTX2 or Narp) encodes a protein that has been associated with excitatory synaptogenesis, AMPA receptor aggregation, and the onset of critical periods. Here, we show that Narp is a direct transcriptional target of brain-derived neurotrophic factor (BDNF), another highly regulated activity-dependent gene involved in synaptic plasticity. Unexpectedly, Narp is bidirectionally regulated by BDNF. Acute BDNF withdrawal results in downregulation of Narp, whereas transcription of Narp is greatly enhanced by BDNF. Furthermore, our results show that BDNF directly regulates Narp to mediate glutamatergic transmission and mossy fiber plasticity. Hence, Narp serves as a significant epistatic target of BDNF to regulate synaptic plasticity during periods of dynamic activity